Home Forums Hunting & Shooting Ask The Gunwriters Does re-focusing a scope affect zero?

JB and all, if I re-focus the eye-piece after sighting in, is it possible that the zero will be changed? I'm guessing no but I'd like to hear what the experts say.

My vision changes enough (probably due to RK surgery years ago) from am to pm that I sometimes need to change the focus.

Thanks

Shoot some in the morning , and some in the afternoon , post adjustment of course . If they all shoot to the same POA , then focus has no effect on POI !

No

If one has a bore sighter, he can install that and check before and after.

Power changes though on less expensive scopes can have a significant effect on POI.

From what I've seen, it depends on the scope.

Assuming that you do not have an adjustable objective (front lens) focus adjustment, adjusting the focus of the rear lens should not affect POI/POA. It will slightly affect the parallax-free distance of the scope.

As previously mentioned, changing the power setting on some scopes will greatly affect POI/POA, becoming a PIA. One Simmons scope that I had would shift every time I adjusted zoom, and setting the zoom back did not restore POI. That scope does not live here anymore.

You have to understand how a riflescope works. The objective lens (the big-ass front lens) focuses the image of the objective into the first focal plane in the riflescope. The erector lens group turns the image upside down and left to right. The zoom lenses increase the magnification by a factor of 1 to 3 or 4, or higher in some newer scopes. The image is the focused onto the second plane of the scope, where the reticle of the second focal plane is located. (As opposed to the first focal plane.) That second focal plane image is adjusted by the adjustable objective, and may be modified by the zoom lenses. However, by the time the image is merged with the reticle in the second focal plane, there is nothing that will mix it up. The last lens group is the eyepiece and it simply magnifies the second focal plane image. You adjust that eyepiece so the picture is crisp to your eye.

To add:

A first focal plane reticle should not be able to shift the relationship between the reticle and the image from the objective. So, even if the zoom lenses are crappy, it will move the combined image the same way. It just depends on the where the reticle is located in the scope, but once the image is merged with the reticle, nothing can change it between that and your eye.

An experiment that demonstrates some of FTR's post:

Get a piece of ground glass and a relatively long focal length lens. Focus an image of a scene on the ground glass. Now, get a relatively short focal length lens to magnify the image on the ground glass, with the new lens placed behind the glass. Without moving anything else, remove the ground glass. Nothing changes except that the texture of the ground glass disappears from the image you see.

Now you can add crosshairs to the situation. If your crosshairs are in the focal plane, they will not move with respect to the image as the eye is moved left/right/up/down. If they are not exactly in the focal plane, they will move. We call that parallax.

Denton, that is a good point. Let me just add that the objective lens is the one that you adjust to eliminate parallax error. Once your objective picture is focused properly on the same plane as the reticle, everything that happens to the image after that will not change the relationship of the reticle to the image.

Anson, it really depends on the scope. I had a Tasco that walked all over the place when I adjusted the parallax setting, I didn't discover that until I clamped down the rifle and watched the crosshairs. By then, Tasco had folded and I was out of luck.
Theoretically speaking (thanks Dent and FTR), because of where the crosshair is in the component stack within the scope, you SHOULD be able to play with the eyepiece all you want and not move zero much if at all. But some cheaper scopes appear to wander. Most good ones don't, at least not enough to matter.
The best way to find out is to try. Either shoot at two sessions as suggested above, or figure out some way to rigidly clamp down your rig (not necessary to actually shoot) and see how things move when you move the eyepiece.

Thanks to all who responded. I didn't think it would cause a problem, especially with a quality scope, but it's good to hear others agree. Next range session, I'll experiment a little.