Adjustable Objective lens

[rldarmstr]

This is going to be a real basic question to some of you folks.  I don't know if its 'old age' or that pitcher of Margerita's I had last night but could someone please explain the phsyics involved with a targert scope and the adjustable objective? That is to say; I have 2 targert scpoes and they have to be adjusted to a set distance (or close) to focus properly, at a given distane. My hunting scopes focus on any distance without adjusting anything.

Why the difference?  What am I missing here?

Thanks,

Robert L.

[Graybeard]

Well NOT REALLY.

On hunting scopes or for that matter most scopes intended to be used on centerfire rifles parallax is minimized at about 150-200 yards. On any scope not having a means to adjust for parallax it is set to be minimal or hopefully none at one distance only. At all other distances there is some parallax. Now by setting it on out there at 150-200 yards the effects of it on your shooting are minimized. The closer ranges it's not enough to cause a miss on game and at longer ranges it's just not so much an issue.

But once they put that adjustment mechanism on a scope then it MUST be used. If you don't it gives a seriously blurred image at all other distances than the one to which it's set.

Parallax really isn't so much of an issue below about 9X in rifle scopes and below about 6X or 7X in handgun scopes. For me any scope with more magnification desperately needs a means to adjust it. I'm NOT gonna hunt with a scope with adjustable objective or other means for adjusting parallax as you really just don't have time to be messing with it when you need to be shooting at game. So I don't carry hunting rifles that have more than 9X magnification nor handguns with more than 7X.

[PA-Joe]

Another factor is that the eye adjustment is used to focus the hairs so that you can see them clearly while the objective adjustment is used to focus the target so you can seee it more clearly.

[wncchester]

The "out of focus" range is larger with higher power scopes due to what photographers call "depth of field".  Below about 10X parallax is almost unnoticeable but it's still there. 

But ajusting for parallax is not required, per se, at any power or range IF the shooter positions his eye in the center of the scope's exit pupil.   Adjusting the objective for parallax does improve the sharpness of the target at high magnifications but it isn't automatically neccessary for good shot placement.  That means parallax, as such, only shows up when the eye is moved towards the edge of the scopes ring of light and the target moves relitive to the cross wires.