some scopes like the 6x24x56 Diavari are very accurate in the ranges printed on ao knob and actual. However the increments in the calibration are too wide to be of actual use when compared to the 1 yd or meter of a LRF. Tunnel vision or vignetting is caused by the lens elements including the light from the end of the scope in the sight picture and show a design flaw. Sometimes this is seen more often on FFP scopes because of the need to focus on the front reticle. Front ao are easier to tune because of a finer thread pitch, and when combined with a greater radis in the tuning knob-- A best example is the big 50 schmidt and bender with the large side wheel used in field shoots. In opposition to the position that distances can't be read from the ao side wheel, this is exactly what this scope was designed to do. Depth of field is a function of non refracted light lens from lens, so a larger objective will have less depth of field than a smaller, all else being equal. The more parallel the light rays the greater the depth of field, that is the purpose of an iris. This also depend of the magnification. The more the mag. the less the depth of field. About 15-20 years ago Burris built a scope with an adjustable iris for exactly this purpose to increase depth of field on higher mag scopes for daylight pd hunting. Worked great.
