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Old Jan 31, 2003, 9:50 PM   #1
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Default Camera tests

Did some test shooting and am going to try to explain this graph:

I shot the same gray card with constant light, cropped out a 200x200 chunk of the middle of each one, and used Max Lyons’ Thumber’s (http://www.tawbaware.com/index.html - shareware) Noise Analysis Report to measure the average RGB value with all the f/stops and shutter speed available to me (I dropped many that stuck at zero or 256 from the graph). The X-axis zero value is the exposure the in-camera light meter read as the proper exposure.

I suspect that much of the “scatter” is due to the camera not reporting the real value of the f/stop. In any case it is less than plus/minus half a stop.

I think that graph shows why tending toward underexposure is a good idea with a digicam – or at least with mine. At plus three stops, the image has gone to a pure white – blown out. Though it is tailing toward zero, at minus four stops, there is still some variability left. Blowout (256 – pure white) happens abruptly while blocking (zero – pure black) happens gradually.

The graph clearly shows why it is possible to dig an image, albeit noisy, out of an underexposed image while an overexposed one is simply gone.

The newly announced Fuji CCD (http://www.steves-digicams.com/pr/fuji_01222003_pr.html here at Steve’s and http://www.dpreview.com/news/0301/03...superccdsr.asp at dpReview) looks like it is designed to deal with the issue of the sharp “knee” to blowout. Should be interesting to see in a real camera.
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