What is a scope's eye box? - Definition attempted

glock24

Gunny Sergeant
Belligerents
Sep 14, 2006
1,996
32
154
West Michigan
As I read different people's reviews on a particular scope's performance, the more certain I become that the correct definition of a scope's "eye box" is becoming lost in translation. Below is my version of the correct definition, and I'm hoping this thread can help serve to confirm, deny, or further refine it. Unfortunately, explaining "eye box" really requires explaining two other terms;


<span style="color: #3366FF"><span style="font-weight: bold">Eye Relief:</span> </span>
This is the specific distance behind the scope's ocular bell at which the target image and reticle can be seen in the scope's <span style="text-decoration: underline">full</span> field-of-view (FOV). A distance that is either too short or too long results in blurry edges around a FOV that is <span style="text-decoration: underline">smaller</span> than optimum. <span style="text-decoration: underline">Eye relief is a listed value by all scope manufacturers</span>.


<span style="color: #3366FF"><span style="font-weight: bold">Exit Pupil:</span> </span>
This is the cylinder-shaped beam of light that exits the scope and contains the target image. The diameter of this cylinder varies in size depending on the size of the scope's objective lens, and the particular magnification setting. In order for a shooter to see the entire FOV offered by the scope, the shooter's eye must be; 1)at the correct eye relief distance, and 2) INSIDE the diameter of light defined by the exit pupil diameter. An eye position outside (or on the edge of) the exit pupil diameter results in a blacked-out or blurry image.

I like to think of the exit pupil as a "floating disk" behind the ocular bell. The shooter's eye must remain inside this "floating disk" to see the entire FOV. This is no easy chore when a scope's magnification increases, because the diameter of the "floating disk" can shrink significantly.

<span style="text-decoration: underline">Exit pupil is also a listed value by all scope manufacturers</span>, or can easily be calculated by dividing the scope's magnfication setting by the objective lens diameter



<span style="color: #3366FF"><span style="font-weight: bold">Eye Box:</span></span>
Eye box is defined as the amount of "allowable error" in a shooter's eye relief position, yet still offering a clear target image and <span style="text-decoration: underline">full</span> FOV. In other words, an optimum target image and full FOV despite a shooter's eye being slightly in front of, or slightly behind the optimium eye relief position.

<span style="text-decoration: underline">Eye box is NOT a listed value by any scope manufacturer.</span>


What confuses me about a scope review that trys to describe a scope's eye box is that it usually ends up sounding like a description of the exit pupil.

"<span style="font-style: italic">The eye box is great at low power, but at high power it becomes very tight</span>". NO! This is simply a function of the exit pupil, and one doesn't need a scope review to understand this. Exit pupil can be calculated for any scope without even looking through it.

What I think is more accurate is a review that describe's a scope as "<span style="font-style: italic">easy to get behind</span>". This tells me the shooter can be relatively inaccurate in cheek weld distance from the scope, yet still see clearly through the scope. This describe's the scope's abiliy to allow eye relief error without sacrificing FOV or image quality. This is NOT something can be looked up in a scope's technical specs, or calculated based on the scope's design. This is the scope's eye box.

Assuming my definitions are reasonable to this point, the two questions I have for our resident optics gurus include;

1. Can eye box size change with magnification, or does it maintain a constant "error distance" throughout?
2. Would it be possible for a manufacturer to measure and list this eye box value? What value is it?

 

billyburl2

Sergeant
Belligerents
Jun 22, 2009
880
0
18
48
Cottonwood AZ
Re: What is a scope's eye box? - Definition attempted

Of course it can change with mag. While the definitions you have provided are correct it also depends on how the scope handles all these at once...I.e. if it has a variable eye relief or the ocular lens arrangement. Kinda why the Vortex scope just went through an upgrade, even though the scope is relatively new to the market
 

T2CH

Sergeant
Belligerents
Dec 7, 2010
517
1
22
Va, USA
Re: What is a scope's eye box? - Definition attempted

And now I'm starting to understand why it is easier for me to get behind my NF 5.5-22x56 than it is to get behind my S&B 4-16x42. Mike (CS Tactical) was trying to explain it to me on the phone but it went over my head.

From what your saying, the product of the objective and the mag setting is X. The higher value of X the lower the exit pupil?

I would think as the exit pupil shrinks with magnification so would the size of the eye box.

 

dcjs

Sergeant
Belligerents
Mar 27, 2003
388
0
18
Re: What is a scope's eye box? - Definition attempted

A couple of points:

1. Exit pupil (EP) does not necessarily equal objective diameter/magnification because the internal pupil may be restricted at some magnifications in variable scopes so EP may be smaller than calculated at low magnifications.

2. There is an upper and a lower threshold for eye relief (ER), it is not a fixed value but a "band" that may be wider or broader.

3. ER values can and will change with magnification changes. The amount of change depends on the optical design and may range from negligible to significant.

4. ER may be regarded as technically "constant" if there is one value that provides a clear view throughout the magnification range, the width of the "band" may change with magnification though.

5. ER values (min/max) can be measured with a standardized procedure.

6. The eye box is a function of the allowable lateral and axial movement of the eye pupil while still maintaining a clear view of the target. It is dependent on min/max values for eye relief, EP diameter and the diamter of the eye pupil.

7. Because of 6., it is easy to understand that the eye box will be perceived as more "forgiving" at lower magnifications in a variable scope because of the larger exit pupil (the scope EP will usually be larger than the eye pupil), the eye pupil can be moved inside the larger EP.

I'm not sure if there are other optical aspects contributing to the perception of "ease of getting behind a scope", but these are the "geometrical" basics so to speak.
 

TiroFijo

Sergeant
Belligerents
Dec 4, 2002
732
10
22
Asuncion, Paraguay
Re: What is a scope's eye box? - Definition attempted

Thanks David, very educative comments


I've seen this ER "band" mentioned also as "non critical eye relief", seems like the same concept.
 

heldebrant

Private
Minuteman
Jul 15, 2013
3
0
0
Sorry for digging up an old thread but I am a n0ob and have just recently come across this 'feature'. I am handicapped and can't always get into the perfect position for the various scopes I have and am looking to purchase some more scopes. Are there brands/models that lend better to a larger eye box?
 

Diver160651

Who cares
Belligerents
Feb 7, 2013
3,834
2,704
219
BELMONTCA
You might want to look into pistol scopes or scopes made for scout riffles and dual purpose CBR rifles. You will not generally get super high magnification, but tons of "Eye relief". I think that is the term you might be looking for.


Jt

Sent from my iPad using Tapatalk HD
 

Spuhr

Gunny Sergeant
Belligerents
Sep 25, 2009
1,126
0
138
48
Sweden
www.spuhr.com
When the exitpupil gets bigger the eyebox also gets bigger.
Therefor is it right to take the exitpupil in account when discussing the eyebox.

The example above is a agood example betwen the S&B and the nightforce.
As the S&B have smaller objective the exitpupil at a given magnification is less and the eyebox becomes also less.
 

scudzuki

Gunny Sergeant
Belligerents
Jul 1, 2012
2,045
99
154
54
Philadelphia suburbs
Exit pupil is the greatest factor but not the only one; two scopes of different design with identical exit pupil and eye relief can have different tolerance for side to side and up and down misalignment ("eyebox").

Joe
 
Last edited:

teamnichols

Private
Belligerents
Jan 6, 2014
5
0
0
The best explanation of "EYE BOX" I've come across:
Understanding Eye Box, and More, with Leupold - YouTube

And, IMHO, "YES" ... there are brands/manufs of scopes which absolutely provide a more forgiving EYE BOX THRESHOLD than others. Not sure how the internal mechanisms and lenses of the scopes work, but I do believe the Eye Box "Threshold" and/or "Band" can differ significantly between manufs. It seems to have far more to do with than just Exit Pupil.