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Re: [OM] [OT] Why am i an idiot - what is wrong with this calculation

Subject: Re: [OM] [OT] Why am i an idiot - what is wrong with this calculation
From: HI100@xxxxxxx
Date: Fri, 17 Nov 2000 23:41:44 EST
 atk@xxxxxxxxxx writes:

<< According to these calculations an 8meg pixel camera should be
 on par with typical high (but not optimal) 35mm system. I thought the number
 was much higher -- what is wrong with my calculations ? 
>>

Alan,
        this is a topic that has been discussed a fair bit on the list and 
you could probably profitably check the archives. 

Very briefly:
1)  the digital camera vendors specify the pixel count for the (RGB) three 
color sensor pixels, used so that for an effective full color pixel the pixel 
count is reduced by a factor of 3. So a "3Megapixel" camera actually is at 
most a 1Megapixel resolution. In reality it is less than this for a number of 
reasons some of which are discussed in 2. They also try to reduce artifacts 
by interpolating pixels by averaging using nearby pixels of the same color 
this generally limits the resolution further.

2) The two samples per line of resolution is a theoretical limit ("the 
Nyquist limit") for sampling. You actually need to sample more pixels to 
resolve a line pair.
Think of this: if your sensor pixels in the "film plane" exactly line up with 
the lines you are imaging you get the correct lines resolved. *But* if you 
offset the sensor by 1/2 a pixel you get the average of half a black and half 
a white line or a medium grey - no image at all! Further if the pixels are at 
just a slightly different pitch than the image you get a picture with nothing 
but very wide dark and lighter bands. This is termed aliasing or moire. With 
seperate color pixels this can lead to even more strange artifacts. see below 
urls for an example

Bottom line you need a fair number more pixels than you might at first think.

Check the Foveon Website for some insight on color sampling issues and 
moire/aliasing:
http://www.foveon.net/interp.html
http://www.foveon.net/moire.html

                  Regards,
                 Tim Hughes
                >>Hi100@xxxxxxx<<

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