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Old May 12, 2006, 12:05 AM   #11
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

DocX wrote:
I had a camera that did this. it was dust on the ccd. You could clone them out in Photoshop or any other imageing program.
Those are hot pixels. It has nothing to do with dust. ;-)

Dust on a CCD is going to be a darker spot, usually more visible at smaller apertures (higher f/stop numbers) against a bright background (a sky is where you see them often).

Hot pixels are when photosites record a higher value than they should, usually with longer shutter speeds. When the problem occurs at faster shutter speeds, too, they're considered "stuck" (always bright).

Virtually all CCDs will have them in some conditions. The longer the shutter is open, the more likely a photosite won't be linear with it's fill factor, and it will record a higher value faster than it should.

You just don't see them with most newer cameras because of built in noise reduction to subtract them out on longer exposures, since the ones that occur at typical shutter speeds have already been mapped out at the factory.

With many older cameras, there is no dark frame subtraction system.

Unfortunately, sometimes with age, a CCD develops more bad pixels (ones that are not linear in the way they respond to light at typical shutter speeds). Sometimes they're too bright (hot), and sometimes they don't respond at all (dead).

Fortunately for the Original Poster, there is software to map them out in the D100 (updates a bad pixel table in EEPROM, so that the camera knows about them and replaces them with values interpolated from adjacent pixels).

With newer Nikon models, a trip back to Nikon would have been required to fix it (they don't release their service software to the public).

JimC is offline   Reply With Quote

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