Diopter correction on a RDS

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Originally Posted by JiminAZ

Bill, this point that you make raises another question that you as an industry guy may know the answer to. Within an RDS, is the dot at a different optical distance than the target image? I would expect them to be the same or at least far enough away optically that diopter correction differences would be minimal.

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Unfortunately my experience with red dots is limited, but my understanding is that the dot is displayed within the sight itself, which is quite close to the eye as compared to the target, or even as compared to an iron front sight. The relevant distance would be the distance from the plane in which the dot is displayed to the eye. So, putting a corrective lens in front of the red dot sight to best see the dot would be like trying to see distance using the near portion of your reading glasses - it simply isn't designed to focus on that plane.

Do you have astigmatism?? Does your script correct for this condition or is it mild enough to not require correction at this time? I've had astigmatism for years but at onset it was so mild that an extra 1/4 power of diopter correction was what worked. This may be why you're getting good results with the magnifier and your glasses. The added diopter sharpens thing up.

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For sighting-in my Aimpoint Micro I have used a variety of homemade diopters.

Now, using a divider, I find the center of the factory provided cover for the rear lens. Then, I take a red hot needle and melt a hole about 1/16" diameter at that center point.

Over the years, I've made a wide variety experimenting with these diopters for different eyed people and everyone has seen an improvement. Some needed a larger hole but I always start out small and increase diameter.

Because it is hard to find the red dot with such a tiny hole, this doesn't have much practical use other than sighting in but you will see that red dot turn from a comma (or whatever your eyes make you see) into a precise red dot. It is kind of neat to have the diopter in place then flip it off and see that dot get instantly transformed into some other shape your eye makes you see.

This also helps you see what part of the comma you should be aiming with. Aim on a rest and then flip it off and see what part of the comma is on target. I use the bottom right end of the comma to be on target.

But maybe it's just me and my old eyes.

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The red dot is collimated, projected onto the same plane as the target image, which is usually infinity. You don't need diopter correction on the RDS, you need diopter correction on your eyes. I.E.....glasses.

Inability to focus on the red dot is an astigmatism issue...again, corrective lenses (glasses) are the only practical way to address that issue. Presbyopia is a separate issue, doesn't really affect the use of an RDS since presbyopia is a close focusing problem, not an infinity problem.

Thanks to all for suggestions and interesting points.

WRT the optical design of the RDS, I don't think that the ideas presented can be true at the same time.

- BillSWFA essentially says that the target is presented optically at whatever distance it is, and the dot is at some closer optical distance (inferring that it is the distance between my eye and the RDS lens). Hope I summarized your thinking OK Bill.

- Hmac says the dot is collimated or projected onto the same plane as the target image, in which case the dot is optically at infinity.

I don't have astigmatism (or at least that's what the doc told me). I wonder though because the blurry dot is not round but slightly oblong. My distant vision is better than 20-20. Generally I just have trouble focusing close, more so in my right eye than my left.

Makes sense to me that the RDS is just a pass through image of the target with the dot projected at infinity. That would make the most sense in a design whose objective is to project the dot onto the target.

But if that is true why am I having trouble if I have better than normal distant vision and no (or very little) astigmatism?

If I run the Pro with the front cover on and both eyes open and focused hard on the target, I still see a blurry dot. The target is sharp.

My current (speculative) thinking is that the focal plane of the dot is at some fixed optical distance that works for most people with good eyes, that the target optical distance is just whatever distance away that it is, and that RDS designers somehow get away with it because the dot is essentially a point source.

Quote:

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Originally Posted by JiminAZ

Thanks to all for suggestions and interesting points.

WRT the optical design of the RDS, I don't think that the ideas presented can be true at the same time.

- BillSWFA essentially says that the target is presented optically at whatever distance it is, and the dot is at some closer optical distance (inferring that it is the distance between my eye and the RDS lens). Hope I summarized your thinking OK Bill.

- Hmac says the dot is collimated or projected onto the same plane as the target image, in which case the dot is optically at infinity.

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http://ssequine.net/rdscoll.jpg

Aimpoint, in their literature, describes the working principle of their red dot sights as "reflex collimator sight - red dot sight". The red dot is projected in same plane as the target (collimated), therefore infinity, so close-focusing, as in wearing your bifocals/trifocals/progressive lenses is not only not helpful, it's counterproductive and, especially in someone with presbyopia, trying to focus on the dot will leave it blurry, blobby, and comma-shaped. My shooting glasses for rifles with an RDS are ground only for basic sphere (distance vision). No "near-add" for reading or close vision since that isn't helpful. If focusing on the target leaves the dot misshapen, then that's going to be astigmatism.

Pistols are a different story because you have to focus on the front sight. So some sort of compromise in sphere is going to be important. If you use your distance correction, you can't focus on the the front sight. If you use close-vision correction, you can't focus on the target. A good optometrist/opthalmologist will recommend a compromise correction somewhere in the middle that allows "pretty good" focusing on both.

Yup, dot is at infinity.

Sent from my VS980 4G using Tapatalk

With no glasses I see a fuzzy oblong dot. I have to conclude I have more astigmatism than the doc told me, because I have (otherwise) good distant vision. Or that the astigmatism that I have is minimal for normal vision requirements, and just noticeable when looking at the dot.

Thanks for the clarification guys. Learned some things on this thread.

Just like with nearsightedness or farsightedness, correction for astigmatism is in increments. Your eye dr probably picked what he felt was the closest increment, but it might not be a perfecton correction, or it could possibly be an increment off. It might be worth asking the eye dr about it.

Bill, the correction in my lenses is perfect as the dot is perfect with my glasses on. My whole disconnect had to do with only needing near vision correction and why that affected my ability to focus on an optically distant dot. I think TAZ's comment above may have nailed it. Whatever the case, all is good. I have some things to try with the rear sight and now know what to ask my eye Dr.

Currently traveling for business/vacation so I won't be able to play around with my gear for a few weeks. When I get the chance I'll post if the rear sight helped.