How is Field Of View designed into a scope?
What are the Scope paramaters (Glass, Objective, Ocular and tube size) that determine FOV?
Is there a coorelation between FOV and eye relief? A trade off?
I'm looking for a technical discussion from folks who know scope design.

Thanks,

Don

DMB: Thanks for starting a discussion on this subject with a "blank sheet of paper".

I'm not competent to get into the details of optical engineering, but do want to point out the obvious relationship between eye relief and field of view. Think of the view through your living room window. When you stand close to it ("short eye relief") you can have a much wider angle of view. When you stand further away, your angle of view decreases. The same principle applies to an optical instrument (actually, your living room window is a non-magnifying optical instrument of sorts). The longer the eye relief that is built into it (though whatever method), the narrower the field of view must be. That is why your binoculars have a wider angular field of view at the same magnification than your conventional rifle scope, and why your conventional rifle scope has a wider angular field of view than your handgun scope.

I'll be delighted to see the responses and observations of others on this subject. Hopefully, no one will contend that enlarging the scope tube will result in its carrying "more light" as if it were a pipe full of water!

Stonecreek,Makes sense ,how ever you know that someone will challenge you just because they like to take an opposing point of view,regardless of the subject.Most have no practical world experience and base their ideas on The Al Gore Global Warming Theories.No real facts ,just try to dazzle you with BS.

Stone,

Thanks for the info.
I wanted to start clean, and develop what an optical design guy would do in his job of designing a new scope, given the things like you just mentioned.
Don't know if we have any optical design guys, or physicists on board to walk through the design process.
I didn't want to pre-load the subject with my thoughts in the matter, but I do know there are trade-offs, as with many things in the design process.

Don

someone post that "eyebox" argument that got me kicked off 24hour.

Not an optics designer or engineer.......my understanding is the ocular size is a determining factor in fov, and of course magnification is also. Theres a design triangle I believe between fov - eye relief - and exit pupil. I understand this trade-off occurs due to exactly the simple to understand reasoning that Stonecreek puts forth. I believe a larger ocular lens allows you to maintain a larger fov while not compromising eye relief or eye position too badly.

Seems to be labeled now as the "eye box" but its really a direct relationship of these different factors, and you can't change one without changing the others.

quote:

someone post that "eyebox" argument that got me kicked off 24hour.

I won't go there......I would enjoy the post probably......bet it had E in it

I have a crude understanding at best....don't really have the background to understand it fully. Hope some more guys post their opinions and knowledge.

woods

I am an electrical engineer by degree and profession, but also have experience in the design of telescopes and optics for electro-optical systems. I would not consider myself an optical engineer.

Stonecreek has an interesting analogy with a living room window. The living room window size and your distance from it absolutely determine your apparent field of view (and because the window has 1x magnification, the real field of view as well). An important difference between a telescope and the window is that the design of the telescope's eyepiece often defines the apparent field of view more narrowly than the "window" size would otherwise indicate. This is usually done to simplify the design (reduce cost), increase eye relief, reduce distortion and abberation, etc.

The exit pupil of a telescope is simply the diameter of the bundle of light exiting the eyepiece from each point in the image. It is calculated by dividing the objective diameter by the magnification. Your eye's pupil diameter relationship (ratio) to the exit pupil's determines how much of the light gathered by your scope is actually transmitted to your retina. From strictly a brightness point of view, more exit pupil than can enter your eye pupil (you cannot receive more than 100% of the light from the scope). However, particularly with respect to use of rifle scopes, excess exit pupil size can increase the diameter of the "sweet spot" behind the scope within which the user will see maximum brightness and under the right conditions, even field of view (i.e. if your eye's pupil lies outside of the exit pupil from a certain part of the image, you will not see that part of the image). I have never seen the term "eye box" used professionally, so I'm not sure exactly what it is, but this may be related. Eye box may also be related to the relative positions of the "bundles of light" from each part of the image. If they all intersect in the same spot (in three dimensional space), the sweet spot that shows the entire available field of view is maximized.

Other than the effects on the size of the sweet spot (eye box?), it is very important to understand that exit pupil and field of view are not related except by magnification (higher magnification reduces both exit pupil size and real field of view, all other factors held constant). Exit pupil and field of view cannot be "traded off" against each other, because they both vary in inverse proportion with their only common factor (magnification).

Andy

quote:

Exit pupil and field of view cannot be "traded off" against each other, because they both vary in inverse proportion with their only common factor (magnification).
Andy

Excellent observation, Andy. Some users do comment that some scopes with a larger exit pupil seem to have a larger FOV, but the phenomenon they are observing is that the larger exit pupil increases the lateral size of the area in which the eye sees the full sight picture, thus it may seem to the observer that the FOV is larger. What they are actually observing is non-critical eye placement, which is very useful in an optical gunsight (although I suppose it can increase the maximum potential parallax error.)

Shooters have come to commonly use the term "eye box" to mean the extent of both fore-and-aft and lateral eye placement that still allows the viewer to see the full sight picture (FOV).

Very few people understand that there is some leeway in lateral eye placement (and it is measured in only a few millimeters between the "tightest" the "loosest") so manufacturers never mention this specification their literature. People do seem to understand eye relief, so manufacturers always list that specification (although they often do so inaccurately). There is also a range of optimal eye relief for a given scope, and scopes with a longer range of acceptable eye relief sometimes call their eye relief "non-critical". On some scopes, especially those which seek to optimize apparent FOV and make eye relief as constant as possible through the magnification range, eye relief tends to be rather critical (eye must be placed within a narrow range of distance from the eyepiece in order to see the full sight picture, ie., smaller "eye box"). The visible FOV on such a scope is usually larger than on a scope with "non-critical" eye relief, but the disadvantages of the trade off are obvious.

Intermediate eye relief (Scout-type) scopes and handgun scopes not only have long eye relief, but need an extended range of optimal eye placement. The combination of long eye relief and generous "eye box" causes these scopes to have much narrower FOVs than conventional rifle scopes, however, the extra generous "eye box" allows the shooter to acquire the sight picture very quickly, thus somewhat offsetting the disadvantage of a narrow FOV. That's one good reason you'll never see a 12x Scout scope -- it would have such a small FOV that it would be nearly useless in practical application.

Thanks guys, I'm getting a better handle on it.
My interest was/is the trade between FOV and eye relief. And, what does a design guy do to adjust for the trade during his design. What changes does he make within the scope to adjust for more FOV and less eye relief, and visa versa?

Don

quote:

Originally posted by DMB:
Thanks guys, I'm getting a better handle on it.
My interest was/is the trade between FOV and eye relief. And, what does a design guy do to adjust for the trade during his design. What changes does he make within the scope to adjust for more FOV and less eye relief, and visa versa?

Don

Sorry, but I think that's a subject more aptly treated by a good optics design text, and beyond the scope of an online discussion, at least for practical designs in rifle scopes. I know a few tricks that are useful in general (astronomical) telescope eyepiece design, but they are not very applicable to rifle scopes, where the apparent field of view is generally much smaller and the eye relief is much longer.

Andy