digital cameras

From: Dwight Elvey <elvey_at_hal.com>
Date: Tue Jul 13 21:19:25 1999

ard_at_p850ug1.demon.co.uk (Tony Duell) wrote:
> > It is determined by the f/ratio
> > and the color that you are trying to focus. It is a wave property
> > of light. A point sourse of light can only be focused to certain
> > disk size. The Airy disk is defined as 1.22 * wave_length * f_ratio.
>
> For a simple pinhole (no lens, but I can't believe it varies in a
> different way), I seem to remember 1.22*wavelength/diameter.

 The equation you remember is 1.22*Wavelength*FocalLength/Diameter.
I just took the FocalLength/Diameter and replace it with the
f/ratio. I forgot to say that the Airy disk was the radius.

>
> Most camera lenses do become unsharp at very small apertures.

Oops, I meant to say that the Airy disk grows in size as
you stop down. Don't know what I was thinking. I was
looking right at the formula and stated incorrectly.
 I should have said:
As you stop down, you get better depth of field because
the lens looks more like a pin hole but the Airy disk
size increase because the diameter of the aperture is
smaller.
 Thanks for the correction.
Dwight

PS
 It is interesting that there was a significant discovery made
just recently about light going through small holes. This experimenter
made a large number of small holes in a thin foil. The amount of
light that got through the foil was several times more than the
amount of light predicted by just the areas of the holes. It seems
that the light was funneled into the holes and expanded on the other
side. The holes were a few wavelength in diameter and the experimenter
made about a million of them.
Received on Tue Jul 13 1999 - 21:19:25 BST