As I mentioned earlier, using the Mojo sights on the Israeli Mauser creates a problem for me with the target fading and becoming blurry. I believe this has to do with the fact that the rear aperture is a full 14-16" away from my eye. So I needed a (cheap) fix.
I have fairly decent eyesight, although I do need glasses for daily life. My numbers from a year ago:
With shooting glasses to match that prescription, I have no problem shooting Master scores, and I cannot say eyesight is to blame for not shooting High Master more consistently. However, the main difference on my Service rifle or smallbore rifle and the Mauser is that the rear aperture is only an inch or so from my eye.
The solution I found is to use an old trick of creating another pin-hole aperture closer to the eye. I accomplished this by simply punching a small (approx. 1/32"?) hole in a piece of thin cardboard, and then looking through that hole, holding the cardboard in place with a headband I already wear (for sweat, and also to hold another piece of cardboard in place that I use to occlude my non-aiming eye). Worked like a charm! The sight picture immediately cleared up, with the target returning to a clearly defined black ball.
This technique is surprisingly a "secret" many target shooters never learn about. There are a couple of manufacturers that sell adjustable or fixed apertures that are held in place with suction cups on shooting glasses, or thin films:
I must confess I read about the cardboard technique somewhere online, and other folks simply use electrical tape over their shooting glasses. What I like about my approach is that I can adjust the position of the hole easily, because I look out of slightly different positions on my glasses depending on the shooting position. Plus, it's cheap, as in free.
Many rifle and especially pistol shooters who think their eyes are too far gone to shoot with iron sights find they can still do it by using a pinhole aperture. This means they can keep shooting in divisions which do not allow scopes or red dots, or it saves the expense of having to buy those optics and modifying the firearm. Indeed, many shooters who need glasses to see but don't want to buy prescription shooting glasses can try this technique with non-prescription shooting glasses.
I don't claim to fully understand why this technique works, although I'm sure a photographer or optometrist could explain it. My basic understanding is that the aperture causes the "depth of field" to increase, whereby the lens in the eye is able to sharply focus objects at a greater range of distances. As to why this works, as best I understand it, the aperture reduces the angles at which light would otherwise hit the lens, as well as the area of the lens that actually receives light, and thus the "circle of confusion" for each visible object is reduced. The downside is that the image appears darker, so it is advisable to play with different aperture sizes for different light conditions. Basically, this same effect accounts for some of the reason why a person without their glasses can sometimes see by squinting, or making an "ok" sign with their hand and looking through the hole made by their index finger and thumb. The wikipedia articles linked give a better explanation, and really, it is not so critical to understand how it works, just that it does!