infint

Apologies. In considering my reply, I belatedly realised that I made a mistake. Not counting artificial mechanisms like sunglasses, the pupil is the only available means by which the eye can adjust the amount of light striking the retina. As worded, the statement is correct. The problem is that everybody seems to accept that the pupil is the main means by which the eye adapts to different light levels and that's the idea I want to challenge. Perhaps I am now "preaching to the converted" but I will present my evidence anyway. Experiment 1. I stand in front of a mirror in a darkened room and use a small torch to adjust light levels. My pupils barely react. At most, the change in diameter is about 2:1. I can just about read a book in bright moonlight and I can read the label written on the glass of a working 40W light bulb. There is no way that the pupil could adjust for that variation in light level. Experiment 2. I use my smartphone camera to take a picture of a working wall light and it doesn't look anything like what I can see with my eyes. Either the surroundings are too dark or the light is too bright. I say that this is because the camera can only adjust the whole image while my eyes can adjust on a cell by cell basis and it's the individual cells of the retina that do almost all of the adjustment for different light levels. I also understand that the pupil dilates or constricts in response to pain, drugs, disease, interest and imagination.

I believe that these statements are true - "When the pupil dilates, more light enters the eye." "When the pupil constricts, less light enters the eye." "In bright light conditions, the pupil constricts a little." "In dim light conditions, the pupil dilates a little." and that this statement is demonstrably and logically false "The iris/pupil controls/regulates the amount of light entering the eye." When I search the internet, I find variations of that last statement everywhere and various doctors and opticians also seem to agree and yet, I can prove to my own satisfaction that it is false. It does depend on what you mean by the words "control" and "regulate". Before I describe the experiments that "prove" my case, I would like to know if anyone else already agrees with me.