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I would like to suggest everyone who will be having their eyes examined in the future to request their eye doctors to show you the RED-GREEN test, also known as the duochrome test. This test may give you an understanding of how colors work within the human eye.
Some doctors may routinely use this test to confirm and finalize your lens prescription. The line of letters is shown half with a red background color and half with a green background color. When your final lens prescription is exact, you will see the line of letters with equal sharpness on both the red and green sides. If a slight plus lens is used, the letters on the RED side become clearer while the green side becomes blurred. Conversely, if a slight minus lens is used, the letters on the green side now become clearer while the red side becomes blurred. Different colors are focused differently within our eyes.
If we now apply this principle to our red dot scopes, I believe this explains why the red dot is focused at a different position or plane from the target. We see colors of different wavelengths at different positions in relationship to our retina. Red does not focus onto our retina the same as other colors. I have found that although many of my patients are sensitive to this red-green test and give good responses, many cannot see any difference at all. This may explain why some see the red dot at the same plane as the bull while others see the red dot closer, and it has nothing to do with being nearsighted or farsighted.
My intuition tells me that there was never any manufacturing designs for the dot to be at any specific distance, but because of color wavelength properties, the red dot may not be seen in the same plane as the target. A picture is worth a thousand words, so have your eye doctors show you this red-green test, which then may make you a believer rather than relying on my words.
Good Vision and Good Shooting To All,
Norman H. Wong, O.D.