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Copyright P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.


Eye Relief - especially for spectacle wearers.

comments measurements recommendations


Eyerelief comments

 As a spectacle wearer I am driven by eyerelief. I find no earthly use for some eyepiece with an apparent field of 65 degrees when I can only see 20% of the area. Unfortunately, one of the major growth areas in optics seems to be adding unusable wide field eyepieces to telescopes for sale to the public. An option is usually a 'special' long eyerelief eyepiece at an exorbitant cost.

The following is a small extract from a much larger document relating to eyepiece design and specifications.

Eye relief - issues.

Obviously, the eye must be manoeuvred so the exit pupil of the eyepiece matches as well as possible the pupil of the eye. This pupil changes from about 2 mm for extremely bright light to approximately 8 mm for a fully dark adapted young eye. Aged eyes normally have a smaller range.

Human physiology places absolute limits on eyerelief. This will be different for various people. Short of surgery or directly bonding lenses to the cornea, observers are almost totally dependent on the whims of designers and the preconceptions of the average buyer. These have been so distorted by poor products and mass marketing that very few people have any real knowledge of good practice. 

I personally find that eyerelief is the main force driving my own designs. Being a spectacle user, I prize it beyond nearly all other attributes, but from a design point of view I know how quickly it straightjackets a design. Even worse, that portion of the population without spectacles see no gain in performance. They, in fact, pay a heavy price in dollars, and have their extra wide apparent field of view compromised. But, unless this is done, a 70 degree apparent field eyepiece may be reduced to a paltry 30 degrees for spectacle users. 

Some uninformed spectacle users simplistically believe that eyepiece designers could produce long eyerelief designs if the bothered, but have simply not put any effort into catering for spectacle users. While this contains a small element of truth, it is grossly unfair to the designer.

Very long eyerelief is always had at the expense of extra wide field performance. The best compromise position is very different if both spectacle and non spectacle users are to use the same design.

What is acceptable eyerelief.

This is a can of worms. The last person to ask is an eyepiece salesman. Similarly, some designers are not honest with themselves. One of the worst problems is an eyepiece prescription originally intended for, say, 40 mm efl. which has been thoughtlessly and rather lazily scaled down to 10 mm efl..

The appreciation of eyerelief is very relative. If users have only seen poor binoculars with eyerelief of 4 mm then 10mm will seem perfect. How many out of every 1000 people have ever seen eyerelief of greater than 10mm. Probably none have seen wide field eyepieces of greater than 15mm eyerelief.


Eyerelief measurements

Out of curiosity I have measured the eyerelief of a series of very diverse optical instruments.

The results are interesting. I feel they partly reflect what observers have been made to put up with rather than an indication of best or even good practice.

There is really only one way for you to determine what is acceptable to you. You should measure the eyerelief of some comfortable optical systems. Refrain from coming to conclusions until you have tried a variety of devices covering different fields of view.  

Remember that the eyepiece must be used in conjunction with some real optical device to assess it properly. And remember that the same eyepiece used differently has different eyerelief. As an example, binocular eyepieces used with long focus telescopes may have eyerelief noticeably reduced..  

The instruments selected have been chosen to represent as wide a range of eyepiece performance and types as possible.

The eyerelief value shown in brackets is the amount the eyelens is recessed. If the mounting was flush with the lens, eyerelief would be increased by this amount. Some manufacturers simply throw away large amounts of precious eyerelief by poor mount design. Many eyepieces can be transformed by simply maching away 'extraneous matter' behind the eyelens.

 

 Device

Eyerelief

Full field Note

Draw observers telescope (1920's)

0 (+4)

35 Just useable without glasses.

Draw observers telescope (1950's)

3 (+5)

35 If Machine away, then fair without glasses.

Draw hikers telescope (1990's)

2 (+8)

40 Hardly useable without glasses. When deep eyecup removed, fair without glasses.

Theodolite (1940's)

5 (+1)

32 Narrow field useable without glasses.

Quality microscope (1960's)

12 (+0)

35 Narrow field fair with glasses.

Good student microscope (1960's)

12 (+0)

15 Narrow field no problem with glasses.

Cheap pocket microscope (1970's)

13 (+4)

30 Narrow field. Glasses OK if remove eyecup

Hilger Autocollimator (1960's)

13 (+6)

15 Narrow field OK with glasses but better if deep eyecup removed.

Quality borescope (1960's)

11 (+8)

12 OK with glasses. Deep eyecup but satisfactory because of very small field.

Barr & Stroud rangefinder (1940's)

8 (+4)

35 Nearly useable with glasses.

25 pownder gunsight (1940's)

40 (+5)

50 Oodles of eyerelief. Too much ?? Many find objectionable without rubber eyecup to help position eye - especially at night.

Artillery panoramic sight (1940's)

12 (+4)

40 Almost useable with glasses. A little field loss

Bushnell spotting scope15x60 (1970's)

17 (+2)

48 Wide. Just OK with glasses. More would be nice.

'Cheap' Russian bino 7X50 (1990's)

15 (+2)

45 Edge unusable with glasses. Nearly OK.

'Cheap' Asian bino 6x30 (1990's)

9 (+3)

60 Just OK without glasses. Need much more with glasses.

'Cheap' Asian bino 7X35 (1990's)

7 (+3)

65 Almost impossible even without glasses.

Naval bino sight 3.5X20 (1940's)

15 (+6)

63 Machine away gives 21 which is just useable with glasses.

Rifle Scopes

75 to 85

30 - 25 For normal viewing too much. Necessary because of recoil !! Difficult to align eye to exit pupil without placement of head on rifle stock - especially at night.

Extreme Field Military sight (1970's)

7 (+0)

70 Even without glasses almost impossible.

Zoom eyepiece @ 7.5 (1970's)

7 (+1)

40 Just possible without glasses.

Zoom eyepiece @ 22.5 (1970's)

1 (+1)

23 Narrow field visible with glasses.

 


Eyerelief recommendations

 

For what it is worth here are my conclusions.

(a) Eyerelief cannot be divorced from field of view. As an example, a 20 view is possible with only a few mm eyerelief even with glasses. Yet 50 field requires at least 18mm eyerelief for use with glasses.

(b) Without spectacles, 10 mm eyerelief permits 60 field fairly easily but feels cramped with wider fields.

(c) Except for quite narrow fields found in some specialist instruments (less so these days) a design figure of about 15 will satisfy nearly all non spectacle users even at extreme fields. At medium fields a minimum of 10 mm would be reasonable.

(d) Medium fields require about 18 mm eyerelief for spectacle users. 20mm would be better.

(e) Wide and extreme fields for eyeglass users require well over 20 mm. An eyerelief of 25 mm would not be too much and in some situations 30 mm could be worthwhile.

(f) Night use on faint objects opens the pupil of the eye. This makes a little less eyerelief acceptable.

Measured eyereliefs taken from actual instruments seldom, if ever, achieves these ideals. Sometimes it is impossible to design for such performance. A more honest approach, however, would be to reduce the wide field of view that cannot be effectively used. In the present sales literature driven society this seems impossible.

From this, and other perceptions, I have constructed the following graph. It is based on very subjective data taken in daylight. The lower lines of each series represent what is comfortable. The top lines in each series represent an uncomfortable situation just before I find the field of view becomes compromised.


 

 


When too little eyerelief exists the apparent field of view is reduced. As seen field decreases, the edges first dim, then eventually they are cut out totally. Thus three areas are visible. The centre is fully visible or unvignetted. Next comes an area of fuzzy decreased illumination. Outside this no illumination exists at all.


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