Ted Bennett <[email hidden]> writes:
Normal attribution styles would make reading your posts much simpler.
Quoted message said:Yes, that is why the variation in pupil size is more for focusing than for light control. As you
say, the sensitive range of the retina is several orders of magnitude greater than the light
variation due to pupil size.
For those who are unfamiliar with the visual system, here some details.
It takes roughly 40 minutes for a normal human retina to be fully dark adapted when transitioning
abruptly from bright light to darkness. Interestingly enough, that's roughly how long it takes for
twilight to deepen into night. Dark adapted vision is about 1,000 times more sensitive to light than
light adapted vision. This is the adaptation at the retina that Ted refers to, as he says it is a
much greater range than can be achieved by opening the iris; indeed, the iris probably has more of a
protective function by narrowing the pupil to protect the eye from too much light rather than from
opening to see better in low light situations.
Rods in the retina are responsible for vision in low-light situations; they are most sensitive to
light of a wavelength of about 505 nm (green, although the rods do not register the color). Cones,
responsible for color vision, are most sensitive to light of about 555 nm (yellow). Cones are
primarily part of our color day vision, adapted to normal sunlight.
Quoted message said:Pupil size certainly does change with accommodation, however, and I think it is significant. Under
constant lighting conditions, the pupil contracts as the eye accommodates, or shortens its focal
length, and this is readily observable. The benefit is the same as with any variable aperture: a
greater depth of focus is obtained with a smaller aperture.
While this makes sense from what we know about camera optics, I can't find anything to support this
in any of the sources I've got on my bookshelf. Do you have a reference?
I found this interesting: from Gibson (1966, p. 172):
"Accommodation can be achieved by either moving the lense toward or away from the retina, as in
fish, or by changing its curvature, as it is in mammals... As Walls (1942) points out, "the small
eye needs no accommodation at all" and not all vertebrate eyes accommodate for distance. The
comparison of eyes with cameras or projectors is misleading, and the investigators of human vision,
where the eye is not very large, have overstated the importance of focusing the lens, as if it were
like the focusing of a picture on a screen. The human eye loses most of its power of accommodation
at the age of 50 or 60, but if it was normal to begin with it remains a fairly efficient organ."
Which must be whey I wear bifocals. ;-) But then my eyes weren't normal to begin with (myopia).
