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?
<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?
