Dark-adapted pupil diameter as a function of age medical study (1 Viewer)

Ed,
My thoughts about magnification as a function of object distance was a response to a question you asked, deep in the bowels of the discussion on the Zeiss HT. My post was #1750!
I'll just paste it right here.


Ed,
I don't know of a special treatment of the focusing lens, but I am certainly not into it enough to think that one doesn't exit. The focusing lens can be either positive or negative, and common binoculars use both. I hope you won't mind some random thoughts.

I get a lot of mileage out of a couple of simple page-1 type forumlas which I'm sure you are familiar with. Where f is focal length and o and i are object and image distances,
there is the "lens equation",
1/f = 1/o + 1/i
and one for linear magnification of a lens (image height/object height),
m=f/(f-o)
Then there's also the fact that combined power (inverse focal length) of stacked lenses is the sum of the individual powers.

Although these are strictly correct only for "thin" lenses, which the modern binocular certainly isn't, conceptually they are useful for figuring out how things generally work, with a little boldness, which is to say, no worries if it's not quite right!

By the third relation, the power of the objective is not changed by the motion of the focusing lens. Therefore, what is being changed is the effective location of the origin of focusing. Moving the focusing lens is equivalent to moving the entire objective back and forth. For various object distances, the image can be made to fall at the field stop of the eyepiece.

The variation in magnification then can be seen to result purely from the variation in the object distance, as described by the second equation.

The next step I think would be gravely major: get a copy of ZEMAX, get the precise optical layout, and go to town on a genuine ray trace.

To qualify my wisdom, I must admit to having taken optics as a sophomore in 1969, but had usually drank so much beer the night before that I was kind of in a daze during most of the classes. I actually absorbed the above information, Snell's law of refraction at a boundary, and the basic bit about thin film refraction and reflection (how coatings work) but buddy that's "IT". Just enough to make me a prime victim for consumer optics!

Ron

Ed again,

I'm breaking my response into two parts because that previous one was pretty busy already.

Yes I see your point that when suddenly going from a dim to bright surrounding, the pupil does contract providing up to about a factor of 10 dimunition in light entering. But, the difference in the dim to bright illumination can, in real world conditions be as much as (sleep shades on in the tent, out into the noonday Sahara) can be 7 orders of magnitude, or a million times greater. Sure a factor of ten helps, and I'll gladly take it under that circumstance. But as you say, changes in retinal adaptation are the lion's share of the eye's response to large changes in ambient lighting.

That little factor of ten just seems so small, that I figured the maximization of acuity was probably the greatest result of pupil variation. But OK, the variation in acuity is not real major either, is it?

Of course it's impossible to say "why" the pupil does what it does. But we have both pointed out reasons that things work out nicely. Thanks for your comment.

Ron

ronh said:

That little factor of ten just seems so small, that I figured the maximization of acuity was probably the greatest result of pupil variation. But OK, the variation in acuity is not real major either, is it?

Ron

Click to expand...

Ron,

Visual acuity does change significantly with light level.

As far as I've been able to glean it looks like most of the pupillary response occurs between 300cd/m2 and about 10 fold lower light levels or approximately corresponding to the change in pupil area. The changes in acuity in this photopic range appears to be mostly accounted for by progressive optical aberrations at wider diameters. How much very much depends on the individual's lens and cornea but somewhere around 50% reduction in acuity appears to be common though probably gets worse as we get older.

I'm sure there must be some retinal modulation within this ten fold range but the majority I've seen described is for much higher and lower light levels.

Koenig's classic study on acuity and light levels can be found about 40% down this page.
http://webvision.med.utah.edu/book/part-viii-gabac-receptors/visual-acuity/

On page 22 of Gijs' report the same data has been transposed into more familiar units.
http://www.houseofoutdoor.com/testr..._AND_CONTRAST_IN_BINOCULAR-IMAGES_highres.pdf

David

. Hi Ron,
yes, I find this discussion interesting.
In the real world illumination values do change by seven orders of magnitude, but this might be 10 million rather than 1 million times.

The protection against very bright light uses several factors.
Yes, pupil size. Also one's eyebrows against a high sun. also the forehead. And averting one's vision. And maybe even squinting.

Incidentally, regarding pupil size. I find that if I am using a binocular at a difficult angle up or down, the exit pupil is often reduced by 50% by one's eyelids or at least my eyelids. This is easily seen with an out of focus artificial star. it can be particularly acute with large pupil size in low light.

I also did some optics about the same time as you but unfortunately didn't consume much beer.

There is actually a relationship between alcohol and bright sunshine in that our natural protection instincts reduce if we are drunk and this is the cause of quite a bit of damage due to direct sunlight.

Hi Ron,

Well it's safe to say you didn't acquire a copy of ZEMAX. It's pretty expensive software.

I don't mean to take this too far, but it might be worth reviewing this Wiki article about Light/Dark Adaptation. Think of the retina as a photo sensor array that can change its ISO sensitivity over an enormous range, well beyond what any modern camera can do. What the article doesn't mention, nor do many others, is that this adaptation is occurring all the time. The visual system does not just seek out and stop at the photopic and scotopic extremes, although one may be inclined to think about it that way. The only reason we know more about the extremes is because they are easier to configure in a laboratory. When we're driving at night, however, our eyes are in an in-between state called mesotopic vision.

Hope you don't mind me pointing this out, but that's what I was referring to above.

Ed

Ed,

Thanks indeed, I will read that.

Typo's estimate suggests that in most cases the acuity variation with pupil diameter, which I was trying to make a big deal of, is in fact far less than the light regulating effect.

Ron