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The topic of eye relief. What is it actually? (1 Viewer)

Swedpat

Well-known member
During many years I have tried a lot of binoculars. Then I have experienced that the stated ER is often to take with a pinch of salt. Not only that the useful ER can vary a lot depending on the eyepiece design. While it needs to be a margin in order to prevent eyeglass lens and eyepiece lens to touch each other, in many cases the eyepiece lens is deeply recessed so several mm of ER is unnecessary wasted.
If I got it right ER is a static value which is measurable. So when it sometimes is described like "eye relief is adjustable in three steps"(the eyecup) it is a wrong description.
We know there is a relation between AFOV and size of eye lens in order to provide a given ER.
The wider AFOV - the larger eye lens is required in order to maintain the ER.
But I have noticed that often a narrower AFOV does not require as much stated ER in order to provide a satisfactory view with eyeglasses.
It should not be so. The AFOV should be included in the equation when measuring ER.
It seems like the manufacturers measure the distance to provide a given AFOV as base for ER, regardless of the actual AFOV.
And in some cases manufacturers carelessly state the same ER for an entire series. For example Zeiss, stating the same 18mm ER for 8x32, 10x32, 7x42, 8x42 and 10x42. We know this is far from the reality. The 8x32 has smaller eye lens while bigger AFOV than 7x42. The same ER is here a total contradiction.

What is your thoughts in this topic? I find it unlikely that high grade optics manufacturers don't understand what ER is and how to measure it. Still they often get it wrong.
 
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The exit pupil is the aperture throught which all the rays from the eyepieces pass through and the eye relief ER is its distance from the closest you can practically get to the eyepiece or some other defined plane like the location of the fully wound in eye cup.

In use you eyepiece exit pupil should coincide with the entrance pupil of the eye. Hence the need for adjustable eyepiece cups to accommodate glasses and different eye socket shapes.

In an ideal world the diameter of the exit pupil should match the eyes entrance pupil for maximum illumination. However, the eyes entrances pupil via the iris is variable.

With wide angle eyepieces in bright light the exit pupil diameter will exceed that of the eyes entrance pupil and you can seem to "look round" the field.

The size of the eye lens is not directly related to ER so a manufacturer could have a series with the same ER and different magnifications and objective sizes.

Regards Andrew
 
The exit pupil is the aperture throught which all the rays from the eyepieces pass through and the eye relief ER is its distance from the closest you can practically get to the eyepiece or some other defined plane like the location of the fully wound in eye cup.

In use you eyepiece exit pupil should coincide with the entrance pupil of the eye. Hence the need for adjustable eyepiece cups to accommodate glasses and different eye socket shapes.

In an ideal world the diameter of the exit pupil should match the eyes entrance pupil for maximum illumination. However, the eyes entrances pupil via the iris is variable.

With wide angle eyepieces in bright light the exit pupil diameter will exceed that of the eyes entrance pupil and you can seem to "look round" the field.

The size of the eye lens is not directly related to ER so a manufacturer could have a series with the same ER and different magnifications and objective sizes.

Regards Andrew

Hi Andrew,

I recently got a link in some thread about the relation between eye relief and eye lens size, but don't remember in which thread it was.
What I understand there is a direct relation at least when comparing the same ocular design.
This is very noticeable when comparing astronomical oculars, for example Plossl.
The ER is very long on 30-40mm focal and becomes shorter (in line with lens size) with shorter focallength.
4mm Plossl and shorter, have so short ER the eye's lens almost touches the eye lens in order to see the entire FOV.
With modern ocular designs the eye lens size is bigger and ER is longer at a given focal length.
My wondering is why the manufacturers cannot get this right if it's possible to objectively measure the ER.
It should not be so relative in the same way focal length is not.

Regards, Patric
 
I think you would need to look at the whole design to be sure exactly what's going on with a particular optic or series. Variations in how focus is achieved, fixed or variable magnification etc.

Personally, I have never had an issue which has encouraged me to measure the ER of my binoculars or spotting scope. In principle it's not hard to do. Have you measured it?

Regards Andrew
 
No, I have not tried to measure eye relief. However, based on the stated ER, different binoculars with the same stated ER still do not work equally well with glasses. One example: Zeiss Conquest HD 8x42 and GPO Passion ED 8x42.
Both have the excellent eye cup design which makes the eyeglasses come very close to the eye lens.
The Conquest provides a perfectly open view and I see the entire FOV evenly "lit", while not with Passion. It lacks 2-3mm.
Therefore, I can state that the Passion does not have as long an ER as the Conquest, even though both have specified 18mm.
 
I can only suspect they don't measure it from the same place. Just goes to confirm one should, if at all possible, try before you buy.

Regards Andrew
 
Hi Patric and Andrew,

The first of these two posts deals with the simple geometric relationship between AFOV, eye relief and eye lens diameter. The second one describes a method for measuring eye relief using ordinary household items. No knowledge of the AFOV or the eye lens diameter is needed.



Henry
 
There is a thing worth to mention about the eye relief. I think everybody wants to see the entire FOV. But it's easy to think that it does not matter if you don't see the entire FOV if the AFOV is very big. For example comparing two binoculars with same magnification: a certain eyepiece provides 20mm ER and 50deg AFOV. Another eyepiece provides 16mm ER and 65deg AFOV.
If you clearly see the entire FOV with 20mm ER you maybe see only 85% of the FOV with 16mm ER. This means it will give you 55deg visible AFOV, which is still more than the other eyepiece. So if the goal is to get as much as possible in the field, one can think the WA model is still to prefer. Yes it works.
But the problem is that the FOV is not evenly illuminated. If you don't come close enough there is an outer vignetting in the image. Actually from the point where you start to see the edges you need to come 2-3mm closer to get the perfect view.
And too close will result in blackouts, as the most of us have experienced when using binoculars without eyeglasses and the eyecups not folded up enough.
Some maybe don't care about a vignetting because the details are there anyway under bright conditions.
But for me it does a lot for the viewing enjoyment. I use to describe it as "climbing into" the image.

The question is here: do manufacturers use different standards to measure eye relief?
Do some base the stated ER from the point where you start to see the entire FOV and other from the point the field is evenly illuminated?
If the standard is different it can result in several mm difference depending on where between seeing the edges and blackouts occures the value is based.
 
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Thanks for the links. The second one is how I would measure it.
On the first it assures the field stop is the eye lens which may not be the case.
In truth this is all very academic. Best try before you buy.
Regards Andrew
 
Andrew, post 9,
It is not academic at all, the binocular company that made a special instrument to measure the eyerelief, was in my case Nedoptifa dr. C.E. Bleeker, a Dutch company that made quite a few optical instruments like binoculars, microscopes, magnifying glasses etc. It is worth to mention that in November 2023 it was 70 years ago that Prof.Zernike from Groningen University received the Nobel prize for developing phase contrast microscopy and Dr. Bleeker en Prof Zernike shared patents with regard to this matter. Dr. Bleeker also had designed and constructed Ramsden dynameters , instruments to measure the diameter of the exit pupil and eyerelief of their binoculars. I received one of this Ramsden dynameters from the head of the optical department of Bleeker optical company when I wrote the biography of Dr. Bleeker. I use this Ramsden dynamter to measure eyerelief and diameter of the exit pupils from the binoculars we investigate. Description of the Ramsden dynameter can be found in "Lehrbuch der praktischen Physik" by F. Kohlrausch, sechzehnte auflage, Leipzig , Berlin, 1930, page 321.
Gijs van Ginkel
 
I think we can be agreed that try before you buy is to recommend. Different designs of the eyepieces and sometimes untrustworthy stated values of eye relief makes it impossible to really know how well it works without trying.
 
Andrew, post 9,
It is not academic at all, the binocular company that made a special instrument to measure the eyerelief, was in my case Nedoptifa dr. C.E. Bleeker, a Dutch company that made quite a few optical instruments like binoculars, microscopes, magnifying glasses etc. It is worth to mention that in November 2023 it was 70 years ago that Prof.Zernike from Groningen University received the Nobel prize for developing phase contrast microscopy and Dr. Bleeker en Prof Zernike shared patents with regard to this matter. Dr. Bleeker also had designed and constructed Ramsden dynameters , instruments to measure the diameter of the exit pupil and eyerelief of their binoculars. I received one of this Ramsden dynameters from the head of the optical department of Bleeker optical company when I wrote the biography of Dr. Bleeker. I use this Ramsden dynamter to measure eyerelief and diameter of the exit pupils from the binoculars we investigate. Description of the Ramsden dynameter can be found in "Lehrbuch der praktischen Physik" by F. Kohlrausch, sechzehnte auflage, Leipzig , Berlin, 1930, page 321.
Gijs van Ginkel
To manufacturers and designers your quite right. However, to a consumer, as long as it's large enough details about it are irrelevant and hence academic as far as they are concerned. This was what I was referring to. Regards Andrew
 
Andrew, post 9,
It is not academic at all, the binocular company that made a special instrument to measure the eyerelief, was in my case Nedoptifa dr. C.E. Bleeker, a Dutch company that made quite a few optical instruments like binoculars, microscopes, magnifying glasses etc. It is worth to mention that in November 2023 it was 70 years ago that Prof.Zernike from Groningen University received the Nobel prize for developing phase contrast microscopy and Dr. Bleeker en Prof Zernike shared patents with regard to this matter. Dr. Bleeker also had designed and constructed Ramsden dynameters , instruments to measure the diameter of the exit pupil and eyerelief of their binoculars. I received one of this Ramsden dynameters from the head of the optical department of Bleeker optical company when I wrote the biography of Dr. Bleeker. I use this Ramsden dynamter to measure eyerelief and diameter of the exit pupils from the binoculars we investigate. Description of the Ramsden dynameter can be found in "Lehrbuch der praktischen Physik" by F. Kohlrausch, sechzehnte auflage, Leipzig , Berlin, 1930, page 321.
Gijs van Ginkel
Isn't it interesting how in the hands of the uninitiated the most basic of facts can take on a plethora of new complexities?
 

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