Unfortunately, optical physics and old wives tales are difficult to tell apart.
Old eyes needing more light is plain wrong. There are a few medical conditions where it is true, but they are rare. The true answer is that a smaller aperture will give you a better depth of field, so you want as small as you can. The limitation is when the image starts to go dim because of inadequate light. You judge focus both based on blur, and on contrast difference, so if you start to get the image dim, it might be in focus, but without the contrast it is difficult to see. THe myth about old eyes needing more light comes from reading menus in dim restaurants. Your brain learns that if you turn up the lights, you can read better, and so you believe more light is better focus. In reality, what is happening is that more light makes your pupil constrict, thereby giving your eye a greater depth of field. In shooting, you don't have the biological reaction of aperture size varying based on light, so you can manually set the aperture smaller.
Optically, there are two elements you want: a big depth of field, which you get with a small aperture, and you also want a relaxed focal point which centers that depth of field between the front sight and the target. This is accomplished by adding +0.50 diopters to any distance correction you have. If you have perfect 20/20 vision, all you need is a +0.50 lens. If you have a distance correction, even a slight one, you take that lens value and add +0.50 to it.
Gehmann and Centra both make rear iris with adjustable focus. I'm on the fence about these. I love them, because you can just dial them in while looking through them, however it is not an adjustment that should change frequently, and once you know the optical math to calculate what you need, you may as well just buy the correct lens and be done with it. So I see them as an added complexity and cost that often is not needed. Also, they typically only correct spherical errors in your eye, not astigmatism errors (I think Gehmann made one that did astigmatic corrections, but it was a beast with 4 lenses that you needed to dial and orient .... you needed a degree in optics just to follow the instructions).
All lenses will both shift your focal point, and will magnify somewhat, so trying to define a lens as a magnifying lens, versus a focal adjustment lens is really one and the same thing. How much of each a lens does is driven by lens power, and distance of the lens from your eye. Eagle eyes are +0.25 diopter and +0.50 diopter lenses that are put in the front sight. From a focus perspective, they are a poor solution - they are too close to the front sight to help you see it any better, and they will blur the target, so you see that worse as well, however their distance from the eye means they magnify about 150%, and a lot of people find this makes it easier to see the number boards.
How you execute the lens has options. Adding it to the contact should work, but if it causes dry eye, that's not good. I make a lens holder that can add the lens into the rear sight, or you can get shooting glasses with the prescription dialled in. Glasses are the least favorite, as you need to look through the lens at a strong diagonal in prone, and that introduces astigmatic errors in the lens.
Aperture distance to your eye does not really make much difference so I would not worry about that - much better to get it so you are comfortable with a consistent cheek weld.
Art Neergaard
ShootingSight llc
Statistics: Posted by ShootingSight — Sat Aug 08, 2015 4:19 am