[tangolima]

I could be wrong as optics is not my forte. But I don't think parallax itself causes any error directly. With zero angle A, there will be no poi shift, however much is the parallax.

Consciously keeping the shooter's eye coaxial with the optics works. The black ring seen around the image in the scope must be even and symmetrical. It works but could be restrictive for certain applications. Some optics designs try to help relax such restrictions.

Angle A is the key. It depends on the shift of the shooter's eye position of course. But it is also function of the distance of the reticle to the shooter's eye. Traditional telescopic sight is not very good as the reticle is rather close to the shooter's eye, 6" or so. Reflex sight (red dot) creates the reticle (the dot) at infinite distance, making it much more tolerant to change of shooter's eye position.

Talking about parallax, it is a common understanding that it worsens with target distance. It is not totally true I'm afraid.

Let's say a scope has fixed parallax setting for 100yd (3600") and its objective lens has focal length of 6". Using the lens equation 1/f=1/u+1/v, we have the following table

u, v, parallax

10yd, 6.102", 0.092"
25yd, 6.040", 0.03"
50yd, 6.020", 0.01"
100yd, 6.010", 0"
200yd, 6.005", -0.005"
500yd, 6.002, -0.008"
1000yd, 6.001", -0.009"

The position of the target image relative to the reticle changes side at 100yd. The parallax will never grow beyond 0.01" afterwards. Parallax is more of an issue for close distances.

-TL

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