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Svbony 3-8mm Zoom Report


Louis D

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With all the recent hype around the APM Super Zoom this year, I was all prepared to get in line and order one for Santa to deliver this Christmas. However, new stock didn’t show up at retailers in time, so that plan went out the window. Thankfully, another exciting new zoom debuted just before the Holidays at a much more reasonable price. Once prices dropped slightly for Christmas sales, I went ahead and ordered one direct from the Svbony website. It arrived direct from China in 2 ½ weeks. It came packed in a nice black box with sturdy, closed cell foam cut/molded to fit the eyepiece. The eyepiece itself comes in a simple plastic baggie with a microfiber cleaning cloth. There are no printed inserts included in the box.

The eyepiece box's exterior and interior are shown in the following image:

618476534_SvbonyZoomBox.thumb.jpg.dd9745a38bf0eaa2db565dd2e93d8687.jpg

Exterior

The eyepiece itself is nice and sturdy, but lighter than its bulk would belie at 171 grams or 6.0 ounces by my scale. That makes it just slightly lighter than the 5mm AstroTech Paradigm (BST Starguider) at 6.8 ounces and just barely heavier than the 8mm version at 5.9 ounces.

The checkered grip ring is nice and broad despite the diminutive overall size of the eyepiece. It makes zooming the eyepiece easy to do. In use, I found that the focal length scale below the grip ring is difficult to read if you have a GSO style 2" to 1.25" adapter that partially submerges the eyepiece's upper barrel to keep 1.25" eyepieces parfocal with 2" eyepieces if they all focus at the shoulder. To be certain of which focal length I was at for each photo session, I racked the zoom collar back to 8mm and counted clicks downward from there.

The top lens cap stays on securely enough, but not nearly as snugly as with other eyepieces’ top caps. The bottom lens cap is very rubbery and fairly loose. I would imagine it could easily get lost in the field, so I make sure to remove it and leave it in my eyepiece case to avoid losing it. This cap is rather unique in having a ribbed ring around the edge closest to the closed end. I would imagine it’s there to give users a better grip to remove it. However, this presupposes that it fits tightly, which it doesn’t.

The fairly short eye cup flips up and down easily but stays up when pressed around an eye socket. The flipped up eyecup stands about 6.5mm above the flat top of the eyepiece.

The following image shows the eyepiece with caps, without caps with the eyecup flipped up and flipped down, and with the eyecup fully removed:

691140638_SvbonyZoomCapsEyecup.thumb.jpg.55f6ac79e9997efbfd7b0fb55f1634b1.jpg

The zooming action is quite smooth with positive detents at each click stop. I had no issue moving between focal lengths. The action felt quite refined. Overall, I found the eyepiece quite well made with obvious signs of precision milling inside and out.

The following image shows the eyepiece at each of the 6 zoom settings (each setting can be seen above the red dot):

335418684_SvbonyZoomSettings.thumb.jpg.9b8878855885275d0fde7153ac66ef61.jpg

The field lens is quite far from the bottom end at the all settings. What appears to be a light baffle ring moves progressively closer to the bottom end when zooming from 8mm to 3mm. I don’t believe it to be the field stop because the eyepiece barely needs refocused during zooming, yet this rings moves many millimeters. The eye lens moves dramatically upward a full 20mm while zooming from the 8mm to 3mm settings.

The following image shows the moving light baffle positions at 8mm and 3mm:

1221379187_SvbonyZoomBottom8mm3mm.jpg.c272ced4026d5809f2475aab03c74f39.jpg

The safety undercut on the chromed insertion barrel is fairly shallow with a tapered bottom edge. No hang-up issues with focuser compression rings were noted over multiple nights with multiple 2” to 1.25” adapters. The insertion barrel is just over 36mm long and may not fully seat in many 1.25” diagonals. It remained about 2mm above the top of my WO dielectric diagonal fully seated. There is the possibility of it hitting the mirror or prism of a 2” diagonal when used with a 2” to 1.25” adapter; although I had no such issues with my 2” GSO dielectric diagonal.

The eye lens is mounted basically flush with the flat top of the eyepiece with the eyecup removed. This lens is a mere 13mm in diameter, vastly limiting potential usable eye relief. The flipped down eyecup uses up less than 1mm of usable eye relief, which is excellent. The top of the eyepiece is just over 38mm in diameter with the eyecup removed. As such, I believe it is a bit small for attaching a Tele Vue Dioptrx. According to Svbony, the top is M28.5x0.6 threaded which sounds about right. The top of the eyepiece does not rotate when zooming, so using a Dioptrx with it is an option if an attachment method can be created. There is a nice wide notch below the lip that the eyecup snaps around, so it may be possible to insert O-rings or elastic hair bands in this gap to build it up to a Dioptrx compatible diameter.

Stray Light Control

The multicoatings appear entirely greenish. There are many ridges between the moving light baffle and the field lens, ostensibly to block stray light. Lens and interior blackening and baffling appears to be good as no obvious reflections were seen with the flashlight test, but it is not quite at the level of premium fixed focal length eyepieces.

Eye Relief

Svbony claims 10mm of eye relief. I measured 8mm of usable eye relief (ER) with the eyecup flipped down. This was measured from the eyecup rubber to where the exit pupil converges to the tightest circle via light projection at all zoom settings. I have read of others online measuring it to have 10mm of eye relief at 8mm, decreasing to just under 7mm at 3mm with the eyecup removed. This seems to actually be closer to reality based on my experience using the eyepiece in a telescope, but I was unable to measure an obvious decrease using my methodology to verify this.

Weirdly enough, that 8mm to 10mm of eye relief was easily sufficient to see the entire field with eyeglasses at the 8mm setting. I didn’t need push in that hard with my eyeglasses to achieve this. I can’t readily explain this since the 13mm eye lens and measured apparent field of view (AFOV, discussed later) should yield just under 12mm of eye relief. It felt more like 14mm of eye relief.

However, the usable, perceived eye relief decreases with the zoom focal length setting. At 7mm it decreases just a bit (maybe 1mm), requiring a slightly greater push against my eyeglasses. At 6mm, even more (another millimeter perhaps). By 5mm, it is becoming impossible to push in enough to see the entire AFOV. By 4mm and 3mm, eyeglasses must be removed. Even then, my long eyelashes are pressed into the flat top of the eyepiece to see the entire AFOV. Thus, I have to pull back to blink to avoid smearing the eye lens with eyelash oil. It feels even tighter than the 7mm of eye relief mentioned above. I can’t really explain why the perceived eye relief mismatches so much with the measure eye relief.

The eyecup height is just about right for use without eyeglasses at 8mm, but is increasingly too tall at decreasing focal length settings. In fact, by 5mm, it really needs to be flipped down. Unfortunately, there's no intermediate setting between all the way up and all the way down. A mechanism to withdraw the eyecup downward as the eyepiece is zoomed toward the shorter focal length end would be ideal.

It's a good thing the top of the eyepiece doesn't rotate, or the rubber eyecup would tug at the skin surrounding the eye while zooming due to the tight eye relief. This would necessitate leaving it down at all focal length settings if it did.

The video below attempts to show the effect of decreasing eye relief in this eyepiece. I zoomed the eyepiece from 8mm to 3mm in 1mm increments and then back again to 8mm with the back of the cellphone in its case resting on the flipped up eyecup. Clearly, the AFOV visible from a fixed position above the eye lens drops off dramatically as the eyepiece is zoomed, indicating dramatically decreasing eye relief. Since the eyecup top is 6.5mm above the top of the eyepiece, and the entrance pupil of the camera is probably 3mm inside the camera lens, 9mm to 10mm sounds about right for eye relief from the flat top at the 8mm zoom setting. I didn’t try to measure the separation at other zoom settings to image the entire AFOV, but I recall it getting quite small by 3mm. I may try to measure this separation someday to get a more concrete eye relief measurement, or at least it's decrease from one end to the other. Please disregard the loud sound of the eyepiece in the video. The camera's microphone was quite close to the eyepiece. In practice, about all you hear when zooming is the clicking of each setting's detent.

SAEP

I have not noticed any spherical aberration of the exit pupil (SAEP) or kidney beaning either photographically or when observing bright objects such as the moon at any zoom setting. This is a laudable achievement.

CAEP

I could not detect any chromatic aberration of the exit pupil (CAEP) which leads to the "ring of fire" as seen in the TV Nagler T5 31mm and ES-82 30mm eyepieces. This is fairly uncommon in eyepieces of this focal length range and AFOV size anyway, but I thought I should mention lack of it to be complete.

AFOVs

Using both the flashlight projection method with the eyepiece mounted in a telescope and my photographic method, I measured a pretty consistent 58° to 61° AFOV across the focal length range. The only easy way to see the variation in AFOV is to rapidly zoom from end to end and observe the field stop in peripheral vision. Svbony only claims 56° for its AFOV which is unusual in this day of exaggerated AFOV claims. A zoom having a constant AFOV of around 60° in this eyepiece's current price range is a fantastic accomplishment.

The field of view measurements, along with several others described in the next section, are listed in the table below.

2022619675_Svbony3-8mmZoomMeasurements.thumb.PNG.4e571d7c6bd2c43b4a217b08da7e0538.PNG

Field Stop Diameters and Focal Lengths

I photographically measured the effective field stop (FS) diameters and central and edge focal lengths at each zoom setting. They are listed in the above table. All but the 3mm setting agree well with the marked focal lengths. This is an amazing showing by such an affordable zoom eyepiece. Radial edge magnification distortion is a bit on the high side for a 60° class eyepiece.  This is visible as stretching of the distance between ruler marks toward the edge in the comparison AFOV images at the end this report.

Under the Stars

Enough about the eyepiece's specifications, how does it view the night sky in a telescope? I spent several nights using this zoom in my 8" f/6 Dobsonian without my GSO coma corrector to eliminate seeing CC induced spherical aberration at high powers and in my 90mm TS-Optics Photoline f/6.6 FPL53 Triplet APO with a properly spaced TSFLAT2 flattener. I need to spend more time with it to refine my impressions of it, but time during the Holidays has been limited due to family obligations. I will simply include the following previously posted recollections for now and update later with more recent observations as I get more time with it:

  • It is close to parfocal, but for critical focus on objects like Jupiter's bands, an eighth turn of the fine focus knob was needed going from 8mm to 7mm. Less was needed for the next few jumps. End to end refocusing was definitely needed no matter which end was focused first. However, on star fields like the Pleiades, the defocus was hardly noticeable.
  • Star fields like the Pleaides looked great across the zoom range.
  • Zooming in to the 4mm and 3mm settings while viewing can be disconcerting as the top of the eyepiece lunges upward toward your eye. I certainly was unprepared for this the first time I zoomed while viewing. My other zooms don't change their physical length while zooming. While my Speers-Waler 5-8mm grows dramatically in length when changing focal lengths, I don't consider it a zoom, so it doesn't count when discussing true zooms. Instead, I consider it a varifocal since there is no effort to maintain parfocality in its design.
  • The fieldstop is nice and sharp throughout the focal range.
  • There was a bit of light leakage through the field stop indicating it is not a physical ring defining it but rather the combination of several lens edges.
  • It is a noticeably sharp eyepiece except maybe in the last 10% of the field near the edge at the shorter focal lengths. Even then, it's relatively unobtrusive in use, and I wouldn't have noticed it without specifically looking for it in my photos and during star testing.
  • It seemed like there was a touch of field curvature (focus change) center to edge across the zoom range, but it was barely noticeable.
  • Field distortion seemed low. Yes, there was a bit of rolling ball effect going on during panning, but it was pretty minor.
  • I specifically looked for light scatter around bright objects, but I didn't see any obvious issues at any focal length.
  • There is minor yellow fringing at the 3mm and 4mm settings. I have yet to compare my 3.5mm Pentax XW to the zoom at these settings to see if the fringing persists. I do see similar fringing in my 2.5mm TMB Planetary clone, so it might exist independently of the eyepiece.

The following notes were made in comparison to my 24 year old 5-8mm Speers-Waler varifocal eyepiece in my Dob without a coma corrector:

  • Both were showing subtle lunar mara contrast details quite well no matter where I put them in either eyepiece's AFOV.
  • Both fields appeared flat of focus center to edge to my presbyopic eyes.
  • 5mm seemed to be the limit of usable magnification to my eyes most nights. Any higher, and my floaters got in the way, and no finer detail was revealed in the Svbony despite the higher power and greater image scale. Again, 8mm in both seemed best as far as contrast and ease of seeing fine details. Those without floaters or older eye issues might have a different experience.
  • The lack of a CC didn't seem to hurt axial details moved to the edge all that much with the moon. What was more important was slanting my view with or without eyeglasses by tilting my whole head and pulling back to get a straight on view of the edge rays. This minimized chromatic aberrations.
  • The S-W's 78° AFOV and 7-11mm of usable measured eye relief felt way more comfortable with and without glasses than the Svbony's 60° AFOV and 7-10mm of measured usable eye relief. There's more to ER than what the measurements say with zooms/varifocals.
  • I could see the Svbony's AFOV receding away as I zoomed in. The S-W didn't seem to do this all that much. This made the Svbony's AFOV feel much narrower at higher powers than it really was.
  • With my 2.5 diopters of astigmatism, the views at 6mm to 8mm were subtly sharper with glasses than without despite the small exit pupils in the f/6 scope.
  • The lack of parfocality of the S-W was far outweighed by the huge AFOV and slightly longer eye relief. Much more of the moon, if not all of it at lower powers, would fit in the S-W's AFOV giving a much more satisfying view.
  • The Dob didn't care how huge and long the S-W was compared to the Svbony as far as balance. This might not be the case with a small alt-az mounted refractor, Mak, SCT, or Newt.
  • The Svbony handily wins out for use in a travel kit by being considerably smaller, lighter, and easier to replace should it be lost or stolen.

The following image shows the size difference between the Svbony 3-8mm zoom and the Speers-Waler 5-8mm varifocal at each eyepiece’s 8mm and 5mm settings:

282711224_SvbonyvsSpeers-WalerZooms.thumb.jpg.378368f4b9b3aa6080b5ae276e2ff9ad.jpg

Obviously, the S-W is huge in comparison to the relatively diminutive Svbony.  It’s also clear that the Svbony gets only a little bit longer at 5mm while the S-W gets massively longer at 5mm. That, and the S-W eye lens diameter at 25mm is nearly twice as wide as the Svbony’s. However, due to the S-W’s much wider AFOV (78°), usable eye relief ranges from 7mm at the 5mm setting to 11mm at 8mm setting, so still very tight with eyeglasses. This is the major reason I rarely use it, not because of its massive size or lack of parfocality.

Conclusion

I have come to the realization that this eyepiece might be a great eyepiece for newbies trying to populate their high power collection as its price decreases from its initial offering price. If eye relief isn't an issue, it can easily replace 3.2mm, 4mm, 4.5mm, 5mm, 6mm, 7mm, and 8mm TMB Planetary eyepieces along with 3.2mm, 5mm and 8mm Starguiders/Paradigms without giving up anything except eye relief. That makes it a bargain at its current price.

As I became more accustomed to it, I really began to appreciate what an optical engineering achievement this little eyepiece really is. It appears to have taken the 3-6mm Nagler Zoom as a starting point, lengthened the focal range upward to 8mm and widened the AFOV by 10 degrees, all while maintaining very good optical quality at an exceedingly affordable price point. That, and the mechanical aspects are top notch as well. It just exudes quality and attention to detail.

Comparison Images

The image below shows the zoom’s AFOV at each of the 6 zoom click-stop settings, all at the same scale as originally taken without resizing.

525896562_Svbony3-8ZoomAFOVComparison1.thumb.jpg.83273ced9fc494f8cecd43dd8a40331c.jpg

Clearly, AFOV doesn't change much, just image scale.

The following images show the zoom at each click-stop setting in comparison to my other eyepieces of similar focal lengths.

1843970769_2.5mm-4.5mmAFOV1.thumb.jpg.492d77b5150bb0c003cb0e7ebb12ce24.jpg

985896394_5mmAFOV2.thumb.jpg.b6eb6949871dc8f7a0906a9169655738.jpg

1308931296_6mm-6.5mmAFOV1.thumb.jpg.6da7ca07402e244596a323f6bab00eeb.jpg

1939046_7mmAFOV1.thumb.jpg.80733ed16b887710be15e85c0aaee454.jpg

1913017594_8mmAFOV1.thumb.jpg.2bc3a71102b5465e3627ddb3aeb07ba9.jpg

All AFOV images were taken through an Astro Tech 72ED telescope with a properly spaced TSFLAT2 field flattener and then composited together in Photoshop. The objective to target distance was approximately 35 feet for all images. All sub-images were taken with a Samsung Galaxy S7 phone camera except for the “Full View” ones which were taken with the superwide angle LG G6 phone camera and then scaled up to match the central image scale of the S7 images, so the entire field of view can be compared for eyepieces exceeding the approximately 76° angle of view limit of the S7 (corner to corner).

The edge images were also taken with the S7 camera, but pointed straight at the edge to best capture the true edge sharpness that would be experienced by looking straight at the edge with the eye.

Edited by Louis D
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Excellent detailed review, and thanks very much for taking the time to write this.  I've been using your reviews as a gold standard for choosing eyepieces.  It's the reason I picked this up whilst your initial thoughts were put up on another thread!

I am basically the newbie you suggest this would be well suited to, 6 months observing with a 130PDS (F5/650mmFL).  I agree that this is an excellent choice for a short focal length eyepiece.  I can directly compare it to the Starguider/Paradigm 8mm with the matched x2 Barlow and found that with the exception of the eye relief the svbony zoom was superior for viewing Jupiter and Mars with my setup at 8 and 4mm (as well as having all the additional focal lengths).  I have yet to try it on lunar or DSO observing.

Given that for two of the Starguiders (or one and the barlow) you are looking at (as of Jan 2023) £100-110, versus £105-115 for the Zoom I would agree that the zoom is the better choice for a short focal length option, provided you don’t need more eye relief.

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And, the zoom is the equivalent of 11 eyepieces spaced a half mm apart, or 21 eyepieces space 0.25mm apart, etc.

Can't beat the price.

It has some issues (eye relief at the short end), a longer-than-average 1.25" barrel, etc., but so do other inexpensive eyepieces.

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  • 2 weeks later...
On 08/01/2023 at 21:36, Louis D said:

There is minor yellow fringing at the 3mm and 4mm settings. I have yet to compare my 3.5mm Pentax XW to the zoom at these settings to see if the fringing persists. I do see similar fringing in my 2.5mm TMB Planetary clone, so it might exist independently of the eyepiece.

Finally compared the 3mm (really 3.5mm) setting to my 3.5mm Pentax XW.  There is no color fringing anywhere in the field with the Pentax, but there is in the Svbony.

After comparing images with my premium eyepieces, I'd say from 5mm to 8mm, the Svbony hangs with the best of them.  At 4mm and 3mm (really 3.5mm), image quality becomes second tier.  So, to get premium level performance from 5mm to 8mm at it's current price is a pretty good deal.  I'd consider the sub-5mm settings as nice to have, but not really showing any additional detail due to aberrations.

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  • 2 weeks later...

Tonight, I was viewing Comet C/2022 E3 (ZTF) with the Svbony 3-8mm zoom getting good views in my GSO 6" f/5 Newt under Bortle 6/7 skies before the full moon overwhelmed my skies.  My Speers-Waler 5-8mm zoom was also showing it well.  However, when I swung the scope over to nearby Capella, I noticed that the Svbony seems to show more scatter around this bright star than the S-W zoom.  I'll have to investigate this further on other nights with other objects.  I don't recall scatter being a big issue with Jupiter in my earlier testing.

If anyone else has seen increased scatter in the Svbony compared to other eyepieces (zoom or not) on bright objects, please post your observations on here.

I was alerted in a PM that there is also the possibility of it viewing slightly darker than other same focal length eyepieces, like the Vixen LV eyepieces of the 90s.  I think I may have seen this effect, but it is subtle if it exists.  I definitely think the Orion Nebula showed up better in the Svbony than in the S-W zoom possibly because of this background sky darkening effect.

Again, post on here if you're seeing the Svbony zoom viewing darker than other eyepieces at the same focal length(s).

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5 hours ago, Louis D said:

Tonight, I was viewing Comet C/2022 E3 (ZTF) with the Svbony 3-8mm zoom getting good views in my GSO 6" f/5 Newt under Bortle 6/7 skies before the full moon overwhelmed my skies.  My Speers-Waler 5-8mm zoom was also showing it well.  However, when I swung the scope over to nearby Capella, I noticed that the Svbony seems to show more scatter around this bright star than the S-W zoom.  I'll have to investigate this further on other nights with other objects.  I don't recall scatter being a big issue with Jupiter in my earlier testing.

If anyone else has seen increased scatter in the Svbony compared to other eyepieces (zoom or not) on bright objects, please post your observations on here.

I was alerted in a PM that there is also the possibility of it viewing slightly darker than other same focal length eyepieces, like the Vixen LV eyepieces of the 90s.  I think I may have seen this effect, but it is subtle if it exists.  I definitely think the Orion Nebula showed up better in the Svbony than in the S-W zoom possibly because of this background sky darkening effect.

Again, post on here if you're seeing the Svbony zoom viewing darker than other eyepieces at the same focal length(s).

That's interesting, Louis.

Scatter was the defect I was most interested in with mine, and I was actually quite pleased (there's that "quite" word again, sorry). It lost out narrowly against a Morpheus 6.5mm, for example, but I'd expect that. It was noticeably better than my Nirvana 4mm, though. I did find it a bit difficult sometimes to exclude the effects of possible high cloud, too thin to see with the naked eye, but enough to increase the scattering.

On the image darkness, again I was happy overall. The Svbony views in good conditions seemed quite vibrant to me.

I have been meaning to post my review, but it's turned into a bit of a marathon. I should finish this week.

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On 09/01/2023 at 03:36, Louis D said:

With all the recent hype around the APM Super Zoom this year, I was all prepared to get in line and order one for Santa to deliver this Christmas. However, new stock didn’t show up at retailers in time, so that plan went out the window. Thankfully, another exciting new zoom debuted just before the Holidays at a much more reasonable price. Once prices dropped slightly for Christmas sales, I went ahead and ordered one direct from the Svbony website. It arrived direct from China in 2 ½ weeks. It came packed in a nice black box with sturdy, closed cell foam cut/molded to fit the eyepiece. The eyepiece itself comes in a simple plastic baggie with a microfiber cleaning cloth. There are no printed inserts included in the box.

The eyepiece box's exterior and interior are shown in the following image:

618476534_SvbonyZoomBox.thumb.jpg.dd9745a38bf0eaa2db565dd2e93d8687.jpg

Exterior

The eyepiece itself is nice and sturdy, but lighter than its bulk would belie at 171 grams or 6.0 ounces by my scale. That makes it just slightly lighter than the 5mm AstroTech Paradigm (BST Starguider) at 6.8 ounces and just barely heavier than the 8mm version at 5.9 ounces.

The checkered grip ring is nice and broad despite the diminutive overall size of the eyepiece. It makes zooming the eyepiece easy to do. In use, I found that the focal length scale below the grip ring is difficult to read if you have a GSO style 2" to 1.25" adapter that partially submerges the eyepiece's upper barrel to keep 1.25" eyepieces parfocal with 2" eyepieces if they all focus at the shoulder. To be certain of which focal length I was at for each photo session, I racked the zoom collar back to 8mm and counted clicks downward from there.

The top lens cap stays on securely enough, but not nearly as snugly as with other eyepieces’ top caps. The bottom lens cap is very rubbery and fairly loose. I would imagine it could easily get lost in the field, so I make sure to remove it and leave it in my eyepiece case to avoid losing it. This cap is rather unique in having a ribbed ring around the edge closest to the closed end. I would imagine it’s there to give users a better grip to remove it. However, this presupposes that it fits tightly, which it doesn’t.

The fairly short eye cup flips up and down easily but stays up when pressed around an eye socket. The flipped up eyecup stands about 6.5mm above the flat top of the eyepiece.

The following image shows the eyepiece with caps, without caps with the eyecup flipped up and flipped down, and with the eyecup fully removed:

691140638_SvbonyZoomCapsEyecup.thumb.jpg.55f6ac79e9997efbfd7b0fb55f1634b1.jpg

The zooming action is quite smooth with positive detents at each click stop. I had no issue moving between focal lengths. The action felt quite refined. Overall, I found the eyepiece quite well made with obvious signs of precision milling inside and out.

The following image shows the eyepiece at each of the 6 zoom settings (each setting can be seen above the red dot):

335418684_SvbonyZoomSettings.thumb.jpg.9b8878855885275d0fde7153ac66ef61.jpg

The field lens is quite far from the bottom end at the all settings. What appears to be a light baffle ring moves progressively closer to the bottom end when zooming from 8mm to 3mm. I don’t believe it to be the field stop because the eyepiece barely needs refocused during zooming, yet this rings moves many millimeters. The eye lens moves dramatically upward a full 20mm while zooming from the 8mm to 3mm settings.

The following image shows the moving light baffle positions at 8mm and 3mm:

1221379187_SvbonyZoomBottom8mm3mm.jpg.c272ced4026d5809f2475aab03c74f39.jpg

The safety undercut on the chromed insertion barrel is fairly shallow with a tapered bottom edge. No hang-up issues with focuser compression rings were noted over multiple nights with multiple 2” to 1.25” adapters. The insertion barrel is just over 36mm long and may not fully seat in many 1.25” diagonals. It remained about 2mm above the top of my WO dielectric diagonal fully seated. There is the possibility of it hitting the mirror or prism of a 2” diagonal when used with a 2” to 1.25” adapter; although I had no such issues with my 2” GSO dielectric diagonal.

The eye lens is mounted basically flush with the flat top of the eyepiece with the eyecup removed. This lens is a mere 13mm in diameter, vastly limiting potential usable eye relief. The flipped down eyecup uses up less than 1mm of usable eye relief, which is excellent. The top of the eyepiece is just over 38mm in diameter with the eyecup removed. As such, I believe it is a bit small for attaching a Tele Vue Dioptrx. According to Svbony, the top is M28.5x0.6 threaded which sounds about right. The top of the eyepiece does not rotate when zooming, so using a Dioptrx with it is an option if an attachment method can be created. There is a nice wide notch below the lip that the eyecup snaps around, so it may be possible to insert O-rings or elastic hair bands in this gap to build it up to a Dioptrx compatible diameter.

Stray Light Control

The multicoatings appear entirely greenish. There are many ridges between the moving light baffle and the field lens, ostensibly to block stray light. Lens and interior blackening and baffling appears to be good as no obvious reflections were seen with the flashlight test, but it is not quite at the level of premium fixed focal length eyepieces.

Eye Relief

Svbony claims 10mm of eye relief. I measured 8mm of usable eye relief (ER) with the eyecup flipped down. This was measured from the eyecup rubber to where the exit pupil converges to the tightest circle via light projection at all zoom settings. I have read of others online measuring it to have 10mm of eye relief at 8mm, decreasing to just under 7mm at 3mm with the eyecup removed. This seems to actually be closer to reality based on my experience using the eyepiece in a telescope, but I was unable to measure an obvious decrease using my methodology to verify this.

Weirdly enough, that 8mm to 10mm of eye relief was easily sufficient to see the entire field with eyeglasses at the 8mm setting. I didn’t need push in that hard with my eyeglasses to achieve this. I can’t readily explain this since the 13mm eye lens and measured apparent field of view (AFOV, discussed later) should yield just under 12mm of eye relief. It felt more like 14mm of eye relief.

However, the usable, perceived eye relief decreases with the zoom focal length setting. At 7mm it decreases just a bit (maybe 1mm), requiring a slightly greater push against my eyeglasses. At 6mm, even more (another millimeter perhaps). By 5mm, it is becoming impossible to push in enough to see the entire AFOV. By 4mm and 3mm, eyeglasses must be removed. Even then, my long eyelashes are pressed into the flat top of the eyepiece to see the entire AFOV. Thus, I have to pull back to blink to avoid smearing the eye lens with eyelash oil. It feels even tighter than the 7mm of eye relief mentioned above. I can’t really explain why the perceived eye relief mismatches so much with the measure eye relief.

The eyecup height is just about right for use without eyeglasses at 8mm, but is increasingly too tall at decreasing focal length settings. In fact, by 5mm, it really needs to be flipped down. Unfortunately, there's no intermediate setting between all the way up and all the way down. A mechanism to withdraw the eyecup downward as the eyepiece is zoomed toward the shorter focal length end would be ideal.

It's a good thing the top of the eyepiece doesn't rotate, or the rubber eyecup would tug at the skin surrounding the eye while zooming due to the tight eye relief. This would necessitate leaving it down at all focal length settings if it did.

The video below attempts to show the effect of decreasing eye relief in this eyepiece. I zoomed the eyepiece from 8mm to 3mm in 1mm increments and then back again to 8mm with the back of the cellphone in its case resting on the flipped up eyecup. Clearly, the AFOV visible from a fixed position above the eye lens drops off dramatically as the eyepiece is zoomed, indicating dramatically decreasing eye relief. Since the eyecup top is 6.5mm above the top of the eyepiece, and the entrance pupil of the camera is probably 3mm inside the camera lens, 9mm to 10mm sounds about right for eye relief from the flat top at the 8mm zoom setting. I didn’t try to measure the separation at other zoom settings to image the entire AFOV, but I recall it getting quite small by 3mm. I may try to measure this separation someday to get a more concrete eye relief measurement, or at least it's decrease from one end to the other. Please disregard the loud sound of the eyepiece in the video. The camera's microphone was quite close to the eyepiece. In practice, about all you hear when zooming is the clicking of each setting's detent.

SAEP

I have not noticed any spherical aberration of the exit pupil (SAEP) or kidney beaning either photographically or when observing bright objects such as the moon at any zoom setting. This is a laudable achievement.

CAEP

I could not detect any chromatic aberration of the exit pupil (CAEP) which leads to the "ring of fire" as seen in the TV Nagler T5 31mm and ES-82 30mm eyepieces. This is fairly uncommon in eyepieces of this focal length range and AFOV size anyway, but I thought I should mention lack of it to be complete.

AFOVs

Using both the flashlight projection method with the eyepiece mounted in a telescope and my photographic method, I measured a pretty consistent 58° to 61° AFOV across the focal length range. The only easy way to see the variation in AFOV is to rapidly zoom from end to end and observe the field stop in peripheral vision. Svbony only claims 56° for its AFOV which is unusual in this day of exaggerated AFOV claims. A zoom having a constant AFOV of around 60° in this eyepiece's current price range is a fantastic accomplishment.

The field of view measurements, along with several others described in the next section, are listed in the table below.

2022619675_Svbony3-8mmZoomMeasurements.thumb.PNG.4e571d7c6bd2c43b4a217b08da7e0538.PNG

Field Stop Diameters and Focal Lengths

I photographically measured the effective field stop (FS) diameters and central and edge focal lengths at each zoom setting. They are listed in the above table. All but the 3mm setting agree well with the marked focal lengths. This is an amazing showing by such an affordable zoom eyepiece. Radial edge magnification distortion is a bit on the high side for a 60° class eyepiece.  This is visible as stretching of the distance between ruler marks toward the edge in the comparison AFOV images at the end this report.

Under the Stars

Enough about the eyepiece's specifications, how does it view the night sky in a telescope? I spent several nights using this zoom in my 8" f/6 Dobsonian without my GSO coma corrector to eliminate seeing CC induced spherical aberration at high powers and in my 90mm TS-Optics Photoline f/6.6 FPL53 Triplet APO with a properly spaced TSFLAT2 flattener. I need to spend more time with it to refine my impressions of it, but time during the Holidays has been limited due to family obligations. I will simply include the following previously posted recollections for now and update later with more recent observations as I get more time with it:

  • It is close to parfocal, but for critical focus on objects like Jupiter's bands, an eighth turn of the fine focus knob was needed going from 8mm to 7mm. Less was needed for the next few jumps. End to end refocusing was definitely needed no matter which end was focused first. However, on star fields like the Pleiades, the defocus was hardly noticeable.
  • Star fields like the Pleaides looked great across the zoom range.
  • Zooming in to the 4mm and 3mm settings while viewing can be disconcerting as the top of the eyepiece lunges upward toward your eye. I certainly was unprepared for this the first time I zoomed while viewing. My other zooms don't change their physical length while zooming. While my Speers-Waler 5-8mm grows dramatically in length when changing focal lengths, I don't consider it a zoom, so it doesn't count when discussing true zooms. Instead, I consider it a varifocal since there is no effort to maintain parfocality in its design.
  • The fieldstop is nice and sharp throughout the focal range.
  • There was a bit of light leakage through the field stop indicating it is not a physical ring defining it but rather the combination of several lens edges.
  • It is a noticeably sharp eyepiece except maybe in the last 10% of the field near the edge at the shorter focal lengths. Even then, it's relatively unobtrusive in use, and I wouldn't have noticed it without specifically looking for it in my photos and during star testing.
  • It seemed like there was a touch of field curvature (focus change) center to edge across the zoom range, but it was barely noticeable.
  • Field distortion seemed low. Yes, there was a bit of rolling ball effect going on during panning, but it was pretty minor.
  • I specifically looked for light scatter around bright objects, but I didn't see any obvious issues at any focal length.
  • There is minor yellow fringing at the 3mm and 4mm settings. I have yet to compare my 3.5mm Pentax XW to the zoom at these settings to see if the fringing persists. I do see similar fringing in my 2.5mm TMB Planetary clone, so it might exist independently of the eyepiece.

The following notes were made in comparison to my 24 year old 5-8mm Speers-Waler varifocal eyepiece in my Dob without a coma corrector:

  • Both were showing subtle lunar mara contrast details quite well no matter where I put them in either eyepiece's AFOV.
  • Both fields appeared flat of focus center to edge to my presbyopic eyes.
  • 5mm seemed to be the limit of usable magnification to my eyes most nights. Any higher, and my floaters got in the way, and no finer detail was revealed in the Svbony despite the higher power and greater image scale. Again, 8mm in both seemed best as far as contrast and ease of seeing fine details. Those without floaters or older eye issues might have a different experience.
  • The lack of a CC didn't seem to hurt axial details moved to the edge all that much with the moon. What was more important was slanting my view with or without eyeglasses by tilting my whole head and pulling back to get a straight on view of the edge rays. This minimized chromatic aberrations.
  • The S-W's 78° AFOV and 7-11mm of usable measured eye relief felt way more comfortable with and without glasses than the Svbony's 60° AFOV and 7-10mm of measured usable eye relief. There's more to ER than what the measurements say with zooms/varifocals.
  • I could see the Svbony's AFOV receding away as I zoomed in. The S-W didn't seem to do this all that much. This made the Svbony's AFOV feel much narrower at higher powers than it really was.
  • With my 2.5 diopters of astigmatism, the views at 6mm to 8mm were subtly sharper with glasses than without despite the small exit pupils in the f/6 scope.
  • The lack of parfocality of the S-W was far outweighed by the huge AFOV and slightly longer eye relief. Much more of the moon, if not all of it at lower powers, would fit in the S-W's AFOV giving a much more satisfying view.
  • The Dob didn't care how huge and long the S-W was compared to the Svbony as far as balance. This might not be the case with a small alt-az mounted refractor, Mak, SCT, or Newt.
  • The Svbony handily wins out for use in a travel kit by being considerably smaller, lighter, and easier to replace should it be lost or stolen.

The following image shows the size difference between the Svbony 3-8mm zoom and the Speers-Waler 5-8mm varifocal at each eyepiece’s 8mm and 5mm settings:

282711224_SvbonyvsSpeers-WalerZooms.thumb.jpg.378368f4b9b3aa6080b5ae276e2ff9ad.jpg

Obviously, the S-W is huge in comparison to the relatively diminutive Svbony.  It’s also clear that the Svbony gets only a little bit longer at 5mm while the S-W gets massively longer at 5mm. That, and the S-W eye lens diameter at 25mm is nearly twice as wide as the Svbony’s. However, due to the S-W’s much wider AFOV (78°), usable eye relief ranges from 7mm at the 5mm setting to 11mm at 8mm setting, so still very tight with eyeglasses. This is the major reason I rarely use it, not because of its massive size or lack of parfocality.

Conclusion

I have come to the realization that this eyepiece might be a great eyepiece for newbies trying to populate their high power collection as its price decreases from its initial offering price. If eye relief isn't an issue, it can easily replace 3.2mm, 4mm, 4.5mm, 5mm, 6mm, 7mm, and 8mm TMB Planetary eyepieces along with 3.2mm, 5mm and 8mm Starguiders/Paradigms without giving up anything except eye relief. That makes it a bargain at its current price.

As I became more accustomed to it, I really began to appreciate what an optical engineering achievement this little eyepiece really is. It appears to have taken the 3-6mm Nagler Zoom as a starting point, lengthened the focal range upward to 8mm and widened the AFOV by 10 degrees, all while maintaining very good optical quality at an exceedingly affordable price point. That, and the mechanical aspects are top notch as well. It just exudes quality and attention to detail.

Comparison Images

The image below shows the zoom’s AFOV at each of the 6 zoom click-stop settings, all at the same scale as originally taken without resizing.

525896562_Svbony3-8ZoomAFOVComparison1.thumb.jpg.83273ced9fc494f8cecd43dd8a40331c.jpg

Clearly, AFOV doesn't change much, just image scale.

The following images show the zoom at each click-stop setting in comparison to my other eyepieces of similar focal lengths.

1843970769_2.5mm-4.5mmAFOV1.thumb.jpg.492d77b5150bb0c003cb0e7ebb12ce24.jpg

985896394_5mmAFOV2.thumb.jpg.b6eb6949871dc8f7a0906a9169655738.jpg

1308931296_6mm-6.5mmAFOV1.thumb.jpg.6da7ca07402e244596a323f6bab00eeb.jpg

1939046_7mmAFOV1.thumb.jpg.80733ed16b887710be15e85c0aaee454.jpg

1913017594_8mmAFOV1.thumb.jpg.2bc3a71102b5465e3627ddb3aeb07ba9.jpg

All AFOV images were taken through an Astro Tech 72ED telescope with a properly spaced TSFLAT2 field flattener and then composited together in Photoshop. The objective to target distance was approximately 35 feet for all images. All sub-images were taken with a Samsung Galaxy S7 phone camera except for the “Full View” ones which were taken with the superwide angle LG G6 phone camera and then scaled up to match the central image scale of the S7 images, so the entire field of view can be compared for eyepieces exceeding the approximately 76° angle of view limit of the S7 (corner to corner).

The edge images were also taken with the S7 camera, but pointed straight at the edge to best capture the true edge sharpness that would be experienced by looking straight at the edge with the eye.

I don't know how I missed your first review, Louis, but I could just have referenced it in mine, and saved on some typing. The test images are interesting, as ever, particularly the comparisons with the 5mm and 8mm Paradigms (the same as our Starguiders, of course).

The one point I might disagree with you is where you say "as its price decreases from its initial offering"  ... I suspect the price may go up, as more reviews get written 😃

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2 hours ago, Zermelo said:

The one point I might disagree with you is where you say "as its price decreases from its initial offering"  ... I suspect the price may go up, as more reviews get written 😃

Interestingly enough, the price before Christmas went as low as just under $100 before taxes on ebay-US for a day before bouncing back up again.  I'm guessing someone was testing the waters to see if the demand would noticeably go up if the price went significantly down.  Hopefully they don't try price testing in the opposite direction to see what the market will bear.  Explore Scientific certainly has done this in recent years with most of their offerings.

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  • 4 weeks later...
1 hour ago, Kobayashi Maru said:

Do these sorts of eyepieces work well (or ok) in F5 scopes?

Yes.  It is better than the BST 8mm and 8mm + BST X2 ED Barlow in every way except eyerelief for planetary imho.

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22 hours ago, nicholasastro said:

Based on my experience with this zoom, on the whole, yes, especially between 8mm and 5mm focal lengths.

I should add that I was particularly impressed with how the zoom performed on Mars this season. I needed a night of very good seeing to appreciate just how good the Svbony is. I had one such night, when I was up at 583x. On that occasion, there was also very little water vapour in the air - which otherwise can create the perception of light scatter. During that evening, I could see no more scatter than I could through my 3.4mm Vixen HR. Very impressed by this little gem of an eyepiece!

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7 hours ago, stormioV said:

Great review. Would this be a useful addition to my LX90 for Lunar observing. Or would it mostly unuseable.

You're going to be starting off at a small exit pupil at 8mm (8/10=0.8mm) progressing to a very tiny exit pupil by the time you get to 3mm (really 3.5mm, so 3.5/10=0.35mm).  If floaters are not an issue with your eyes, and you have really good seeing conditions (going from 2032/8 = 254x at 8mm to 2032/3.5 = 581x at 3mm/3.5mm), you might make good use of it to probe the limits of your scope, seeing, and eyes.  Have you tried experimenting with these high powers using your existing eyepieces and a 2x or 3x Barlow already?  Also, your collimation will need to be spot on for best results at these powers.

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  • 4 months later...

I had my AT72EDII out last night looking at a variety of night time objects across the sky including bright nebula, open and globular clusters, galaxies, and double stars. Included with my kit was a SvBony 3-8 zoom and a SvBony SV135 7-21 zoom.

I started out with the 7-21 zoom but while teasing out one of the tighter doubles  switched to the 3-8 zoom. I hadn’t used it in a while and so was very pleasantly reminded at just how handy and capable an eyepiece it is. The views through it were sharp and relatively distortion free, and it facilitated finding just the right magnification for the object and seeing. It’s not perfect, and my biggest complaint is that it’s stiffer to turn than I would have liked, but beyond that and at its current sale price of $120 here in the states, it’s an absolute bargain.

I had with me several wider field eyepieces, but last night as I hopped from object to object the two zooms with their combined focal length range of 3-21mm were all I used or needed. If you’re on the fence about the 3-8 I’d suggest giving it a try and while you’re at it grab a SV135 zoom as its complement. They’re perfect for a lightweight travel kit.

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