Thought I’d share this because I didn’t know it until recently, and thought maybe others would learn something too. What makes for a comfortable eye box? Well, it’s not one thing, but a few things together. Glass quality can play a role but not as big as you think. Mostly it’s just physics.
First, you need to have the optimal eye relief. It is best to find optimal eye relief on high magnification because the exit pupil of the scope is at its smallest, so the eye box will feel tightest at this magnification. Generally, if it works at the high end, it should feel even better at lower mag.
Second, the exit pupil diameter is determined by the size of the objective lens, divided by the magnification you are on. Therefore, if you have a 40mm objective lens, at 10x mag, your exit pupil should theoretically be 40/10 = 4mm.
Why does this matter?
Well your pupil grows and shrinks depending on how much ambient light you are in. So at dawn/dusk your pupil grows, up to about 4mm+/-. When you have an exit pupil smaller than 4mm, your pupil has trouble lining up with the light exiting the eyepiece, and it can be harder to see the image, it will have shadow, and the eye box will feel much tighter than it would in broad daylight, when your pupil is much smaller.
So I really like lower powered optics that max out at like 6, 8, 10, or 12x, because then you can have a reasonably sized objective and still get good performance in low light situations. For instance a 12x top end with a 50mm objective still puts the exit pupil just north of 4mm. Or a 10x w/ 42mm objective. By contrast, a 16x or 20x top end with like a 44mm objective will be sub-par for low light conditions (but probably okay at the range in mid-day). Even a 56mm objective / 4 = 14x max for low light, and 56 is awfully big. On the other hand, I rarely use high magnification when hunting anyway, so it is unlikely I would choose to purchase such an enormous scope as this. Even if the scope tops out at 14 or 16x, and has as smaller objective like 44mm, I would just choose not to use above 10x during those low light times of day.
One of my favorite scopes for hunting here in the Midwest right now is a Schmidt & Bender Zenith 1.5-6x42; even on 6x you have a comfortable 7mm exit pupil, which is very easy to get behind, and at any time of day. I also really like a 2-10x42 form factor (as there are many really good options in this range). And for maximizing versatility I like 3-15 or 4-16x44 or 50mm; this gives me more distance capability but within certain limits (ambient light permitting). 56mm objectives just feel like blimps to me so I haven’t ever wanted to purchase something in that size.