Minimum Eye Relief for Glasses Wearers

[chrispj]

This is something I imagine will vary depending on your prescription and probably other factors but hey, I think it will be interesting for me. I'm quite short-sighted with a -3 (more or less) prescription and have to wear my glasses when observing.  I'm gradually building up an eye-piece collection, mostly from 2nd hand purchases here and on UK Astrobuysell. Some eye-pieces work much better for me than others - I love the Pentax XW with its 20mm of eye relief, but I don't get on at all with the ES 18mm 82 degree where I got caught out by the 13mm relief. Somewhere between the two is going to be the limit for what I can comfortably use but I don't know where that limit is yet and there's a lot of eye-pieces that fall in between...

 

It's not so much of a commitment buying 2ndhand as it should be possible to move the eye-piece on again at a minimal loss, but it is obviously much more of a commitment to buy new, without knowing if it will work (& I don't like messing a retailer around and returning equipment that is perfectly fine but just 'wrong' for me).

 

So, it would be really helpful (for me at least) if any other glasses wearers out there would post up the lowest eye-relief eye-piece that they can comfortably use (and what the value is).

 

Thanks in advance.

[Louis D]

It's also dependent on your nose bridge to eye socket depth and eyeglass style.  Technically, you could get away with wearing a monocle wedged into your eye socket to use low eye relief eyepieces.

To answer your question, I need 18mm of measured eye relief to avoid touching the top of the eyepiece, 17mm when touching, 16mm when pushing in slightly, 15mm when really pushing in, and 14mm when cramming my eye in to the point of hurting my eye socket with my eyeglass frame.

I say measured eye relief above because manufacturers claimed eye relief is rarely the usable eye relief.

Here are my measured usable eye relief figures for my eyepiece collection:

Eyepiece	Focal length	Measured Usable Eye Relief
Pentax XW	3.5	18
Meade Series 5000 HD-60	4.5	14
AstroTech Paradigm	5.0	12
SW 5-8 Zoom	5.0	7
Pentax XL	5.2	21
Generic Huygens	6.0	3
SW 5-8 Zoom	6.0	7
Meade Series 5000 HD-60	6.5	15
Pentax XW	7.0	18
SW 5-8 Zoom	7.0	9
Surplus Shed 7.2-21.5 Zoom @ 7.2mm	7.2	10
AstroTech Paradigm	8.0	12
Celestron 8-24 Zoom @ 48x	8.0	16
SW 5-8 Zoom	8.0	11
Generic Kellner	9.0	4
Meade Silvertop Plossl	9.0	5
Vixen LV	9.0	18
Meade Series 5000 HD-60	9.0	17
Baader Morpheus	9.0	20
Celestron 8-24 Zoom @ 40x	9.6	15
Televue Delos	10.0	21
Surplus Shed 7.2-21.5 Zoom @ 14mm	12.0	9
Meade MA Astrometric	12.0	5
Celestron 8-24 Zoom @ 32x	12.0	13
Pentax XF	12.0	16
AstroTech Paradigm	12.0	12
Meade Series 5000 HD-60	12.0	17
Televue Nagler Type IV	12.0	16
ES-92	12.0	17
Generic Kellner	12.5	5
Celestron Microguide Ortho	12.5	10
AstroTech AF70	13.0	18
Pentax XL	14.0	18
Baader Morpheus	14.0	18
Meade 4000 UWA	14.0	18
AstroTech Paradigm	15.0	12
Celestron 8-24 Zoom @24x	16.0	13
B&L WF	16.7	19
AstroTech AF70	17.0	17
Televue Nagler Type IV	17.0	15
ES-92	17.0	16
AstroTech Paradigm	18.0	12
Meade Series 5000 HD-60	18.0	19
Gary Russell Konig	19.0	15
Celestron 8-24 Zoom @20x	19.2	13
Generic Huygens	20.0	13
Generic Reversed Kellner	20.0	10
SVBONY 68° Ultra Wide Angle	20.0	14
Orion Centering SWA	20.0	11
UW80	20.0	28
Surplus Shed 7.2-21.5 Zoom @ 21.5mm	21.5	11
AstroTech AF70	22.0	16
Televue Nagler Type IV	22.0	14
Aspheric 62 degree	23.0	18
Olivon 8-24 Zoom @ 16x	24.0	14
APM Ultra Flat Field	24.0	17
Edscorp Abbe Ortho	25.0	16
AstroTech Paradigm	25.0	17
Meade Series 5000 HD-60	25.0	18
Meade Silvertop Plossl	26.0	16
Orion Sirius Plossl	26.0	11
Meade MWA	26.0	10
Televue Panoptic	27.0	14
Edmunds RKE	28.0	26
Rini Modified Plossl	29.0	10
APM Ultra Flat Field	30.0	16
UW80	30.0	18
Explore Scientific 82	30.0	16
Kasai Super WideView 90°	30.0	12
Orion Sirius Plossl	32.0	15
GSO Super Plossl	32.0	15
Rini Modified Plossl	35.0	17
US Military WF	35.0	28
Baader Scopos Extreme	35.0	16
Aero ED	35.0	14
Rini Modified Plossl	38.0	22
Meade Series 5000 Plossl	40.0	27
Meade Series 5000 SWA	40.0	24
Pentax XW-R	40.0	17
Rini Erfle	42.0	23

[chrispj]

Wow, that is a comprehensive list, thanks!

[Don Pensack]

In addition to what Louis has posted, it's important to understand 2 concepts:

Design eye relief, which is the distance of the exit pupil from the top center of the eye lens in the eyepiece, and

Effective eye relief, which is the distance from the top of the rubber eyecup (when folded down) to the exit pupil.

There can be (I've measured) as much as 15mm between these two figures.

In essence, one eyepiece could have 20mm of eye relief and be unusable with glasses, while another could have 18mm of eye relief and be very easily usable with glasses.

The factors that influence that are:

--the flatness or concavity of the eye lens in the eyepiece

--the amount of depth to the eye lens in the eyepiece into the body of the eyepiece

--the shape of the rubber eyecup and how high it rises above the eyepiece body.  One recent eyepiece with a 20mm eye relief has an 8mm tall solid rubber eyecup that can't be folded down, so makes the eyepiece unusable with glasses.  It's usable with glasses if the eyecup is removed, but

         then there is no protection for the glasses from the aluminum top of the eyepiece.

--your tolerance for how hard you are willing to press your glasses against the eyepiece.  I haven't found an eyepiece yet where my glasses don't at least touch the rubber, but some required a hard press that isn't comfortable after a while.

This is why I think about the only way to be sure the eyepiece is usable with glasses is to look for at least 18mm of eye relief, then read reviews of the eyepiece, then try it for yourself.  Otherwise, it will be difficult to know.

 

There is some math you can use to decide whether the manufacturer is lying about the design eye relief:

Apparent Field of View= 2*Tan^-1(eye lens radius/eye relief)

The eye relief in the formula assumes a flat eye lens, so if the center of the eye lens is 2mm concave, and the mfr claims a 20mm eye relief, then you would use 18mm in the formula.

One recent eyepiece claims a 20mm eye relief and the eye lens is ~2mm concave.  It has a 30mm eye lens and a claimed apparent field of 80°.

Plugging 18mm of eye relief into the formula gets an apparent field of 79.6°, close enough to 80° to say the mfr is telling the truth.

Alas, the eyepiece only has 10mm of effective eye relief, so isn't usable with glasses as is.

Had the manufacturer had a smaller eye lens, then you could know they were either lying about the apparent field or the eye relief, or both.

 

[Carbon Brush]

Another consideration.

If you are short sighted - and that is the only issue - you do not need specs.
You just adjust the focus to compensate. This means you can use any eyepiece.
This works provided there is enough focus travel on the scope - which is usually the case.

If you have astigmatism, that does need correcting. However, certain of the 'posh' eyepiece manufacturers do have astigmatism correct add-ons available.
https://www.televue.com/engine/TV3b_page.asp?id=54
This does though lock you into certain (albeit good) eyepieces.

My solution has been to use contact lenses. They can provide dioptre and astig correction in one go.
Giving me a free choice on eyepieces.

HTH, David.

[Don Pensack]

I don't know where he got 14mm as the effective eye relief of the 22mm Nagler type 4.

I need at least 17.5mm of effective eye relief to see the field stop of an eyepiece with glasses on with that apparent field, and I have no problem whatsoever with that eyepiece with the eyecup in the down position and the eyeguard

in its maximum inward position.  My glasses touch the rubber, but they don't press hard against the rubber, and the edge of the field is easily seen in peripheral vision.  I'd put the effective eye relief at or about 17.5mm.

 

Ditto the 30mm APM UFF, which is even easier to use than the 22mm Nagler.  I barely have to touch lens to rubber to use that one.

My personal experiences with most of the eyepieces on that list differs by only about a mm from his figures, but a few are way off, differing by 4mm or more from his figure.

He has the 17mm Astrotech AF70 at 17mm, for instance.  On my sample, the rubber eyecup could not be flipped down, and when it was at its minimum height it put the effective eye relief around 10mm--it was completely unusable with glasses, and only the central 50-60% of the field was visible.

So how was the Effective eye relief figure derived?  Glasses use?  Or subtracting the depth from the top of the eyecup from the mfr's quoted eye relief figure?

 

One thing occurs to me: You may not need as much eye relief to see a narrower field of view.  A 10mm Design eye relief on a 16mm 82° eyepiece, for instance, was so tight that seeing the field stop was very difficult.

A 10mm Design eye relief on a 50° eyepiece, however, seemed comfortable and easy.  So perhaps glasses-friendly eye relief will vary according to the apparent field of the eyepiece.

If that is the case, then using ones glasses to gauge effective eye relief may not be an accurate way to measure the effective eye relief.  Alas, if that is the case, it also means that knowing both Design eye relief and Effective eye relief may

not tell you if the eyepiece is glasses-compatible.

 

 

[chrispj]

2 hours ago, Don Pensack said:

 

--your tolerance for how hard you are willing to press your glasses against the eyepiece.  I haven't found an eyepiece yet where my glasses don't at least touch the rubber, but some required a hard press that isn't comfortable after a while.

 

With the 18mm ES eye-piece I mentioned, pressing hard enough to gain a reasonable field of view left my glasses lens scratching against the aluminium face around the lens, which obviously isn't preferred!

 

2 hours ago, Don Pensack said:

 

This is why I think about the only way to be sure the eyepiece is usable with glasses is to look for at least 18mm of eye relief, then read reviews of the eyepiece, then try it for yourself.  Otherwise, it will be difficult to know.

 

This seems like a reasonable rule of thumb. Unfortunately, for the gap I want to fill around 18mm, it seems to rule out a few tempting eye-pieces with around 16-17mm quoted eye relief (which was the driver behind this thread really). No Naglers for me then! 😂

 

1 hour ago, Carbon Brush said:

If you are short sighted - and that is the only issue - you do not need specs.
You just adjust the focus to compensate. This means you can use any eyepiece.
This works provided there is enough focus travel on the scope - which is usually the case.

If you have astigmatism, that does need correcting. However, certain of the 'posh' eyepiece manufacturers do have astigmatism correct add-ons available.
https://www.televue.com/engine/TV3b_page.asp?id=54
This does though lock you into certain (albeit good) eyepieces.

My solution has been to use contact lenses. They can provide dioptre and astig correction in one go.
Giving me a free choice on eyepieces.

HTH, David.

This is a fair point, I do use contacts for sports but find with the daily disposables my focus isn't as precise as when I wear glasses (& without either I'd be falling over my tripod legs!)  

[Louis D]

4 hours ago, Don Pensack said:

I don't know where he got 14mm as the effective eye relief of the 22mm Nagler type 4.

I need at least 17.5mm of effective eye relief to see the field stop of an eyepiece with glasses on with that apparent field, and I have no problem whatsoever with that eyepiece with the eyecup in the down position and the eyeguard

in its maximum inward position.  My glasses touch the rubber, but they don't press hard against the rubber, and the edge of the field is easily seen in peripheral vision.  I'd put the effective eye relief at or about 17.5mm.

 

Ditto the 30mm APM UFF, which is even easier to use than the 22mm Nagler.  I barely have to touch lens to rubber to use that one.

My personal experiences with most of the eyepieces on that list differs by only about a mm from his figures, but a few are way off, differing by 4mm or more from his figure.

He has the 17mm Astrotech AF70 at 17mm, for instance.  On my sample, the rubber eyecup could not be flipped down, and when it was at its minimum height it put the effective eye relief around 10mm--it was completely unusable with glasses, and only the central 50-60% of the field was visible.

So how was the Effective eye relief figure derived?  Glasses use?  Or subtracting the depth from the top of the eyecup from the mfr's quoted eye relief figure?

 

One thing occurs to me: You may not need as much eye relief to see a narrower field of view.  A 10mm Design eye relief on a 16mm 82° eyepiece, for instance, was so tight that seeing the field stop was very difficult.

A 10mm Design eye relief on a 50° eyepiece, however, seemed comfortable and easy.  So perhaps glasses-friendly eye relief will vary according to the apparent field of the eyepiece.

If that is the case, then using ones glasses to gauge effective eye relief may not be an accurate way to measure the effective eye relief.  Alas, if that is the case, it also means that knowing both Design eye relief and Effective eye relief may

not tell you if the eyepiece is glasses-compatible.

 

 

I used the projection method of shining a bright flashlight into my AT72ED with an eyepiece in the diagonal and measuring the distance from the top of the folded down rubber eye guard to the point where the exit pupil was smallest in diameter when projected onto parallel card stock.  Sometimes with SAEP and CAEP, this covered an entire range of distances (as with the 30mm ES-82), so I use the middle of this range.

For eyepieces with stiff rubber eye cups that are easily removable like AF70s and Aspheric 62, I measure the distance with them removed since that is how I tend to use them with eyeglasses.  I haven't scratched an eyeglass on any of them yet.  If you don't push in hard, you won't scratch your eyeglasses.  I did scratch an eyeglass on the top retaining ring of the TV Panoptic 27mm despite leaving the flipped down rubber eye cup in place by pressing in hard, so leaving the eye cup in place is no guarantee of scratch-proofness if you wear front surface convex lensed eyeglasses.

Yes, some of the numbers don't match up with "feels like" eye relief, and I can't explain it.  I have remeasured multiple of these eyepieces, sometimes just shining a distant flashlight straight into the bottom of them with the same result.  Obviously, there's more to eye relief distance than the point at which the exit pupil is smallest in diameter.  Not being a trained optical designer (I'm a computer chip designer by education and trade), I can't offer any further insights into it.  I agree, the 22mm NT4 has more usable eye relief than the hard numbers would show.  Go ahead and repeat my testing with your setup on such eyepieces and see what numbers you get.  I guarantee a few will surprise you.  Feels like and measured ER sometimes disagree.

I agree that narrow AFOV eyepieces are sometimes fairly easy to use with eyeglasses.  I have little trouble with my short ER Kellners and such with just a bit of head bobbing from side to side.

An alternate method to measure effective eye relief depends on measuring the distance to a phone camera lens once the camera is at the exact position the field stop pops into view while viewing an actual, distant test target.  This seems more accurate because the incoming light actually forms an image.  The distances will need an offset added to them to account for the location of the camera's entrance pupil location with its lens, but it will be a constant.  It can be derived by comparing the two measured ER distances and arriving at mean value that most eyepieces agree on.  This measurement might provide more insight into "feels like" ER distance because the camera is acting like an eye and may need a different distance than the point at which the exit pupil is narrowest in diameter.  This is how I arrived at the 10mm value for the 26mm Meade MWA due to its massive SAEP.  It refers to the ER to see the field stop.  I may work on adding a cardboard wrap around the eyepiece and push it down with the phone body and then measure the distance to the top of the cardboard from the top of the eyepiece once the camera is removed.  All I need is more free time to repeat this dozens of times.  I'd start with the ones that have an obvious disconnect between feels like and measured ER, though.

[Starwatcher2001]

I can't add anything substantive to the excellent replies above, but I've had exactly the same experience with the eyepieces you mention. My Pentax XW40 and XW10 are very comfortable for me with glasses and I intend to get more as finances allow.  But the ES82 18mm was like looking down a straw when using glasses. It was a beautiful eyepiece, but just didn't work for me. I bought it second-hand on here and then sold it later for a similar amount, so it was a worth while exercise. 

If you have a local astronomy club that meets near you, it would be worth while going along and trying various eyepieces to see what works for you.