Home Forums Hunting & Shooting Competitive Shooting prescription focal point for aperature sights. Rost you out there?

In building a prescription for shooting an aperture sight so you can see the front sight and target in focus where should the focus point be?

Most optometrists that I met don't know anything about competitive shooting and thus are no help with building a prescription.

Anyone have any experience with this? I have heard anywhere between 30" to 20 yards past the front sight but have not found anything that has been of much real help.

Well, for anyone who I interested did find this on the CMP forum.

It is not an impossibility for the target and the front sight to be in reasonable focus at the same time, though it takes exact optics. What is needed is for your focal point to be in the right place, and to have sufficient depth of field.

Getting your focus to the right point is done by your eye's lens accommodating to shift focal point, or by adding a lens which shifts focal point. As you get older, you eye loses its accommodation capabilities, so you have to strain the eye muscle more and more to focus at the right spot. In your case, it is likely that your are trying to focus at the strain limit of your eye muscle, and as the muscle tires from repeated supreme exertion, it can't always hold focus to the same spot. This causes varying amounts of blur on the front sight, causing the front post to appear slightly bigger or smaller as the width of the blur line changes, causing vertical stringing.

From an optical physics perspective, you want your eye to focus at the hyperfocal distance of the front sight, which is at 2x the distance from your eye to the front sight. Assume the front sight is 25" from your eye, you want to focus at 50".

To figure out the lens strength you want to shift you there, take 50", convert it to meters 50" = 1.27 meters, then invert the meters figure: 1/1.27 = 0.79 diopters. Always round down to the nearest lens strength, in this case +0.75. If you don't wear distance glasses, this is the theoretical lens power you need. If you do wear glasses, simply add +0.75 to your distance prescription. Note, theoretical values are a good starting point, but don't account for an individual eye, so you might be one step higher or lower.

Next, you want as small an aperture as possible, without going so small the image dims. This small aperture will, just like in photography, give your eye a greater depth of field, so near and far objects are both in focus. You have seen photos of people close to the camera, and mountains in the background, where both are in good focus, despite the fact that it is theoretically impossible. This is done by setting the camera to take a picture with a small lens aperture to give good depth of field. Your rear sight does the same for your eye. The smaller the aperture, the bigger your depth of field, and the better the focus on both the sight and the target.

Net, I recommend the right lens and a small aperture.

How to do it is a question of convenience and money. My favorite is the adjustable diopter rear sight. This has a focusing lens in it, so you can twist the ring, and get the desired balance of focus between the front sight and the target. A cheaper but less convenient option is to figure out the right lens strength, and just put that one fixed lens in the rear sight or in shooting glasses.

Next step is an adjustable rear aperture. Again, you can get cheaper fixed rear apertures, but as light goes up/down on the range, the adjustability is a nice feature.

I do not recommend colored, or polarized lenses. They rob you of light - a polarized lens blocks up to 70% of transmitted light. Colors block various amounts, depending on the color, but upwards of 30% is likely. In extreme sunlight, this might be acceptable, but if I were willing to give up 30% light, I'd rather make my aperture 30% smaller, and get the improved depth of field.

As to getting both in focus, it depends on your eye. There is a spacing of the photoreceptors in the human eye, and once the blur in an image is smaller than about a receptor diameter, it apperars in perfect focus to your brain. Further focus improvement do not help. Thus, there is a theoretical aperture size where both the front sight and target will be at the detection threshold of focus, and both will be practically in perfect focus. In reality, the image will dim unacceptably before you can get to this point, but the closer you get by getting the right lens power and the right aperture, the better the junction between the post and the target will be.

A different option is to move to a front sight ring, versus a post. While this won't fix your focus issues, at least the front sight and target will have the same amount of blur all the way around, so the blur on one side offsets the blur on the other, and you can still center the target in the sight. This will eliminate your vertical stringing.

I took my starting RX, and took the rifle inot the eye doc, and got him to flip lenses while looking the sights.

Then took the closest 4-6 lenses and took em to the range and checked it out. Decided there.

Another issue is tint. Your article says no. I say some days without tint it doesn't matter a damn bit if you can see the sight clearly the target will not be crisp enough to decipher whats what. Mind you my tints were about half or less of what you normally see tint color in shooting glasses but you need the contrast some days.

But then I"m only a high master service rifle shooter XC and LR.

That's a good idea rost I was hoping you would chime in. The range I shoot at the most is an east facing range which is hell for early morning sight picture let alone my aging eyes.

High master at both xc and lr? You do suck

That is some damn good consistent shooting to get to high master. By percentage the difference between High master and master isn't much but in practice it might as well be on another planet.

HM Service rifle to boot. LOL.

Its all just work and dedication. I think anyone can do it if they put their mind to it.

I don't consider myself or my wife exceptional shooters. Just that we were dedicated. She held Master and was on the brink of HM when we quit shooting.

Lense's on the range is the ONLY way IMHO, and IMHO you need a bright day to do it, the overcast ones seem easier to my eyes..

I tried the optician route, too. The guy knew what I was looking for, but overlooked one basic fact-the low light in the examining room dilates an eye to the point where there is little depth of field. On that basis, I ordered a cheap pair of glasses with 0.75 diopters closer focus than my normal prescription. It wasn't enough, and I ended up with 1.00-1.50 diopters closer than my normal prescription, depending on the light, and use the variable rear diopter to adjust so that I am focused on the front sight and the target is just a bit fuzzy.

Zenni Optical supplies cheap (~$20-25) glasses, with ordering over the internet and ~2 week delivery. I'd get 1.0, 1.25, and 1.50 closer focus if I were starting over, experiment with sight picture, and choose the one that works best for you.

Clarence

A friend who is a high master and an MD told me to just measure the distance from my eye to the front sight (at home) and ask the eye doctor to set the focus at whatever distance is 7" in front of the front sight. The idea is to have the front sight in focus, even if it's an aperture, and it doesn't matter if the target is blurry. I did this and it's a pretty good idea, though maybe not perfect.