There are two aspects to getting a proper sight picture: 1) getting your relaxed focus in the right place via lens power, and 2) getting the largest possible depth of field around that focal point. Depth of field is a measure of how quickly things go out of focus if they are not at your perfect focal point, said another way: how blurry are your sight and target when you are not focused on either, but on a point in between the two.
Based on the distance to the front sight of an M14, the correct hyperfocal distance calls for a +0.50 diopter added to whatever distance prescription you use.
For depth of field, smaller is better, limited only by the amount of light coming in - which is the open area of the aperture. Most people find that a 0.040 or 0.042 is about as small as they want to go, so about 0.00125 square inches. My patented rectangular aperture is based on two findings: first is that depth of field is axial. vertical blur in an image is determined by the vertical size of the aperture. Horizontal blur is driven by the width of the aperture. We think of round apertures because our pupils are round. The second discovery is that for shooting a front post, horizontal focus is not as important as vertical focus. You have two vertical edges to the front sight, so even if they are blurry, and even if your focus drifts, you can still see the symmetry of if the bull is centered left-right. However for vertical, there is only one top horizontal edge, so if your focus drifts, the width of the blur line shifts, and your front sight looks taller/shorter. So you NEED good vertical focus. You do not need good horizontal focus.
Given that people want about 0.00125 square inches of open area, I decided to take the 0.040 round aperture and make it smaller top/bottom, so you have a 0.025 or 0.027 vertical opening, and then make it oversized left/right, to preserve the open area and brightness. So I am robbing Peter to pay Paul. I am improving focus on the more important top horizontal edge of the post by giving up on some of the less critical horizontal focus.
Testing with one of the military rifle teams, where they took 20 shots, then I replaced their sight and they took 20 more suggested that I did not give up anything in horizontal group size, but gained about 0.25 MOA in vertical dispersion. So not a huge benefit, but a real and measurable one.
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