Very basic optics question

[Berg]

What is the condition called when the following happens:

While aiming the rifle at a stationary target, if I shake my head slightly the reticle cross hairs appear to move slightly on the target. This is most noticeable when the rifle is in a solid rest and nothing is moving except my head.

Do some scopes do this more than others? If I wanted a scope that didn't do this what feature would it have that I should look for?

 

[NeuseRvrRat]

parallax

every scope is set for zero parallax at some range. usually it's 100 yds, except it may be 50 yds on rimfire/airgun scopes. higher end scopes can be sent back to the manufacturer to have the zero parallax range set to whatever you would like.

there are two types of scopes with user-adjustable parallax: adjustable objective (AO) and side focus (SF). AO scopes have a ring on the objective end that you turn. SF scopes have a third knob on the left side of the scope similar to the windage adjustment that you can turn.

parallax is what screws you up when you're riding in a passenger seat and you look over at the speedometer and it looks like the speedometer is reading higher than how fast you're going. it's because the needle and the gauge face are in different planes. parallax in a scope is because the reticle and the target are on different optical planes, as perceived by your eye. AO and SF simply allow you to get them on the same plane.

 

[Sky]

parallax

every scope is set for zero parallax at some range. usually it's 100 yds, except it may be 50 yds on rimfire/airgun scopes. higher end scopes can be sent back to the manufacturer to have the zero parallax range set to whatever you would like.

there are two types of scopes with user-adjustable parallax: adjustable objective (AO) and side focus (SF). AO scopes have a ring on the objective end that you turn. SF scopes have a third knob on the left side of the scope similar to the windage adjustment that you can turn.

parallax is what screws you up when you're riding in a passenger seat and you look over at the speedometer and it looks like the speedometer is reading higher than how fast you're going. it's because the needle and the gauge face are in different planes. parallax in a scope is because the reticle and the target are on different optical planes, as perceived by your eye. AO and SF simply allow you to get them on the same plane.

Excellent!!!!

 

[jeeptim]

Most exellent info!! Good job Thank You!!

 

[Berg]

Thanks for the great explanation.

So with an Adjustable Objective I would be able to "focus" the image to the appropriate target distance? What I'm not sure I understand is the target is already in focus.

For example if my scope is pointing to a target 75 yards away it looks as sharp as when pointed to one 150 yards away. How would I know what setting the AO ring should be and wouldn't I be changing my ability to see a clear image?

 

[NeuseRvrRat]

most AO rings are marked for ranges. some SF knobs are as well.

one method for SF knobs that aren't marked is to simply turn it until both the reticle and the target are in focus. ever had a scope where you could turn the ocular focus and get the target crisp, but the reticle would be all blurry or vice versa? it's hard to explain, but you kinda learn what to look for. another option is to adjust it until the reticle doesn't move on the target when you move your eye off-center. you really only need to be close anyway. i've got an AO rimfire scope on one of my 22s that i squirrel hunt with a lot. i might have a shot anywhere from 10 yds to 50 yds, so i just set it to 25 or 30 yds (using the marks on the AO ring) and i don't screw with it while hunting. my 22-250 has a leupold with a SF knob. i shoot groundhogs with it. whenever i see one, i get on target and reach up with my left hand and adjust the SF until the target and the crosshair are both crisp, move my left hand back under the buttstock, and squeeze one off.