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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2017 at 13:03
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I've gotten very tired of trying to set diopter on scopes.  I want to confirm whether I'm understanding the theory correctly, and then see whether I'm missing a more practical approach.

The reason that I think I have a diopter problem is that often when I'm behind a powerful scope (i.e., magnification > 10x) and I try to dial the "parallax" for target focus, that by the time the target is in focus the reticle is not in focus, but if I shift my eyes to focus on the reticle it will appear sharp (and the target will go slightly out of focus).

Evidently my eyes still have a decent capacity for accommodation, because I can change the diopter over a significant range and the reticle will still appear sharp.  I know the goal is to set diopter so that the reticle is in focus when my eyes are focused at some distance, but what distance?  Is there a standard focal distance at which a sport optic is supposed to produce project its image?  Is this the thing that diopter changes?

When I take photos through a riflescope I will often discover my camera lens is focused at just 1-3 feet!  To confirm this, I just took a NightForce NXS 3.5-15x50mm FFP scope and setup my camera to look through it at 15x.  I cranked the bell (diopter adjustment) at least 10 full turns and the reticle focal distance only changed from 2 to 3 feet.

In any case, I haven't found a reliable way to trick my eyes into not accommodating to whatever distance the reticle happens to be focused at so I can tell the difference as I adjust diopter.

I was thinking that I could get this set "correctly" for my eyes by focusing on a target, and then adjusting the diopter so that the reticle and target are in perfect focus at the same time.  Is this a theoretically valid approach?  The problem is that especially at high powers a change to diopter will take the target out of focus, so I have to work both diopter and parallax, and I don't know which way to go with the diopter because I can sort of see "not in same focal plane" but I can't tell whether the reticle is in front of or behind the target plane, or which way to turn the diopter to bring the planes together.
Post Options Post Options   Thanks (1) Thanks(1)     Back to Top Direct Link To This Post Posted: September/12/2017 at 14:29
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It is a bit different for different scopes.  If you are dealing with a reasonably high magnification design, set the parallax to infinity, point the riflescope at something COMPLETELY featureless (blue sky, monotone wall a couple of feet away from you, etc) and adjust the eyepiece for optimal reticle sharpness.  

When you adjust the eyepiece, keep looking far away.  The reticle should be sharp when your eye is set for looking fairly far.  So look away from the scope at something distant.  Then glance through the scope.  The reticle should be IMMEDIATELY sharp with no adjustment period.

And please stop pointing your camera into the eyepiece.  That will really get you wrapped around the axle.

And to answer the three questions in bold: no, no and no.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2017 at 15:50
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So, as far as my eyes are concerned, the image in the scope (both reticle and subject) should be in "distant" focus?  I.e., if both my eyes are focused on the horizon and I bring the "eye box" into my field of view (and could keep my scope eye from shifting focus), the image should be in sharp focus?

And if I am experiencing the phenomenon in which the reticle is out of focus while the target image is in focus, does that mean my diopter is set incorrectly, or is that indicative of some other user error, or is it an optomechanical failure?
Post Options Post Options   Thanks (1) Thanks(1)     Back to Top Direct Link To This Post Posted: September/12/2017 at 15:53
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Correct. Diopter is set incorrectly. If there is no setting where parallax is absent for a reasonable distance, the scope has issues.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2017 at 08:18
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Is there such a thing as "neutral" or "zero" diopter?  Some of my scopes have a zero mark on the diopter, and some don't.

For a person with correct(ed) vision, should the scope diopter value always be "zero"?

Is it true that diopter adjustments are only necessary and appropriate when a scope is being used by an eye that is near- or far-sighted (i.e., lacks the flexibility to focus on distant or close objects)?  And if so, given that diopter should be set using a "distant gaze," is it only helpful for near-sighted eyes?  If not, why not?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2017 at 12:42
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Zero mark is the center of adjustment. It should be close to the right position for an average eye, but it varies and is also subject to manufacture tolerances. Both farsighted and nearsighted people need adjustment in the eyepiece.

Since just about none has an "average" eyeball, everyone needs adjustment.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 11:10
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It would still be great to get some of the theory behind what's going on with diopter.  So far I see resistance to the notion of specifying a focal distance.

Importantly: I'm still stuck in practice – especially with the NightForce that I'm looking at where the diopter does not have a perceptible effect over a number of full turns of the ocular bell.  Everywhere I read instructions that essentially say the same thing suggested earlier: "Look away from the scope at something distant.  Then glance through the scope.  The reticle should be IMMEDIATELY sharp with no adjustment period."  The problem is that my eyes focus before I can process the clarity of the image.  (This results in very tedious visits to optometrists: Instead of being able to decisively answer "better, or worse?", once we're past the coarse corrections we have to label every test "A" and "B", and I have to ask to flip between two candidate images many times before I can offer even a tentative assessment of which causes less focal strain.)

I think machines exist that can image a relaxed eye and spit out a prescription.  But even if I know my ideal prescription, no scopes are labelled so I can just dial it in.  I imagine there are devices and tricks that could determine the absolute diopter setting of a scope.  Are there any that would be accessible to someone without an optics lab?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 11:46
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This is a purely empirical process, I am afraid. You have to take your time and do it right.

As far as the focal distance goes, I can try to answer your question, but I do not understand what you are asking.

What focal distance are you talking about?

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 12:44
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To elaborate further:  the eyepiece has a focal plane and it is inside the scope.  When you adjust the eyepiece you move that focal plane a little in order to match it to the location of the reticle in the second focal plane of the scope (or to its image if the reticle is in the FFP).

Basically, eyepiece adjsutment focuses the eyepiece on the reticle or its image in the focal plane, while sidefocus/AO focuses the image of your target onto the same focal plane where the reticle and the eyepiece are. 

With thousands of riflescopes out there, which exact focal distance do you want specified?  How is focal distance of an eyepiece in any way relevant to any of this?

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 13:52
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Hopefully you'll indulge a reference to a camera lens (the reason why to be explained shortly):  Scenes or images that are closer than half the hyperfocal distance of the lens require that focus be adjusted to some point short of "infinity" to be maximally focused.  Call that focus setting the "focal distance" of the image.  With a typical camera lens looking at a real-world object – say a target – we can actually read that distance right off the lens, and if the lens is well calibrated it will correspond to the real-world physical distance between the camera's focal plane and the target.

The reason I'm starting with a camera is that I don't know what the default or resting focus of a human eye is.  Human eyes can have all sorts of variations from an "ideal" eye, but let's at least assume going forward that we're interested in human eyes that have successfully been "corrected" using glasses.  This means, among other things, that:
  1. They can focus at infinity
  2. They have a hyperfocal distance
  3. They can focus on any target from infinity to some near distance limit (which ranges from perhaps 6 inches in children to a few feet in older adults).
So first there's the general question: What is the typical focal distance of the image projected by a riflescope with "zero" diopter?  My ex ante guess would have been "infinity."  This would mean that if I'm staring at the horizon, and then a properly-adjusted optic is popped in front of my eye, I would not have to change focus to see the reticle and image clearly.  If I were designing sport optics this is what I would do, because users are typically looking out at hyperfocal distances when looking at targets.

However, you suggest that the focal distance of riflescope images is much closer than that, and that it can vary substantially by make/model.  That's a very helpful piece of information, but also confusing given the instructions to "set diopter while focused at infinity."  My eyes (separately) don't do a good job of telling me what distance they're focused at, but my camera does (sorry, I don't know how else to approach this), and sure enough, it reports that at least some riflescopes present their image at 2-3 feet.  I.e., when I look into these scopes my eye has to shift its focus exactly as it would when reading a target held at arm's length.

And I thought this is what diopter is all about: Some people can't focus on objects that close, even with corrected vision.  And so what I thought diopter does is change the focal distance of the image presented by the scope.  E.g., for someone who is far-sighted and can't focus on objects closer than 5 feet, dialing out the diopter will finally push the scope image to a virtual distance at which that person's eyes can bring it into focus.

Now, what about the majority of people who (at least with glasses) can focus from a few feet all the way to infinity?  The scope can present its image at any virtual distance in that range and the person's eye will be able to bring it into focus.  Perhaps I have misinterpreted the instructions for setting diopter, and they should instead say, "Relax your focus" instead of "focus at infinity."  This would make sense if each person's eye has a "resting" focal distance that is not always "infinity," and the purpose of diopter adjustment is to bring the scope's image close to that "most relaxed" focal plane.  But it's essentially useless to someone like me who is not sensitive to the strain of accommodation.  I would go back to my earlier guess and say, "Hey, I can focus anywhere from a foot on out, but when shooting at a distance I'd like to keep my eyes in the hyperfocal region, so tell me how to turn the diopter to get the scope image's focal plane as far away as possible."

Now I'm confused when you say, "When you adjust the eyepiece [diopter] you move that focal plane a little in order to match it to the location of the reticle in the second focal plane of the scope (or to its image if the reticle is in the FFP)."  But that sounds like a static calibration that should be done at the factory.  If the diopter's purpose is to align the focal plane of the scope (i.e., the image leaving the eyepiece) with the focal plane of the reticle, then there is exactly one correct diopter setting, and anyone or anything who can focus on whatever the scope's focal plane is should agree whether the reticle is in focus.  Likewise, if (as I suspect) on my FFP scopes I don't have the diopter set correctly, anyone with corrected vision should say, "Hey, when you dial the target in focus using parallax, the reticle doesn't seem to be in focus."  I.e., in both of these cases diopter should be independent of the user/eye for anyone who can focus at the distance the scope presents its image.  And, furthermore, there should be an optomechanical way of setting and verifying it correctly.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 15:29
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Wow it seems that we are making this more complicated that it is.  The diopter is adjusted so that with your vision the reticle is clear and in focus.  

Diopter adjustment:  All the scope manufacturers directions that I have read say to adjust power setting to maximum and parallax to infinity.  Point the scope at a plain background such as a smooth wall or the sky.  Is the reticle sharp and in focus?  If not adjust diopter to the right until the reticle is definitely out of focus.  Now adjust the diopter back to the left 1/4 to 1/2 turn.  Glance through the scope.  Is the reticle sharp and in focus now?  If not repeat until it is.

Nightforce specifically will tell you to screw the diopter all the way to the right, then start working your way back to the left.  Once you have the diopter set properly, the reticle will be in sharp focus.

Next will come adjusting for parallax.  Pick a target at least 50 yards or more away.  Turn the parallax knob until the target remains stationary when you move your head up/down or left/right.  Is the reticle in focus?  If it isn't you did not adjust the diopter correctly.  The target may or may not be in perfect focus.

If you are lucky your target will be in perfect focus when parallax is set correctly.  Most scopes are not perfectly in focus.  If your Nightforce parallax knob has numbers on it for distance, they may not correctly correspond with the target distance when your parallax is set.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 15:30
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Eyepiece is not a camera lens.

Riflescope is an afocal telescope. Eyeypiece does not firm an image. It puts out collimated light. The lens in your eye forms the image on your retina.

Adjusting the parallax knob does not do anything to the reticle in FFP or SFP designs.

The reticle has a fixed position inside a riflescope. You adjust the eyepiece to see it clearly.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 19:22
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What do I do if I can "see the reticle clearly" with the eyepiece adjusted at any point across a wide range?

As best I can understand, this would almost certainly not allow me to produce a "correct" diopter setting.  (The cause of this is that I'm not able to inhibit my eye's accommodation long enough to assess the reticle's sharpness during each trial glance between diopter adjustments.)

I haven't heard anything yet to refute this hypothesis: One of the consequences of this inability to correctly set diopter is that, when I am looking at a target at high power (say, 100 yards at 15x), I am not able to focus on both the target and the reticle at the same time, even though I have dialed out perceptible parallax.

I assume that with a functional scope I should be able to get the target and reticle to be in focus simultaneously.  If I can only see the diopter is incorrect when I am trying to superimpose it on a target, is there an algorithm I can use to reach a correct diopter setting?  (The trickiness here is that a change to diopter also changes target focus, and I could end up chasing my tail if I make iterative adjustments to diopter and focus/parallax.)
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 19:32
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Originally posted by koshkin koshkin wrote:

Eyepiece is not a camera lens.

Riflescope is an afocal telescope. Eyeypiece does not firm an image. It puts out collimated light. The lens in your eye forms the image on your retina.

This suggests I didn't communicate a key analogy correctly in my earlier post: I was substituting a camera for my eye, not for any part of the scope.  The point was that, yes, the scope puts out collimated light.  A lens (whether in a human eye or on a traditional camera) focuses the light coming from the scope's eyepiece to produce a sharp image (on the retina or camera sensor plane, respectively).  But the light emerging from the scope can be collimated by the scope's optics so as to appear to come from some range of focal distances.  What is that virtual distance?  My eye (especially my eye, as noted by my inability to inhibit rapid accommodation) can't tell very well, but my camera's lens is reasonably calibrated to indicate the distance at which it is focused, so I substitute the calibrated camera lens for my eye's lens to determine the distance to the (virtual) focal plane produced by the rifle scope.  
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 22:04
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If you can 'see the reticle clearly' with the diopter adjusted to any point across a wide range you are not doing this right.

Look at a distant object.  Then look through the scope briefly, only for a second or two.  If you look through the scope too long your eye will adjust to accommodate an out of focus reticle.

When you are looking through the scope at a target your eye cannot focus on both the target and reticle.  Focus on the reticle.  Place the reticle on your target.  If you focus on the target you will make poor shots.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 22:21
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Originally posted by SEMO Shooter SEMO Shooter wrote:

If you can 'see the reticle clearly' with the diopter adjusted to any point across a wide range you are not doing this right.

Look at a distant object.  Then look through the scope briefly, only for a second or two.  If you look through the scope too long your eye will adjust to accommodate an out of focus reticle.

Again: I know you are offering the standard guidance for setting diopter.  I am trying to understand this more deeply because this standard guidance doesn't work for me: I can't perceive the reticle before my eye accommodates to bring it into focus!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/15/2017 at 22:25
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Originally posted by koshkin koshkin wrote:


The reticle has a fixed position inside a riflescope. You adjust the eyepiece to see it clearly.

Here's another attempt at illuminating my underlying question: Is adjusting the diopter on a riflescope optically equivalent (to first order) to leaving the diopter fixed on the scope and instead adjusting the diopter of the eyeglasses worn by the user?
Post Options Post Options   Thanks (1) Thanks(1)     Back to Top Direct Link To This Post Posted: September/19/2017 at 12:25
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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.

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.

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?

ILyta
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I think this thread gave me diabetes.

Is that possible?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/19/2017 at 14:19
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Originally posted by Rancid Coolaid Rancid Coolaid wrote:

I think this thread gave me diabetes.

Is that possible?

Not really, no.  Migraines, on the other hand, are quite possible.

ILya
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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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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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