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Re: [OM] Nope, It missed it by few hundred miles (aliasing)

Subject: Re: [OM] Nope, It missed it by few hundred miles (aliasing)
From: Joe Gwinn <joegwinn@xxxxxxxxxxx>
Date: Mon, 22 Sep 2003 22:02:52 -0400
At 3:05 PM +0000 9/22/03, olympus-digest wrote:
>Date: Mon, 22 Sep 2003 07:26:47 -0400 (EDT)
>From: "Boris Grigorov" <alienspecimen@xxxxxxxxxx>
>Subject: Re: [OM] Nope, It missed it by few hundred miles
>
>Moose wrote:
>Just for the record, these are not filters in the sense that you mean. 
>Nobody is trying to take away your spectrum. I have no idea how well or 
>poorly digital sensors respond to IR or UV, but, whatever the effects of 
>these filters on the transmission of IR or UV, they are secondary to the 
>primary purpose of limiting light that oscillates between light and dark 
>at a high enough frequency to cause aliasing effects. 

I think Moose meant by "oscillates between light and dark" the spatial 
frequency, measured in cycles per millimeter on the film surface.

Most digital sensors respond well to IR light out to about 1000 nanometers 
(green light is  about 500 nanometers, red light is about 600 nm wavelength), 
unless the imager has a built-in filter.  Response to the UV isn't usually all 
that great, unless one has bought a special UV-enhanced CCD.


>Me writes:
>As far as I know and I am in no means an expert, aliasing occurs when you 
>oversample.  

Actually, aliasing happens only when one undersamples (samples are too sparsely 
spaced).  Oversampling (samples close together) is OK, and is sometimes done to 
make the electronics simpler, but it's a long story.


>The broader the spectrum, the harder to control.  All CCD's are sensitive to 
>IR and UV, but this is cut off by a filter in all professional 
>cameras...except for the Dima*ge 7, but note that this feature was "fixed" in 
>the subsequent models at no cost.

The optical spectrum (color of light) has no effect on aliasing, if that was 
the issue.


>How much would it cost to implement a variable filter? Five hundred per body?  
>Would that be a revolutionary camera?

Variable wavelength filters do exist, but are larger and costlier than the 
camera.


Joe Gwinn


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