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Discussion Starter · #1 ·
Hi ShootingSight,

I saw a post you made on the "being trolled leads to vertical stringing" thread, it was about focal distances as a person ages.

So I'm wondering if you'd share some knowledge. I was having some trouble focusing on my front sight today (M14 and M1). I started fine but after about an hour I couldn't do it anymore. I'm 30 years old and I weld for a living. I use a dark lens (shade 12) and I usually weld about 10-12" from my face. I'd sometimes like to be closer but my hood gets too hot. Anyhow, I weld like that all day with no problems. But I couldn't handle more than an hour of focusing on my front sight, and that was intermittent, though I suppose the welding is too.

Do you think this issue is simply that the focal distance is different enough that the muscles in my eyes aren't use to it, or maybe something else? I would just go to an optometrist but last time I did that I got sold a pair of glasses that did nothing, and was told I had 20/20 vision in both eyes.
 

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Eye Master
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Yah, most people think 20/20 means you have perfect vision. It doesn't! 20/20 is a test of vision at 20 feet, it is a distance vision test, and measures if your eyes can resolve 1 MOA at infinity which is the standard test of visual acuity.

The test does NOT measure your ability to focus close up, so you might have 20/20, and still need reading glasses.

With time, the lenses in your eyes get hard and you have trouble focusing up close. I don't know if UV from welding can accelerate this, or if your DNA simply did it to you early.

For your eyes in shooting, it is like being told to do 1 pullup, but that the definition of a pullup is that you have to hold at the top for 20 minutes. We can focus out eyes close/far 1000 times a day, but it is only in reading or shooting that we need to focus up close and hold close focus with great accuracy. This exhausts the eye muscle, and like a pullup, after an hour, the muscle is exhausted. You can do it in the morning, after the muscle has rested all night, but by mid-day, you are done.

So what you want to do is see what your eye doctor prescription said. The math to get you from a prescription to shooting vision is easy and well understood. If you don't have a prescription, the math still works, except that some people have almost good vision, where the error is so small they just live with it, rather than wearing glasses, but if we are going to calculate a lens for you, we may as well start with your real prescription, and not the 'almost' perfect vision concept.

Bottom line is that for an M14 or M1, if you have good distance vision, you will need a +0.5 diopter lens to see the front sight without blurring the target out. This is not a standard reading glasses power, so I make safety glasses with these custom lenses in them, and I can get them made in quantity for cheap because 50 - 75% of the population, this is right for them. If you do not wear glasses for distance vision, they could well work for you.
 

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Eye Master
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By the way, it is a misnomer that you want to focus on the front sight. Wherever your focal point is, you have a depth of field (the size of which is driven by the aperture size). This depth of field ranges from in front of your focal point, to somewhere beyond your focal point.

Your real goal in shooting is to centralize this depth of field, so the near edge just touches the front sight, at the same time as the far edge of the depth of field touches the target, so you can see both sight and target simultaneously. This perfect focal point is referred in photography as the hyperfocal distance of the front sight.

So, given the sight radius, I can calculate the hyperfocal distance, and given your distance vision prescription, I can calculate the delta that your lens needs, so your relaxed focus lets you see the target and the front sight together.
 

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Discussion Starter · #4 ·
Well thank you very much for that insight. I'll re-read this when I'm less tired. I don't have my prescription anymore but I guess it wouldn't do any good now as I got it 5 or 6 years ago.
 

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I agree with Art's comments, and here is how I understand (and do) it.

With a post front sight it is very important to see the top edge of the post clearly enough so it can be consistently aligned with the target. Of course that also means that the view of the target has to be reasonably sharp.
I don't try to actually 'focus' on the front post - if I do that I can't see the target. So I adjust my mental concentration and vision to get a distinct view of the front post and a reasonable view of the target. As SOON as the sight picture and my hold is acceptable, I fire the shot. For me, spending time trying to 'dress up' the sight picture causes eye fatigue and makes things worse.

Jay Kosta
Endwell NY USA
 

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Eye Master
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In another thread, someone called out the difference between 'focus' on the front sight in the optical sense, and 'focus' on the front sight as in concentrate on that part of your sight picture.

I am referring purely to the optical part of the discussion.

Of note, if your lens shifts your relaxed focal point to where it wants to be, you will not get muscular eye fatigue, as the muscle remains relaxed. There are other sorts of eye fatugue, so you don't want to stare too long, but those have to do with the cell chemistry of the retinal photo receptors, and I am not educated on that subject. I stick to the optics.

Also of note, your relaxed focal point is not constant. As you hydrate or dehydrate slightly, your ocular pressure can go up/down. From an anatomical perspective, I don't know how much your eyeball size changes when this happens, but I ran the calculation assuming a 1/100 inch change in diameter, and found that this made a 0.50 diopter change in your relaxed focal point, which is significant, so a day on the range sweating without staying hydrated can influence your vision. Also, glucose level in the ocular fluid will change its refractive index, and influence focal point. Again, from an anatomical perspective, I don't know what the swing rates of blood sugar are, nor do I know the time lag as blood sugar drops, or mounts, how quickly does the ocular sugar level go up/down, but I know it is a factor. Also, alcohol. Things that don't look so good sober look a lot better when you are drunk. In my youth, I did a lot more research in this field, and found that many scores that look like a 9 or 10 when you are drunk, actually look more like a 6 or less the next morning.
 

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Art,
Is there a way to figure that out by doing an actual eye test using a Snellen chart at the prescribed distance, whatever it is (you posted on the actual distances and I saved them, so the number here is not the important issue)? What I mean, is could you put the 20' Snellen chart at the 40-50" or so distance that you posted in other postings? I am getting a new eye doctor soon and my shooting issues will come up. I always end up experimenting a lot even though I know about the general 1/2 diopter decrease in diopter I should need for my needs. I can see close, not far (and have astigmatism). You posted on my astigmatism in the vision board.

I keep trying to run into you at the Nationals, but you are always busy with someone, or away (mostly away), or probably shooting. I shoot Long Range and am probably going through Trigger check, inprocessing, packet pickup and walking Commercial Row when you are shooting.

Thanks,

Danny

By the way, it is a misnomer that you want to focus on the front sight. Wherever your focal point is, you have a depth of field (the size of which is driven by the aperture size). This depth of field ranges from in front of your focal point, to somewhere beyond your focal point.

Your real goal in shooting is to centralize this depth of field, so the near edge just touches the front sight, at the same time as the far edge of the depth of field touches the target, so you can see both sight and target simultaneously. This perfect focal point is referred in photography as the hyperfocal distance of the front sight.

So, given the sight radius, I can calculate the hyperfocal distance, and given your distance vision prescription, I can calculate the delta that your lens needs, so your relaxed focus lets you see the target and the front sight together.
 

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Yes, there is.

Optically, the distance you want to focus at is 2x the distance from your eye to the front sight. Take a ruler, and without poking yourself in the eye, measure from your eye to the front sight.

For a pistol, this might be 24". So you want your eye to have its relaxed focus at 2x, or 48".

If I know your infinity distance correction (ie your 'prescription' from the eye doc), I can do easy math to determine that the addition of a 0.82 diopter lens would shift your relaxed focus from infinity to 48". Lenses come in steps of 1/4 diopter, and you usually want to round down, so I'd round your solution to a 0.75 diopter lens.

Or, the other approach, rather than calculating the shift from infinity using math, you could just put the eye chart at 48" in the first place, and determine what lens works best. Some people prefer this, and that is why I sell test lens kits, so you can try all the different powers.

Bottom line is that putting the chart at infinity and doing the math shift, or putting the chart at 48" should both yield the same answer, the real grain of knowledge in here is that the distance you want is at 2x the distance to the front sight. In photography, this distance is called the hyperfocal distance of a subject.

To make the math easy: because lenses are available only in 1/4 diopter steps, the two answers that shooters normally get are:

1 For long rifles, smallbore, M1, M14, Palma, etc. the correct answer is +0.50 diopter shift from infinity.

2 For AR, carbines, pistols, and other short sight radius stuff, the correct answer is +0.75 diopter added to infinity.

For this reason, I have worked with a safety glasses company and have convinced them to make safety glasses in both +0.50 and +0.75 diopters exactly for the shooting market. As long as you don't need glasses for distance, or wear contacts, these glasses are just what you need to fix your shooting vision.

They are not on my web site yet, but should be in a week or so.

Art
 

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M1A aperture

Art,
Installed and tested 0.460" aperture yesterday on my older SA super match-worked very well for my senior eyes. Ordered another one last night to try on my Garand, and will probably order another one for my Ted Brown LRB M14.
Thanks,
beginner
 
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