Warning

 

Close

Confirm Action

Are you sure you wish to do this?

Confirm Cancel
BCM
Member Login

Posted: 11/19/2016 10:54:05 PM EST
It seems to me like the term eye relief is getting increasingly used in this forum for what I call eye box.

To me "eye box" is the distance between the closest point your eye can be to a scope and the farthest point your eye can be while the scope is still useable. Eye relief is the distance from your eye to the scope when the image is optimal. Eye box is a range. Eye relief is a point. A long eye relief scope can have a small eye box, a short eye relief scope can have a big eye box, and vice versa.

Are the terms becoming interchangeable?
Link Posted: 11/20/2016 10:53:46 AM EST
Quoted:
It seems to me like the term eye relief is getting increasingly used in this forum for what I call eye box.

To me "eye box" is the distance between the closest point your eye can be to a scope and the farthest point your eye can be while the scope is still useable. Eye relief is the distance from your eye to the scope when the image is optimal. Eye box is a range. Eye relief is a point. A long eye relief scope can have a small eye box, a short eye relief scope can have a big eye box, and vice versa.

Are the terms becoming interchangeable?
View Quote


Yes. Eye box isn't just eye relief, it's also includes the exit pupil measurements allowing you to not have a perfect cheek weld. Eye relief is great but if the exit pupil is small, it's still going to have a small eye box.
Link Posted: 11/20/2016 9:36:57 PM EST
Dumbed down explanations as I understand.

Eye Relief: Distance from scope ocular lense to shooter's eye that allows the whole image to be unobstructed.

Exit Pupil: Amount of left - right allowed. Take the objective diameter and divide by the current magnification. (Take a 3-9X40 power scope. At 3x the exit pupil is 13.3mm, and 4.4mm at 9x.)

Eye Box: the combined Eye Relief (back and forth) and Exit Pupil (left and right) that still allows for a clear, unobstructed view through the optic.
Link Posted: 11/21/2016 8:32:59 AM EST
The terms should never be used interchangeably.  We need to know and appreciate the meaning of the differences.

While on tbe subject of exit pupil, we need to know not only how it affects lateral head movement.  It also has an effect on low light performance.

In low light our own eyes respond by increasing pupil diameter.  This allows more light to contact our retina, where the light activated sensors, rods and cones, are located.

Our pupil expands to about 5 or 6mm in size, decreasing with age.  Some people can reach 7mm in total darkness.

If the exit pupil of the scope is smaller than the pupil in our eye, our retina is not receiving all the light it can use.  Fewer rods and cones are being activated.  The image becomes dim and more brownish.  This why large objective scopes perform better in low light, because they can produce a  5 or 6mm exit pupil at higher magnification.

It is also why a 1-6x24mm scope (4mm exit pupil at 6x) will not be as bright at 6x as it is if you dial it back to 4x where it will have a much more retina filling 6mm exit pupil beam.

Example:  I have a Kahles 2-7x36mm hunting scope.  At 6x it has a 6mm exit pupil and still a bit over 5mm at 7x.  The large exit pupil at higher power and very good Austrian glass make for outstanding image quality in low light.  Unless you really need 1x bottom end, where you almost always must accept a 20mm or 24mm objective, you will almost always get better optical performance with slightly higher magnification scopes with 32-40mm or even 50mm objective lens, glass quality being equal.
Link Posted: 11/21/2016 8:52:32 AM EST
Eye relief is a range.  It's how close or far your eye can be from the scope and still see through it properly.  



Eye box is also a range, it's how far off center your eye can be and still see through the scope properly.  




Top Top