Dealing with Hot Pixels in Digital Photography



Hot pixels, also referred to as dead, stuck, or defective, are those undesireable artifacts from a malfunctioning image sensor element (pixel). The following image, a full resolution crop, shows 2 such hot pixels. They are most annoying on somewhat dark photos such as this one. All images on this page were taken with a Nikon D100.




Notice that when the photo is re-sized (re-sampled) to 25%, the hot pixels are there but not easily noticed. The image was sharpened after re-size which makes the hot pixels more apparent.




The following image show the same two hot pixels magnified 16 times, the top being orginal exposure and the bottom being 2.5 midtones levels adjusted in Photoshop. Note that in-camera JPEG compression exasperates the problem somewhat.




The following images show the same two pixel locations but captured under a dark frame environment (lens cap on in a darkened room) at 1/10 second exposure at ISO 200. The imaged was originally stored as an un-compressed TIFF. The top pair is normal exposure, the bottom pair is +5.0 midtones levels adjusted.

I intend to determine if the in-camera mapping out of the central hottest pixels would reduce/eliminate the neighboring ones and whether this would be due to in-camera processing or as part of the inherant functioning of the CCD itself.




The use of a dark frame to overlay the original, using the "Difference" (subtraction) blending change layer attribute, helps somewhat. Dark frames should be taken close to the original exposure for best results, this time it was not.




highly amplified (levels) dark frame (1/4 size) at 1/10 sec. exposure shows just how much "noise" exists in a digital image. Normally, this noise has very little if no impact on picture quality.




highly amplified (levels) dark frame (1/4 size) at 1 sec. exposure. It is interesting how in-camera noise reduction removed many of the brightest hot pixels, suggesting that a degree of noise reduction applied to all images could have the effect of elimating the high noise pixels that do affect the visible quality of a photo.




highly amplified (levels) dark frame (1/4 size) at 10 sec. exposure. The increased noise at upper left is due to warm electronics in the camera near that corner of the CCD sensor which is picking up IR "data".




Below is a false color image depicting all 3 of the above frames, the red being the 10 sec. exposure, the green being the 1 sec. exposure, and the blue being the 1/10 sec. exposure. Again, this is an exaggeration and not meant to suggest that final processed images will contain this much noise. It does, however, demonstrate how in-camera processing affects images of different exposures differently.




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