I'm new to muzzleloading - actually, I don't even do muzzle loading. I'm a Service Rifle shooter, retired mechanical engineer who studied optics at MIT, photographer, and I'm old enough my near vision went, so I studied the optics of the human eye and developed the solution for shooters. I have been doing corrective lenses for shooters for 10 years, and was recently told at Camp Perry that I need to talk to the muzzle loading community, especially since I live in Cincinnati, right next to Friendship. So ...
Definition 1: Depth of Field (DoF). Your eye only has one theoretical focal point at any particular moment, however if the width of the blur line of an 'out of focus' object is smaller than the distance between two photoreceptors on your retina, you cannot see it. Hence the theoretical focal 'point' is actually a range including some distances closer than the focal point, and some distances further than the focal point. How big your DoF is, is determined by the size of the aperture you are looking through.
Definition 2: Aperture. The smallest opening in your optical path. Within reason, does not matter where it is. It might be your pupil, it might be an aperture sight on the rifle, or it could be a sticker with a small hole drilled in it that you have stuck onto your eyeglasses. When people find that they can focus better with more light, it is not actually because of the light, it is because the increased light makes the pupil in your eye constrict, improving your eye's natural depth of field.
A sight picture needs two things: for your eye to have a good depth of field, and your point of focus must be at a distance so the depth of field is centered between your sights and the target (referred to as the hyperfocal distance in photography). Ideally, your depth of field is big enough that the sights are in the near edge of your depth of field, at the same time as your target is in the far edge of your depth of field.
The relaxed human eye focuses at infinity. You exert the ciliary muscle in your eye to squeeze the lens, which brings your focal point closer. You relax the muscle, and the lens' elasticity restores focus to infinity. At around age 40, the lens loses it's elasticity, and the muscle has to struggle harder to focus up close. For really close, you cannot do it, for medium close, you can do it for a few seconds, and the muscle tires and fades. Another way to bring your focus in closer is to add a positive diopter lens in front of your eye, aka reading glasses or a bifocal if you have distance correction. Lens power relates to focal distance, reading glasses are MUCH too strong to shoot with.
Seeing the front sight is a 'middle distance' where shooters struggle. With a rear aperture, the correct answer on a rifle is to add +0.50 diopters of lens. Weaker than reading glasses. With pistol, where the sights are closer, you want to add +0.75 to +1.00 (depending on how blurry you like the target. I have never experimented with buckhorn sights, but imagine it will be at least +1.00. If someone can tell me the distance from your eye to the buckhorn, I can run the math and give a quick estimate.
Bottom line, to see your sights like you did when you were 18, you need to use a +0.50 lens, and an aperture sight, or put a sticker with a small hole on your glasses (like eye pal, or make it yourself).
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