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Diopter theory

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dbooksta View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dbooksta Quote  Post ReplyReply Direct Link To This Post Posted: September/22/2017 at 20:03
Originally posted by koshkin koshkin wrote:

If all is set up well, the diopter should be projecting the image as if it is coming from some reasonably distant point.  Probably not infinity, but distant.

The only people I have seen who can not use the eyepiece to adjust for reticle sharpness had some vision problems, i.e. everything they saw was blurry enough to not be able to see the difference.  I have seen a lot of people who simply did not have the patience to do it.  You do not sound like you do not have the patience, so this is puzzling.

Indeed, to confirm your impression: Over recent weeks I've spent several hours trying to make sense of this.  I have played with diopter on a 15x50mm NightForce, 24x56mm IOR, and 32x56mm Sightron.  In every case I can dial the diopter to a point that it is *not* in focus, but then there is a very wide range where it *is* in focus using the "relax and glance" tests.  (I have no idea if it's relevant, but as background: I was medically qualified to enter pilot training with the USAF 20 years ago, after half a day of state-of-the-art eye testing at (then) Brooks Air Force Base.  Since then I have developed slight astigmatisms (last prescription was OD PL=+025x072; OS -125=+125x082).)

While doing these tests I actually found an internal defect in one of the NightForce scopes and while talking to one of their technicians for RMA'ing it discussed my problem.  He said that especially at max magnification there is no guarantee that the target image is going to be in sharp focus with the parallax removed, but that with diopter I can always get the reticle sharp, and that in such a situation I should focus on the reticle (just like one focuses on the front sight when shooting iron sights).

Originally posted by koshkin koshkin wrote:

You can try to do this in reverse: find a very distant object, set you scope side focus to infinity (hoping it is actually infinity) and go backwards: adjust the eyepiece until there is no parallax.  Then, without touching the eyepiece again, work through the side focus adjustment range to make sure you can get parallax free sight picture for a range of distances you are likely to use.

This sounds interesting!  I haven't checked the effect of the eyepiece on parallax.  Are you saying that with side-focus and a target at virtual infinity a correct diopter will remove parallax, and then (if correct) the parallax should be removable using the side focus at all other distances within the scope's specified range?  Should I then conclude that if that is not the case then either I have diopter set incorrectly or else the scope is defective?

Originally posted by koshkin koshkin wrote:

With the camera looking into the scope, you results are difficult to correlate to what your eye sees simply because with very different lens and imager, depth of field and depth of focus are very different and physical position with respect to the eyepiece makes a difference.

Which camera re you trying to use?

When I've done camera tests I use a Sony A77M2 (APS-C sensor).  I have found that I have to shoot with around a 50mm lens with the objective right about where my eye would be (i.e., about an inch from the eyepiece when scope is at full magnification), and that I have to open the aperture to at least f/2.8 to get a full image through the scope.  But I have a range of lenses and could use something else (or a combination).

Oh, and thank you for your patience!
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Post Options Post Options   Thanks (1) Thanks(1)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: September/23/2017 at 17:36
Since you CAN get the reticle defocused, figure where it is at the beginning of the defocus region on both sides of the deficits region. Once you have those two spots figured out, count how much you have to rotate the eyepiece to get from one to another. Then set your whoever at exactly middle of that adjustment range and go shooting.

ILya
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MZ5 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MZ5 Quote  Post ReplyReply Direct Link To This Post Posted: September/30/2017 at 16:27
This has been an interesting thread, because I have the same sort of difficulty:  Diopter adjustment leaves the reticle sharp and clear over a wide range.  I can tell that my reticle is not at the correct apparent focal distance, because when I come back in from featureless infinity and get on a target, focusing on the target causes the reticle to blur or fuzz on me rather noticeably.  Looking at the sky and trying to get the scope & reticle suddenly in my view makes my eye adjust very quickly to bring the reticle into focus unless the diopter adjustment is _way_ off.

Koshkin, you last post's advice (to find the ends of the diopter adjustment range that yields a focused reticle, and center diopter adjustment between those ends) has been about the best I've found thus far, though it's still not perfect at all.

The comment from the Nightforce guy is interesting.  Is that reasonable, from a physics or optical or technical point of view?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dbooksta Quote  Post ReplyReply Direct Link To This Post Posted: November/15/2017 at 13:26
Originally posted by MZ5 MZ5 wrote:

The comment from the Nightforce guy is interesting.  Is that reasonable, from a physics or optical or technical point of view?

I have gotten so irritated with this failure to align focus with parallax that I just posted that question explicitly here.
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