Ring Sizes   #1 Rear Iris (2) A, B, and C are where your eye sits. Because of geometry, the closer your eye to the rear sight, the larger the ring will appear. It is important to find a balanced rear sight to front sight ratio to make consistent aiming and ease of focusing better. The rear iris is often adjustable to control the amount of light entering the eye. This will not dramatically change the the aforementioned ratio, but it does take off strain on the eye. The Stiles-Crawford effect: . . . . . Light that enters through the center of the pupil stimulates vision better than light entering from the sides. This means that the rear iris can act as an artificial pupil and improve clarity. Having a smaller rear aperture increases depth of field. Depth of field is all about the distance between furthest and closest points where two objects can be clearly viewed when the eye's focus is set on a certain point. For example, if you are focusing on an object 25 cm away, and you can clearly see objects from 15 to 35 cm, the depth of field is 20 cm. As you focus further and further away the depth of field increases. You can demonstrate this by holding three fingers perpendicularly to your face, one touching your nose, and focusing on the middle finger. The outside fingers are not in perfect focus. Now try moving your hand out to its limit of reach. While focusing on the middle finger, all three should be clear. For most people, focusing on an object at 20 or more feet results in a depth of field of infinity past 20 feet. In general, if you focus on an object beyond 20 ft your eye does not need to accomodate. This being said, narrowing the aperture of the eye is known to increase depth of field (i.e. squinting). Reducing the size of the rear iris has the same effect! Bonus!   #2 Aperture thickness Thickness of aperture is dependent on amount of light. In order to keep a favorable contrast, different sizes are needed. Although this can mostly be remedied by adjusting the rear iris, sometimes extreme situations call for extreme measures. (2)     #3 Aperture size This adjustment is highly variable of the individual eye, but there are several rules that always apply. Obviously, too large an aperture makes judging alignment more difficult and too small an aperture makes seeing a clear shot tricky... Specific sizes are dependent on the amount the barrel moves in a particular hold. In the following diagram the bulls are displaced equal distances and aperture sizes are varied. (2) In the first set, the bull has space to move around inside the sight ring without "squashing" into the sides, while it is still clear to see when it is centered. This is probably a good size aperture for this position. In the second set the bull touches the sight rings often. This is problematic because it makes a good shot appear rarer, increasing anxiety and the tendency to overcorrect for any imbalance. The aperture size should be increased. One problem with an aperture size too small that in bright light, light will refract around the black surfaces of the ring and bull, and make the gap between them appear greater. When the aperture is so small you should only see black, sometimes the refraction makes it look like you're seeing a perfect shot! Needless to say, this is not good for you. The third set is opposite of the second. Instead of seeing too much movement, you don't see enough. What may appear to be an okay shot is actually not. The eye has a hard time discerning what is centered and what is slightly lopsided.