P3D Re: Filters in digital cameras?

[Tom Hubin <thubin@xxxxxxxxx>]

Brian Reynolds wrote:
> 
> Dr. T wrote:
> > Why do people want to use filters in a digital camera?
> >
> > I thought all the "filtering" is done after the picture it taken.
> > That's the beauty of digital vs. film.
> >
> 
> Perhaps someone would want to use a special effects filter (e.g., a
> starburst), or a polarizer to cut down on reflections, or a graduated
> neutral density filter to balance light levels across the image, or an
> IR filter or IR blocking filter to enhance or reduce the CCD's
> sensitivity to the IR portion of the spectrum.  These are all easier
> to do before capturing the image.
> 
> --
> Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
> reynolds@xxxxxxxxx              | "Oooh!"
> http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
> NAR# 54438                      |       "Dexter's Laboratory"

Not just easier to do. In some cases post processing cannot be done. For
example, polarization data is lost in the film. You cannot filter it
after the film is exposed.

Dynamic range is another issue in all forms of communications and
recording. And make no mistake, optical imaging on film qualifies as
communications and recording. If a bright spot saturates your film you
will never know what would have been there if the bright spot had been
filtered out. That bright spot could be polarized, IR, UV, red, blue,
green, coming from a certain direction. All potentially avoidable but
only with a filter between the light sources and the film.

What George said about postprocessing the photo is equivalent to running
a radio or TV without an antenna and trying to clean up the image
afterward. You might have two signals with the same characteristics but
from different directions. A directional antenna would enhance one and
diminish the other. Post processing will not sort them out.     

Actually, a lens is really the directional antenna for your camera. The
filter is part of the antenna here. It improves signal to noise ratio in
ways that post processing cannot.

When post processing alone is sufficient we will not only eliminate the
need for filters we will also eliminate the need for lenses.  

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design


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