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Old February 27, 2013, 04:13 AM   #35
Join Date: August 23, 2006
Location: Los Angeles, CA
Posts: 53
Parallax Knobs; A Misnomer???"

Originally Posted by Bart B. View Post
AR15Barrels, that issue of focus and parallax mismatched can happen. The cause is the range scale on the adjustment's not exact for the actual objective lens' focal length.

Let's say the scope's optical system designer chose the objective (front) lens system to have a 200mm focal length. There's a tolerance in making lenses; ones of that type can easily have a few millimeters spread. The mechanics of the focus system are exact; a given threads per inch of the adjustment moves the lens mount exactly the same for all scopes with that design. So the range marks can be put on each and they'll all be perfect for the designed lens focal length.

But the one in your scope is different. You set your range focus at 200 yards but the objective lens actually focuses that target in front of the reticule if it's actally 198mm, behind the reticule if that's a 202mm lens.

Depending on how much error the objective lens' focal length is off from spec, the amount of parallax will vary.

Note that if you're teaching people to not focus their scopes, they'll never be able to adjust parallax. These terms both relate to the same thing. It's physically impossible for the reticule be properly focused on by the eyepiece and when the range focus is on the reticule, to have any parallax at the target range. If I understand what you're saying, I think the scope's eyepiece wasn't focused perfectly on the reticule but instead in front or behind it.

What did you use to "focus" the scope on the target? I ask because all the scopes I've seen only have one adjustment to focus at the target range and it cannot be the eyepiece.

What make and model of scope do you have? This interests me very much and I'd like to look into it deeper.....thanks.
I know about reticle focus, scope image focus and parallax.
In the particular nightforce scope I was talking about above, my reticle was perfectly focused.
It was just that particular scope (I owned 3 of them at the time) which had the parallax and the image focus out of sync.
My other two would put the image in focus and the reticle parallax free at the same adjustment.

When I teach new precision rifle students, I have them first focus their reticle using the sky as the background.
Then I teach them to adjust the parallax for focus first (gets you close) then continue adjusting so that when they move their eye behind the rifle without moving the rifle, they do not see any reticle movement on target.
Some scopes will coincide image focus with parallax.
Other scopes will not.

Shooters looking for precision grouping will do better with soft focus and zero parallax error than they will do with sharp focus and parallax error present.
The parallax markings on the scope are rarely correct.
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