Using lens math from photography, you can calculate the offset in lens power between perfect distance vision, and perfect shooting vision.
Your eye has one theoretically perfect focal point, if you take an object and move it away (or closer) than the perfect focal point, it will develop a line of blur around the edges. However, because the limit in resolution of the human eye is about 1MOA, if you shift an object’s distance such that the blur around the edges remains less than 1 MOA wide - your eye cannot see the blur, and to your brain the object stays in perfect focus.
So while your eye has only one theoretical focal point, there is actually a range, from somewhere closer to your focal point, to somewhere beyond your focal point, where the object appears to be in perfect focus. This is called your depth of field.
As a shooter, what you ideally want is to set your focal point such that your depth of field is centered between the front sight and the target, so the front sight falls just inside the near edge of your depth of field, while the target falls just inside the far edge of your depth of field, and you can see both clearly at the same time to aim. This magical distance that exactly centers the focal point is known in photography as the hyperfocal distance, and we know how to calculate it very precisely.
The problem older shooters face is that the focal power of the lens in your eye is variable, driven by the Ciliary muscle in your eye. When the muscle relaxes, your lens is low power and focus goes to distance, when you exert the muscle, it deforms the lens so it adds power to the lens, and your focus comes in close. Unfortunately, past about 40 years old, the lens in the eye gets hard, and you lose this adjustability range – so much so that bringing in your focal point to be able to see and hold the front sight is beyond what the eye muscle can deliver. Or at best, the eye can do it for 1-2 seconds before the muscle is overly fatigued, and it starts to give up, so you can see the front sight fade if you take more than a few seconds to break the shot. So this is the problem we are trying to solve in shooting – how to allow the eye to focus at the hyperfocal distance without straining the eye muscle.
Luckily, there is an easy solution. Lenses are additive; if you stack one lens on top of another, their powers add. So another way you can add power to your eye’s lens, without exerting the muscle, is to place a positive power lens in front of your eye. This will shift your relaxed focal point from being in the distance, to being closer in. The stronger the power of the lens, the closer it shifts your focal point. So if we know the hyperfocal distance for a rifle, we can calculate the necessary lens power to shift our focal point from infinity to this hyperfocal distance.
Without actually going through the math, the answer is that for a long rifle, M1 or M14, you need to add a +0.5 diopter lens to focus. For an AR or pistol, which have shorter sight radius, you need to add +0.75.
Unfortunately, the easiest lenses to come by are reading glasses, and these are all too strong. The lowest power is usually +1.25. This lens is powerful enough to bring your focal point back to arm’s length, for reading. But for shooting, the focal point is so close that the target is completely fuzzy.
This is why I have worked with a safety glasses company to make shooting glasses with the correct powers molded in. If you have good distance vision, and just need cheaters for reading, these are the glasses for you. If you do need distance correction, but wear contacts, these glasses will also work for you.
If you have distance vision correction and wear glasses, the answer is to talk to your doc (or order glasses on-line at places like Zenni Optical), and get a pair of glasses that have your distance vision prescription in them with either +0.5 or +0.75 added to the lens strength.
With these glasses you will be able see the front sight as well as you did when you were 18.
