Originally Posted by peterh
But here comes the headache.
This is all irrelevant, because (as I remember mentioning earlier) the true depth of focus of a lens system... IS NOT DETERMINED (directly) BY THE TRUE FOCAL LENGTH! The real depth of focus is only determined by the lens opening, relative to the focal length.
And yes, I know that a 200mm tele at 2.8 gives a much shallower PERCEIVED depth of focus than a 24mm at f2.8, but that is the PERCEIVED depth of focus.
I will see if I can find something that describes this elsewhere, because I really think that this would go way beyond the scope (hmm... pun not intended) of this forum. For us, it is really enough to understand that, the higher the magnification, the shallower the perceived depth of focus, and the easier the rangefinding.
You can't change the lens opening on a rifle scope. A 56mm lens will always be a 56mm lens the same as another 56mm lens. But a 56mm lens with a focal length of 1000mm will have a different depth of field to that of a 56mm lens with a 500mm. It will have the same exit pupil if you use a 50mm ocular as the other would with a 25mm, and the same derived mag, but the 1000mm will have a shallower depth of field as you see it.
Size of lens is not an indication of focal length.
On a camera you have an aperture which opens and closes, and by the act of making it smaller, steeply angled light rays are cut out, only letting those more parallel though. This means you see less blur. But because less light comes through you see less light. You don't have this on a scope. The aperture is fixed. So with 2 lenses of the same diameter but different lengths with the same mag as another with only the focal length changing you see different effects.
On a camera the depth of field on a 300m focal length lens is the same as 100mm lens with the same f stop/aperture only if you enlarge the 100mm shot to the same size as the 300mm. But on a scope the mag is derived from the focal lengths. So if you zoom 10x on a 1000mm scope it's the same as zooming 10x on a 500mm scope, they both zoom 10x but the 500mm scope wi present a wider field of view to the eye, and thus the depth of field in relation will appear less because the image will be the same size as presented to the eye.
Look here. http://www.luminous-landscape.com/tutorials/dof2.shtml
The 400mm lens has the same blur amount as the lower focal lengths as the author demonstrates because the aperture remains the same. But as the focal length lowers the image gets smaller. And because it gets smaller the ability for you to pick out differences in focus between the gremlin and the tower becomes more difficult. This is because the field of view changes with focal length and you are packing in a wider view for the same image size.
With scopes the image size is fixed by the mag and which fixes the exit pupil/image size.
So for two scopes, both with 56mm lenses, both with 50x mag, we will see different images with different field of view, with different presentation of the same depth of field, if those scopes use lens clusters with different focal lengths.
Because we don't know focal lengths of lenses, it's therefore difficult to say how a scope presents its image and thus depth of field. Thus scopes which may have apparent smaller objectives, with the same mag, may work better and why other scopes with less mag may work just as well or worse.
Generally the trend is larger objective larger mag is better. But there are scopes that buck that and I have 2 right here from the same manufacturer but different releases that do that. Same mag, same lens size, different field of view. Different ability to determine depth of field.
I also hold that a march set on 50x will have an apparent easier time in showing differences in focus over the same ranges as a s&b 50x ft, assuming I'm correct in my memory that a s&b has a wider field of view than the march at 50x