Re: [Phys-l] FW: Science show "Humanly Impossible" on NationalGeographic channel.

["John Clement" <clement@hal-pc.org>]

OK, maybe I should have said the depth of field is better.  But since most
objects are not just flat, better depth of field equates to better sharpness
overall.  Also since the lens is much less than perfect, limiting the
aperture to the central region should enhance the sharpness.  Yes, it is
true that diffraction decreases sharpness, but I doubt that is the limiting
factor in the human eye with an imperfect non-achromatic lens.  To a certain
extent the mind sorts out the problems so you do not see the color fringes.

Actually most eyes focus differently with dilated pupils.  At night the
pupils dilate and the focus shifts nearer so that people tend to focus on
the windshield rather than the road ahead.  This is one contributing cause
for nighttime accidents.  There have been proposals for different nighttime
distance vision glasses.  So narrowed pupils give better focus.

The physics analysis of sharpness needs to be severely tempered by the
reality of the lens in the eye.  It is true that letting in a bit more light
may increase apparent sharpness by helping the person see shadow detail.

The usual textbook treatments of many practical things leave out the
important details.  Another example is color vision where the reality is
that the eye-mind system compensates for lighting so that color constancy of
objects is maintained.  The simple 3 color model is very wrong in natural
lighting situations.

I am used to using very old cameras with singlet lenses.  Larger f-stops
corresponding to smaller apertures do enhance the sharpness of the image.
The eye probably has worse optics than most Kodak cameras manufactured in
the 40s.  OK, modern well designed cameras can produce a slightly sharper
image with larger apertures, but only up to a point.

Very old cameras in the 1800s had large lenses to compensate for the slow
film speed.  From trying to bring out details in old pictures, one realizes
that the focus is very different at points in the pictures.  A sleeve in
front may be in perfect focus while the tie is slightly out of focus.  A
smaller aperture would give better detail.

John M. Clement
Houston, TX


> John Clement wrote:
>
> "Dilate means to make larger, so pupils do the opposite to sharpen vision.
> Remember the optometrist dilates the pupils to be able to look inside the
> eye.  But pupils dilate when you see something of interest which increases
> the amount of light at the same time the sharpness is decreased."
>
> Is "sharpness" here equated with angular resolution?  If so, wouldn't a
> larger pupil/aperture result in better resolution?
>
> Mike Meyer
> Lecturer/Lab Coordinator
> Michigan Tech University
> mrmeyer@mtu.edu
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
> No virus found in this incoming message.
> Checked by AVG - www.avg.com
> Version: 9.0.707 / Virus Database: 270.14.72/2511 - Release Date: 11/18/09
> 01:50:00