Scope Shift

[horseman2]

https://www.canadiangunnutz.com/forum/showthread.php/2275068-Scope-shift

Some time ago I read an article on "Scope Shift" suggesting there was a shift in point of impact when the power was changed on a variable scope.
Using a Bushnell 6500 4.5 - 30 x 50 on a rimfire of known reliability, groups were shot at 30X, 8X, 16X and returning to 32X.
No appreciable shift was detected. This may have been due to the scope having a side focus and eliminate parallax.
My thought is this shift might be more pronounced when there is no parallax adjustment and shooting outside of the parameters of most fixed power hunting scopes that are pre-set at 150 yards.

This post is mainly to make shooters aware of this phenomena and no I do not have any evidence this does occur. My testing may have been overly simplified.

 

[Clocked92]

If your zero shifts when changing magnification on your scope, the scope is broken.

 

[cosmic]

If your zero shifts when changing magnification on your scope, the scope is broken.

That may be true for a relatively high end scope, but at the lower end of the consumer range its fairly common. Couple that with shifting POA between shots, erratic adjustment, non-repeatability in tracking, etc - all fairly common. I would suggest that a fair percentage of scopes owned by the average recreational hunter display some of these shortcomings, the owner just doesnt know it.
Years ago, I decided to set my minimum standard to the Bushnell 3200 series. They seemed to perform quite well based upon the results of my homemade scope tester. However, I did notice that many scopes, even 3200's and better, did not track well in cold weather. I've also had a 3200 fail on me, fortunately Bushnell replaced it with a similar quality scope.

 

[grauhanen]

That may be true for a relatively high end scope, but at the lower end of the consumer range its fairly common. Couple that with shifting POA between shots, erratic adjustment, non-repeatability in tracking, etc - all fairly common. I would suggest that a fair percentage of scopes owned by the average recreational hunter display some of these shortcomings, the owner just doesnt know it.

This is a very good observation. Less expensive scopes may not always behave exactly the same way in all circumstances. Individual scopes of the same model may not always behave exactly the same way. It should be expected that among the best scopes, there will be a greater level of uniformity of behaviour between models and individual scopes.

 

[barrett50cal]

I read this as two different possible problems being described:

(1) an error arising from a lack of concentricity of the erector lens system with the reticle behind it, so that the reticle doesn’t precisely correspond to the centre of the image as the erector lens /zoom system changes magnification. This is seen almost always — but not necessarily — in a 2nd focal plane scope (constant reticle size as image changes apparent size), and more commonly in either an older scope that didn’t benefit from modern manufacturing processes, or a cheaper scope of substandard quality.

(2) an error arising from parallax. Parallax error usually isn’t a problem under around 10x power, but the exception to that is when using the scope (a) at a range much closer than the factory-set parallax distance and (b) with higher magnification. So for example, parallax error will likely be induced when using a 3-9x40 at 50yds on 9x power. Even experienced shooters can run into this problem. The simple solution is to reduce the magnification power at this close range, or use a side focus / adjustable objective set to 50yds.