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Old Oct 12, 2004, 1:55 PM   #1
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Canon EOS 20D Sample Image Quality Concern

I noticed a strange pixel area when looking closely at the orangutan poster between the eye and ear from your preproduction camera's long exposure "Can D20 img_9956.jpg." What is the reason for this?
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Old Oct 12, 2004, 3:40 PM   #2
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You are seeing a hot pixel, which is common on long exposures.

The Canon EOS-20D does have a long exposure noise reduction system that can be turned on to interpolate them out of the image. But, I don't know if it was enabled for these test shots or not.

Even if it was enabled, these systems can't always find all of them. The longer the exposure, the greater the chance the noise reduction will leave some.

They work by taking two photos. The first photo is of the scene you are shooting. The second photo isa "black frame" photo, using the same exposure settings that the real photo was using. Then, the camera's software looks for hot pixels in the black frame and tracks their location. It then masks out the same pixel locations in the real photo by using values interpolated from surrounding pixels.

It works because two photos taken in rapid succession using the same settings, usually have the hot pixels appear in the same locations. So, it knows where to look for them in the real photo, by where they appeared in the black frame.

But, it can't always be 100% accurate (you may get hot pixels in the real exposure, that don't appear in the black frame exposure, and vice-versa). This is known as "black frame subtraction" noise reduction.

See my post in this thread for more information on hot pixels, dead pixels, stuck pixels, and dark frame noise reduction:


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