“Having said all that, I have never heard of “depth of field””

Sure you have, you have just heard about it in different contexts terminology applied specifically – for example: ‘focal plane’, ‘focus range’, ‘focus axis distance’..

Regarding light gathering capability and exit pupils with optical systems:

Exit pupil refers to the diameter of the cylinder of light exiting the scope and entering your eye. The human eye at maximum dilation opens up to about 7mm in children and somewhat less as we age. In order to see as much detail as possible, we want the scope’s cylinder of light to shine on as many receptors (rods and cones) in our eyes as possible, especially in low light. Since our pupils dilate to their maximum diameter of 7mm in low light, that means (ideally) we want our scope’s exit pupil to be 7mm under maximum magnification. In that situation, the image illuminates as many rods and cones in our eyes as possible and gives us the most detailed image possible.

Thankfully, the method to determine an optic’s exit pupil is super simple–simply your objective lens diameter divided by your magnification. For example, if your magnification is 6 times (e.g. 6x) and your objective lens is 42mm diameter, your exit pupil is 42mm / 6x == 7mm. The practical implication of that example is that your scope’s exit pupil will shrink to less than ideal as you increase magnification beyond 6x, and your eye will not be able to perceive/resolve as much detail. Images will also start to darken. (Continuing with the previous example scope with an objective lens that is 42mm diameter, going to 9x magnification would create an exit pupil of 42mm / 9x == 4.7mm which is less than ideal.)

So, the takeaway is simple: choose a rifle scope with the largest diameter objective lens practical/possible keeping in mind your goal of an exit pupil of 7mm at the highest magnifications.

Okay, I researched “depth of field”. That refers to the range of distances that an optical system can focus simultaneously. For example, depth of field is an important factor if you want your optic to simultaneously keep very close (relatively speaking) and very far (relatively speaking) objects in the same field of view in sharp focus.

When it comes to rifle scopes with fixed focus, in reality you focus your scope so that distant objects are in sharp focus. At that point “field of view” simply refers to how close an object can be and still be in sharp focus. In my experience with rifle scopes, everything past 30 yards (give-or-take) is in sharp focus. Personally, I don’t care if objects within 30 yards (give-or-take) start to become blurry because they will be plenty large and discernable for accurate shooting.

In my opinion, light gathering capability and exit pupil are vastly more important considerations in a rifle scope. That being the case, I recommend that people optimize their choice of optic for light gathering and large exit pupils–which means larger objective lenses for higher magnifications.

(Note: all of my optical applications have been fixed objects at very long distances, thus “depth of field” was/is not a factor and I had never learned about it.)

I have researched optics in depth for shooting applications, terrestrial spotting applications, photography, and intensive astronomical applications.
I am well versed in basic Physics including both the particle and wave properties of light.
I am familiar with glass and lens properties (such as chromatic aberration).
I am aware of “fully multi-coated” lenses and why that is important.
I am familiar with “diffraction limited” optical systems.
I know all about exit pupils.

Having said all that, I have never heard of “depth of field” (which goes to show that no matter how much you know or think you know, there is always a LOT more to learn). I will have to research depth of field.