"The eye relief property should not be confused with the
exit pupil width of an instrument: that is best described as the
width of the cone of light that is available to the viewer at the exact
eye relief distance. An exit pupil larger than the observer's pupil wastes some light, but allows for some fumbling in side-to-side movement without
vignetting or clipping. Conversely, an exit pupil smaller than the eye's pupil will have all of its available light used, but since it cannot tolerate much side-to-side error in eye alignment, will often result in a vignetted or clipped image.
The exit pupil width of say, a binocular, can be calculated as the objective diameter divided by the magnification, and gives the width of the exit cone of light in the same dimensions as the objective. For example, a 10 × 42 binocular has a 4.2 mm wide exit cone, and fairly comfortable for general use, whereas doubling the magnification with a zoom feature to 20 × results in a much more critical 2.1 mm exit cone.
Optics showing
eye relief and
exit pupil
1 Real image
2 Field diaphragm
3 Eye relief
4 Exit pupil
Eye relief distance can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the
eyepiece so needs a longer eye relief in order to still see the entire
field of view. A simple practical test as to whether or not spectacles limit the field of view can be conducted by viewing first
without spectacles and then again
with them. Ideally there should be no difference in the field.
For a shooter, eye relief is also a safety consideration. If the eye relief of a
telescopic sight is too short, leaving the eye close to the sight, the firearm's recoil can force the optic to strike the skin around the shooter's eye, leaving a cut. This is frequently called the "idiot cut" due to the obvious and long-lasting nature of such a mistake. Typical eye relief distances for telescopic sights are often between one and four inches (25 to 100 mm), as opposed to the much shorter 15 to 17 mm for typical binoculars. The exit pupil widths in rifle sights are designed to be larger than the eye's pupil, to allow for a range of motion without vignetting."
The above from wikipedia. One of the scope manufacturers had an ad campaign (many) years ago that summed all this up as the "optics triangle".
"In discussing of optics, there must have a basic understanding of the "compromise solution" that goes into engineering any optical gun sight. This is what we call the
Optical Triangle, since any scope represents a compromise point between three competing values: magnification, eye relief and field of view."
That from
https://www.jprifles.com/1.6.3.php
A scope with a 'forgiving eye relief', that is, one where the exit pupil exceeds the shooters pupil diameter, will of necessity have low magnification. As magnification increases, exit pupil decreases, and eye relief becomes more exact, and closer to the scope, unless objective lens size increases. This is why high power variable scopes have large objective lenses. At the low end , they have larger exit pupils and more forgiving eye relief, and at the high end, those are smaller, but keep the scope usable by keeping the eye relief from coming too close to the scope. (usually a minimum of 3")
I recommend the VX-1, Nikon Prostaff, or Bushnell Elite or Trophy, in that order.
And many other names. My favorite is "the Red Badge of Ignorance"
