Lenses and apertures

[boats]

Art

Have enjoyed your post and think we have learned something useful, Just one more question,

Seems to reason if a + diopter say a + .25 moves my eye focus to sharpen up the front sight it would take less stop down on the rear aperture to sharpen it up. In other words a spin off effect of better front sight focus using a diopter would be allowing a more open real sight aperture which has the effect of giving the sight picture more light and letting you see the target better.

Or am i putting too much logic into it.

Boats

[chuck d]

Having fought the vision problem in NHA high power for the last 17 years, I'd like to offer the following:
1. Art is a BIG HELP!
2. You can get around lenses in sights with glasses. 20" barrel take your precription and have a lense ground with a +.5 diopter. 26" barrel, +.75
3. Art also sells a set of lenses that you hold/look thru to figure out what works for you. It is the best way to get the balance of post/target. Neither will be perfect, but you can make it happen.
4. Bob Jones (his website was mentioned earlier in this post) makes a very inexpensive pair of shooting frames. Take a look
5. I would make sure if you are into the world of bi-focals (speaking from experience here) Have a lense ground for your distance. Don't try to shoot with bi-focals. (open sights)
For high power I have as much in glasses, frames and lenses as a new Leupold. In fact that is why I am shooting Silhouette this year. But you can make it work. My vision struggles were made worse by position and 600 yards. Off hand and 200 yards, vision has not been a problem.
Seeing makes shooting a lot more fun!

chuck

[ShootingSight]

Chuck,

Thanks for the compliment.

Boats,

You are not overthinking, you are exactly correct. You have a focal point,and you have a depth of field that extends front and back of that focal point. The further away from the focal point an object is, the blurrier it gets. How quickly (or badly) it blurs as you move it is driven by aperture size.

So, if I have a front sight 24 inches away, and my eye is focused at 100 inches (inventing numbers here for the example), you will have a certin width of blur line that you see around the front sight. I actually have the optical formulas on my web site that will let you calculate the width of the blur, in MOA, on my website, but it involves apertures size, focal length of the lens, object disance, and other things that are not pertinent to this post. Bottom line is that we have some blur. To reduce this blur, you can do one of two things, you can either move your focal point closer to the front sight, which will reduce blur on the sight, but will increase blur on the target, so changing the lens is a trade off. The other thing you could do is to reduce your aperture size, which reduces the rate at which the front sight goes out of focus as you move it from the 100 inch perfect focal point back to 24" to be on your barrel. The good news is that reducing the aperture expands your depth of field in both directions, so not only will the front post get less blurry, but your target will get less blurry as well. Unfortunately, there is no such thing as a free lunch - as you reduce the aperture size, you let in less light, and the image dims, robbing you of contrast. THis problem was the basis for my inventing the slit shaped apertures. Once I recognized that focus on vertical lines is not critical, I developed apertures that are only small in the vertical dimension, but oversized them in the horizontal direction, so I preserved image brightness.

So, to answer your question directly: if you have trouble seeing the front sight, and reduce your aperture to compensate, you will find that by shifting your focal point back, you could open up your aperture. You would not improve your focus, but the image would get brighter. Alternatively, you could shift your focus back and keep the same aperture, and then youd get the same brightness, but improve focus (OK, or you could do something between and get slightly better focus, and slighlty brighter).

The recommendation to focus at 2x the distance to the front sight exactly comes out of that optical math, as this gives you the greatest possible depth of field possible with whatever aperture you have.

Art

[boats]

Thanks Art

That makes it crystal clear. I think I mentioned I have a diopter for my MVA Tang sight used on a Schuetzen and have used them in Anschutz adjustable rear disk on Smallbore prone rifles. It's just the one on my MVA tang is not working well for me. Fellow that made it said he had never tried one on that sight and some of the problems may be mounting or it might not be the correct diopter. And of course aperture front and rear sights on a bullseye target present a different problem than post on Silhouettes

Merrits looks to be a simpler Anschutz style set up.

Boats

[ShootingSight]

I'd lean towards havig the wrong diopter as the reason for the tang sight lens not working for you. Of all the people selling self-serve lenses out there, few of them have sat down with the optical math to determine what is necessary, and to give you an objective method for chosing the correct one. I imagine there are LOTS of incorrect lenses being used by shooters who have come to the erronius conclusion that either lenses don't work, or that their eyes are simply too old to play the game, when in fact they were simply sold the wrong lens.

This is why I originally started selling diagnostic test lens sets, so a shooter could actually try all the different powers, and see what worked for them.

Art Neergaard
ShootingSight LLC
www.shootingsight.com