Extract of article from NRA mag on glasses & shooting below - take this & your prescripion to an optician & they will make up a lens that will fit the bill. As you read you will see a shooting lens is not a reading glass lens
For frames look up fleabay & type in Varga they do basic frames for ~£75. Or as suggested get a circular lens & basic specs then bend them so the lens is planer to the line of sight & you are looking thru the middle of it.
Good luck to the 2nd phase of your shooting career.
ShootingSight
03-24-2010, 11:52 PM
It is not an impossibility for the target and the front sight to be in reasonable focus at the same time, though it takes exact optics. What is needed is for your focal point to be in the right place, and to have sufficient depth of field.
Getting your focus to the right point is done by your eye's lens accomodating to shift focal point, or by adding a lens which shifts focal point. As you get older, you eye loses its accomodation capabilities, so you have to strain the eye muscle more and more to focus at the right spot. In your case, it is likely that your are trying to focus at the strain limit of your eye muscle, and as the muscle tires from repeated supreme exertion, it can't always hold focus to the same spot. This causes varying amounts of blur on the front sight, causing the front post to appear slightly bigger or smaller as the width of the blur line changes, causing vertical stringing.
From an optical physics perspective, you want your eye to focus at the hyperfocal distance of the front sight, which is at 2x the distance from your eye to the front sight. Assume the front sight is 25" from your eye, you want to focus at 50".
To figure out the lens strength you want to shift you there, take 50", convert it to meters 50" = 1.27 meters, then invert the meters figure: 1/1.27 = 0.79 diopters. Always round down to the nearest lens strength, in this case +0.75. If you don't wear distance glasses, this is the theoretical lens power you need. If you do wear glasses, simply add +0.75 to your distance prescription. Note, theoretical values are a good starting point, but don't account for an individual eye, so you might be one step higher or lower.
Next, you want as small an aperture as possible, without going so small the image dims. This small aperture will, just like in photography, give your eye a greater depth of field, so near and far objects are both in focus. You have seen photos of people close to the camera, and mountains in the background, where both are in good focus, despite the fact that it is theoretically impossible. This is done by setting the camera to take a picture with a small lens aperture to give good depth of field. Your rear sight does the same for your eye. The smaller the aperture, the bigger your depth of field, and the better the focus on both the sight and the target.
Net, I recommend the right lens and a small aperture.
How to do it is a question of convenience and money. My favorite is the adjustable diopter rear sight. This has a focussing lens in it, so you can twist the ring, and get the desired balance of focus btween the front sight and the target. A cheaper but less convenient option is to figure out the right lens strength, and just put that one fixed lens in the rear sight or in shooting glasses.
Next step is an adjustable rear aperture. Again, you can get cheaper fixed rear apertures, but as light goes up/down on the range, the adjustability is a nice feature.
I do not recommend colored, or polarized lenses. They rob you of light - a polarized lens blocks up to 70% of transmitted light. Colors block various amounts, depending on the color, but upwards of 30% is likely. In extreme sunlight, this might be acceptable, but if I were willing to give up 30% light, I'd rather make my aperture 30% smaller, and get the improved depth of field.
As to getting both in focus, it depends on your eye. There is a spacing of the photoreceptors in the human eye, and once the blur in an image is smaller than about a receptor diameter, it apperars in perfect focus to your brain. Further focus improvement do not help. Thus, there is a theoretical aperture size where both the front sight and target will be at the detection threshold of focus, and both will be practically in perfect focus. In reality, the image will dim unacceptably before you can get to this point, but the closer you get by getting the right lens power and the right aperture, the better the junction between the post and the target will be.
A different option is to move to a front sight ring, versus a post. While this won't fix your focus issues, at least the front sight and target will have the same amount of blur all the way around, so the blur on one side offsets the blur on the other, and you can still center the target in the sight. This will eliminate your vertical stringing.
This means I have a single vision pair for reading and a varifocal pair for distance/reading.