When does 50 mm + objective become useful?
A large objective diameter has two useful effects:
a) it increases the size of the exit pupil of the riflescpe
b) it increases the theoretical resolution of the riflescope
The effect (a) in turn does two things:
(a1) It gives you a larger more comfortable "eye box" behind the scope so precise eye position becomes less critical.
(a2) It provides a brighter image at dawn or dusk if your eye pupils enlarge enough to be able to use the wider exit beam.
To me, the effect (a1) is the most desirable for hunting. It makes it very easy to look through the riflescope and acquire the target. Effects (a2) and (b) are good to have too. If you have a high-power scope (say more than 10X), then you do want to have a large 50 or 56 mm objective so that the image resolution is sufficient. With power more than 20X, the air turbulence effects become so noticeable that adding more power or more objective diameter doesn't add much to image quality. Also, hand shake becomes very distracting.
In the good old days, the diameter of the exit pupil of a riflescope was it objective diameter divided by its magnification. With the advent of large zoom-ratio scopes such as Swarovski Z6 and Z8, this is no longer true. At low power, the internal apertures limit the light beam diameter passing through the scope so, for example the Swaovski Z8i 2-16X50, has an effective aperture of only16.3mm at 2X so it is actually a 2X16 scope at its low power!!
As a result, it only provides an 8mm exit pupil at 2X. For comparison, a 2.5-10X50 Zeiss Varipoint riflescope provides a 15mm exit pupil at its lowest power, making it it a much "faster" scope for shooting at running game at low power.
While the extra zoom ratio is loudly advertised, the limitation of the internal apertures is not.
-Omid
