What is a diopter?

A diopter on a scope is the adjustable eye piece that you look through to see the reticle and whatever you are looking at through the ocular bell (the front of the scope). 

You adjust the diopter by turning the ring around it to adjust your eye to the reticle in the scope. It brings the reticle into a crisp focus. This s not to be confused with the focus on the scope or what we call the ocular focus or parallax adjustment. 

The diopter merely works on the reticle to bring it into sharp focus and should be the first thing you adjust when trying out a new scope. To adjust you want to be looking at a flat white wall or a clear sky. Bring the scope up to your eye level and take a quick look through the lenses. If the reticle is not in sharp focus, you turn the ring around the eyepiece until it (the reticle) is sharp. Bring it up and down repeatedly, don't stare through the lense too long. You want it to be sharp as soon as you get it up to your eye. 

Hope this helps,

Shalom

John

A diopter can be simply a disc with a hole in it. They can be very helpful at bringing a guns sights and the target into focus at the same time. A good diopter will have an adjustable aperture to help with changing light conditions. I have one of these, it attaches to my glasses, and it is a great piece of equipment especially for pistol shooting but it werqs well with rifles as well. These are wonderful for our aging eyes and bring back most if not all of what we have lost over the years. The units I have seen are all lens free so no powers.

http://www.meritcorporation.com/products.html

Since you mentioned peep sights, I assume you are asking about the rear diopter sight found on 10 meter target rifles. It is an adjustable aperture sight mounted close to the eye and having an aperture measuring 1.2 mm (0.047") or smaller. According to Wikipedia, apertures larger than this will not provide the optical characteristics of a true diopter, self centering and critical sharpness of the front sight image. This must be why Williams, Marbles and Lyman all offer target disks measuring 0.050". I didn't know that. You learn something every day.

https://en.wikipedia.org/wiki/Diopter_sight

https://gehmann.com/english/products.php?kategorie=20

just some reading and pictures

not rear diopter sights fit all rifles, of course same brand will in most cases but say a Diana rear diopter will fit just Diana's and even they don't fit all of the Diana's

Anschutz seem to have the best universal mount ability but again the dovetails of different manufacture are different and I'm speaking of old match style sights

newer sights are far more interchangeable and when that happened I'm not sure

if you own a FWB 300 you would look for that model sight and they are easy to find, Walther, Anschutz, Diana, Weihrauch and FWB all had different rear sight and the way they mounted but then again some used the same front sight inserts

I spent a hell of lot of time chasing inserts and sight over the past years to the point I can tell you whats, what from a distance

one thing about the rear iris is using a rubber eye cup makes the sight work much better and even a front screw on shade tube, on the front of the rear sight, some will take those

will in any case good luck

mike

So you have a 300s with its standard sights and you are considering replacing the standard iris disk with one of the fancy Gehmann jobs, some of which have diopter correction or magnification, correct?

First, this is probably the wrong forum for detailed feedback. Not many 10 meter or three position shooters here. Try TargetTalk.org.

What I make of it is that some of the Gehmann irises provide 1.5 diopter magnification of the front sight and target. In addition some can provide spherical correction like your glasses in the range of -4.5 to +4.5 diopters. Finally, at least one can also provide correction for astigmatism. All seem to have an adjustable aperture.

If you wear glasses, they already correct for near/farsightedness (spherical correction) and astigmatism. But the prescription might not be optimum for shooting, so the adjustable spherical correction could possibly help make the front sight sharper even while wearing your glasses.

The adjustable aperature is a must for outside shooting where light conditions change a lot, perhaps not so much for indoors. Same I think for the filters

If you want to try one of the Gehmann products, I would start with the 530 Iris. It has the adjustable aperture, +\- 4.5 correction and 1.5 magnification. Also get the 577 Adaptor so you can remove the 1.5 magnifier in case that doesn't work for you. And be sure to let us know if you find it helpful!

In case you or someone else get one, here are general instructions on how to set up a Gehmann 530 Iris.

1. Adjust the aperture to 1.1 mm.

2. Adopting your standard shooting position adjust the position of the sight on the receiver so that the OD of the front sight takes up one third of the view through the aperture. Moving the sight forward reduces the visible area around the front sight, moving it back increases it.

3. Open the aperture fully.

4. Adjust the correction to maximize sharpness of the front sight.

5. Close down the aperture to a value between 0.8 and 1.5 mm giving good definition of the front sight and target bull without reducing brightness more than necessary.