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Diopter theory |
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dbooksta
Optics GrassHopper Joined: July/01/2009 Status: Offline Points: 31 |
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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).
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?
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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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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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.
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MZ5
Optics Apprentice Joined: July/03/2012 Location: Arizona, USA Status: Offline Points: 126 |
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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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dbooksta
Optics GrassHopper Joined: July/01/2009 Status: Offline Points: 31 |
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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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