Re: Noise levels as a function of pixel size

[A complimentary Cc of this posting was sent to
Roger N. Clark (change username to rnclark)
<username@xxxxxxxxx>], who wrote in article <43AA38D4.5080704@xxxxxxxxx>:
> > focus plane. The sensor which will collect ALL the information
> > available in the focal plane should have the step 1/2 of this. This gives
> >
> > lambda/(4*sin(phi)*n)

> There are several factors that are being ignored in this discussion.

Sure. Ignoring the irrelevant factors, and keeping all the relevant
is the key of the "scientific approach".

> 1) To do color photography, you need to focus the visible spectrum
> onto the pixel. That means from about 0.38 to 0.7 microns.

The estimates are for capturing the information at 0.55 microns. They
will, or course, work for longer wavelengths than this. At shorter
wavelength some information is lost; however, note that lenses usually
have a very sharp decrease of performance near the violet (this is the
reason for purple fringe); so maybe the information is not there in
the first place ;-).

> 2) You can't use pixel sizes less than the wavelength of light
> (0.7 microns for color photography). The equations for diffraction
> spot size you have been using are for conditions resulting in larger
> than the wavelength of light for the diffraction disk. When you
> approach the wavelength of light (this means above the wavelength),
> more complex equations will apply.

I'm afraid you do not know what you are talking about. It is the
Maxwell equation; it works for any scale (well, until about 1e-16 cm,
when quantum fields effects appear).

Or do you know some better equations?

> 2a) As pixel size becomes smaller, other problems arise in the
> sensor, including leakage of electrons into adjacent wells.

Sure. This should be taken into account. Actually, this affects MTF,
so it IS already taken into account...

Smaller pixels have some technical problems; larger pixels have some
other technical problems. Both are resolvable,

> 3) You state that you can get these fast lenses with great performance
> in the small form factor. This may be true for monochromatic
> wavelengths, but fast f/ratio lenses in the small form factor
> notoriously have poor chromatic aberration.

Chromatic aberration is visible on resolution charts. Please try to
find some for the lens I investigated (one of KM A200). [Do not know
about other lenses for small format.] DPreview shots show MUCH
less visible chromatic aberration with this lens than with 35mm primes.

Hope this helps,
Ilya
.

Relevant Pages

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