When I was into astronomy, I quickly learned about what is called the "exit pupil" of an eyepiece. The reason that this is important is because if the exit pupil is larger than the diameter of the open iris, then light is wasted in the periphery. In the course of learning this, I learned that at about the age of 35 the ability of the iris to dilate becomes compromised. A 25 year old has a dilation of around 7mm. A 45 year old has a dilation of about 5 mm. This is what results in "night blindness" for many people. There has to be the right combination of rods and cones but also enough light to see with. M.K.
Posted on Apr 12, 2012, 6:47 AM from IP address 18.104.22.168