Ursa Major 7-21mm Zoom

[John]

I notice that FLO have added this zoom eyepiece to their modestly priced Ursa Major range:

Ursa Major 7-21mm FMC Zoom Eyepiece | First Light Optics

I have the Svbony branded version of this eyepiece and it works very well. As an example, using it to observe Jupiter this evening with my ED120 F/7.5 refractor, the images of Jupiter at the 7mm setting seemed just as sharp and contrasty as those served up by the Svbony 3-8mm zoom. The image held up well right across the field of view as well with the Great Red Spot and the details of it's region of the South Equatorial Belt still showing well as it slipped behind the field stop of the eyepiece. 

The apparent field of view of the Svbony 7-21mm zoom seems to match FLO's description of the Ursa Major 7-21mm as well. At 7mm the Svbony seems to show around the same AFoV as their 3-8mm zoom does, so around 56-ish degrees.

So a good buy for under 50 quid I reckon.

This addition to FLO's eyepiece range makes me curious to see whether this will be followed by a FLO version of the 3-8mm zoom ? 🤔

Watch this space, maybe ?

 

 

 

 

[AlcorAlly]

I really like the svbony zoom 7-21 for Solar Ha. Works great in the PST. I tried so many eyepieces including plossls, orhos and a Nagler and this modestly priced zoom is the winner. 

[John]

28 minutes ago, AlcorAlly said:

I really like the svbony zoom 7-21 for Solar Ha. Works great in the PST. I tried so many eyepieces including plossls, orhos and a Nagler and this modestly priced zoom is the winner. 

Yes, mine works well with my PST too !

[Second Time Around]

I was very disappointed with my Svbony 7-21mm zoom with my Quark.  It was much inferior to the Baader 8-24mm zoom on prominences, seemingly because of lower contrast.

[John]

Just now, Second Time Around said:

I was very disappointed with my Svbony 7-21mm zoom with my Quark.  It was much inferior to the Baader 8-24mm zoom on prominences, seemingly because of lower contrast.

That's interesting to know Steve.

[Zermelo]

I also have the Svbony version, and it performs well for me.

I think it did very well in Ernest's bench tests.

[Second Time Around]

Mine did too on sunspots, but not on prominences.  I don't think Ernest tests for contrast, just aberrations.

Of course it may be a quality control issue and I had a bad example. 

[John]

3 minutes ago, Second Time Around said:

Mine did too on sunspots, but not on prominences.  I don't think Ernest tests for contrast, just aberrations.

Of course it may be a quality control issue and I had a bad example. 

I think you are right about Ernest's tests. My focus this evening has not been on faint, low contrast targets so I'll need to see how it does on such stuff on another occasion.

I have owned a few Baader zooms but not at the same time as the Svbony 7-21 so I could not compare the two directly.

 

 

[PeterStudz]

I wasn’t impressed with my Svbony 7-21mm zoom. It was on low contrast targets like the planets (which I primarily purchased it for) that didn’t impress, as @Second Time Aroundsuggests. Although the Svbony appeared sharp, low contrast, or what I sometimes call colour contrast, wasn’t anywhere near as good as the BST StarGuiders that I compared it to at the time. I’m no eyepiece expert but I could see the difference immediately on both Jupiter and Saturn. 

Edited January 7 by PeterStudz

[Zermelo]

10 hours ago, Second Time Around said:

I don't think Ernest tests for contrast, just aberrations.

Yes, I suspect I've tended to use it on higher contrast targets.

[John]

Interesting range of views on the Svbony 7-21mm zoom. Unsurprising though - it's rare for an eyepiece to attract 100% consistent feedback I've found, over my years discussing them on forums 😉

At least for less than £50 a shot, it's not going to cost an arm and a leg to find out if it's for you. Unlike some we discuss 😏

[AlcorAlly]

It's not my favourite for night time, but it plays so nicely with the PST, like it was made for it.

[Louis D]

I wonder what contributes to low contrast with eyepieces?  Poor polish and coatings?  Poor stray light control?

I have noticed that cheap filters scatter light from bright objects, decreasing contrast, negating any advantage to using a filter.  I chalk it up to them either using a dyed polyester sandwich or poorly polished and dyed glass that contains micro-bubbles.  Cheap Chinese astro color filters tend to be the worst in this respect.  Vintage Japanese, American, and European made photographic filters from well known brands such as Hoya, Tiffen, and B&W tend to be the best.  I haven't tried more modern photographic filters because the 48mm size hasn't been made in years.

[Don Pensack]

On 07/01/2025 at 06:22, Louis D said:

I wonder what contributes to low contrast with eyepieces?  Poor polish and coatings?  Poor stray light control?

I have noticed that cheap filters scatter light from bright objects, decreasing contrast, negating any advantage to using a filter.  I chalk it up to them either using a dyed polyester sandwich or poorly polished and dyed glass that contains micro-bubbles.  Cheap Chinese astro color filters tend to be the worst in this respect.  Vintage Japanese, American, and European made photographic filters from well known brands such as Hoya, Tiffen, and B&W tend to be the best.  I haven't tried more modern photographic filters because the 48mm size hasn't been made in years.

Mostly:

poor polish on the lenses

lack of control of light scatter (poor baffling)

poor coatings

micro inclusions in the glass

poor adherence to the design specs in manufacturing

poor design (large spot size)

chromatic aberration

spherical aberration

use of eyepiece in short f/ratios it's not designed to handle.

 

[Zermelo]

56 minutes ago, Don Pensack said:

Mostly:

1) poor polish on the lenses

2) lack of control of light scatter (poor baffling)

3) poor coatings

4) micro inclusions in the glass

5) poor adherence to the design specs in manufacturing

6) poor design (large spot size)

7) chromatic aberration

8) spherical aberration

9) use of eyepiece in short f/ratios it's not designed to handle.

 

I've numbered your points for ease of reference.

I'm interested to pursue this question of contrast.  I understand that Ernest's tests are (deliberately) designed to be objective and, even then, some physical properties (such as no. 1 above) may not be accessible without either very specialized equipment, dismantling or destructive examination.

It seems that factors 6, 7, 8 would be apparent in such tests, while 9 could obviously be controlled for in equipment selection.
i.e. measurement of the optical effects, rather than their physical causes. Presumably 5 could also be rolled into 6, as the observed spot size would reflect both the design effectiveness and the QA variance of the test subject.

Here are the measurements for the 7-21mm zoom, with the Baader Mark IV for comparison (I don't think he tested these in slower OTAs):

Clearly the Svbony fared well in spot size, though the aberration types noted were different from (and perhaps more noticeable than?) the Baader's.

So, I think my question is this: could poorer quality wrt the other factors (1, 2, 3, 4) reduce the contrast in the Svbony relative to the Baader, while still being consistent with those spot sizes?

[John]

It would be interesting if some data was available on the light throughput of zooms vs fixed focal length designs. Some independent light throughput figures were available for a range of eyepieces but this info was at least 15 years old, possibly more. Pentax state 96% for the Pentax XW series but I've not seen similar info for other manufacturers eyepieces.

That out of date list that I mentioned had some interesting examples of eyepieces that were considered very sharp but with less impressive light throughput figures. 

Lower throughput could of course be caused by a number of the factors discussed above. Personally, I've suspected that optical polish, coating quality and light scatter control were the primary influencing factors but it could well be more complex than that. 

My original post here was prompted by the performance observing Jupiter rather than fainter. more nebulous targets.

Wherever they rank in the current eyepiece performance scheme of things, zoom eyepeices have come quite a long way today IMHO. I recently acquired a much older, unbranded zoom, which came with a scope, and I estimate covers the 7-21mm range. It's performance is really pretty awful to be honest 🙄

@Don Pensack - I hope you are not too badly affected by the LA fires Don. The footage on the news channels here is really quite scary 😮 

 

 

Edited January 8 by John

[Don Pensack]

On 08/01/2025 at 14:15, Zermelo said:

 

I've numbered your points for ease of reference.

I'm interested to pursue this question of contrast.  I understand that Ernest's tests are (deliberately) designed to be objective and, even then, some physical properties (such as no. 1 above) may not be accessible without either very specialized equipment, dismantling or destructive examination.

It seems that factors 6, 7, 8 would be apparent in such tests, while 9 could obviously be controlled for in equipment selection.
i.e. measurement of the optical effects, rather than their physical causes. Presumably 5 could also be rolled into 6, as the observed spot size would reflect both the design effectiveness and the QA variance of the test subject.

Here are the measurements for the 7-21mm zoom, with the Baader Mark IV for comparison (I don't think he tested these in slower OTAs):

Clearly the Svbony fared well in spot size, though the aberration types noted were different from (and perhaps more noticeable than?) the Baader's.

So, I think my question is this: could poorer quality wrt the other factors (1, 2, 3, 4) reduce the contrast in the Svbony relative to the Baader, while still being consistent with those spot sizes?

Yes.

[Louis D]

I have some handmade Rini eyepieces made from surplus lenses.  While the views can be sharp due to the quality lenses (many are military surplus), the lack of top notch multicoatings and proper internal baffling really cuts down on their contrast.  This is most notable with regard to sky glow.  They seem to enhance the visibility of it.  Many of the lenses are either singly MgF2 coated or uncoated altogether.