Shooting Glasses
The iris is not the correct answer, it may compensate up to a point, but it is not the advised route for target shooting.
Firstly it is impossible to focus on the foresight and the target at the same time, the foresight is short focus and any thing greater is considered long focus, in young people they have the ability to flick back and forth between the two, once older its not possible due to old muscles, but doings so by any one is not a good principle for target shooting.
With a rifle which is from what you've said you are shooting, the principle is that you should focus on the fore sight and of course the aiming mark will be out of focus, BUT, the effect of the peep sight is similar to the iris in that it will compensate approximately so that with a sharp focus on the foresight looking through the rear sight you will see the aiming mark clearly enough to shoot accurately, that is of course if your eyesight is 20/20 ish.
Sadly most of us are not so we need a correctly compensated lens to see the best sight picture. I would advise proper shooting frames which will place the lens centrally and will have a non shooting eye blinder. The correct lens prescription is a comprimise of short vision and long vision looking through a rear sight, it works out to a focal length of 2 mts as the ideal comprimise. To get that is a standard adjustment for all shooters, it is long vision prescription plus +0.5 of a diopter, so for example if you are +1.75 in your shooting eye on long vision then for you it will be a prescription of +1.75+0.5 = +2.25 diopters. If you are short sighted and have a minus prescription it will be -1.75 +0.5 = -1.25 diopters. The lens should also include an astigmatism correction which is just as important. This is a standard correction and is perfect for rifle, I would also advise doing an eye test with a shooting savvy optician but the result will be the same. There is guidance on this on the Knobloch glasses website for opticians which is a good idea to take for them. The best shooting optician I have found is Stephen Hing at Shefford near Bedford.
Why should you focus on the foresight? Several reasons, primarily as raised, it is not fossible to sharp focus on two focal lengths, the foresight is the moving one and is with in your control, the target is fixed and stationary so why focus on that. The perfect sighting technique is to focus on the foresight and flick the eye around the white gap between it and the aiming mark to judge centralisation, (another reason to not have a too small foresight ring, but that's another subject!), that will produce the best end results.
Have you ever had what you thought was a perfect sight picture and the shots were going any where? That was because you had allowed your focus to go to the aiming mark, you were looking at that, the foresight was out of focus and the white was not even but you could not see it as you were staring at a perfectly still aiming mark (still, as its fixed!), result, bad shot.
Incidently, for pistol, the correct focal length and prescription is easier, its the distance to the foresight with the pistol on aim, same principle, sharp foresight, fuzzy target.
Have fun and good shooting
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?