Why do hot pixels show up as small crosses in long-exposure photos?
Asked 4/29/2014
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When shooting long exposures on a Canon EOS 600D/T3i, I sometimes see tiny bright white cross-shaped marks in the image. They behave like hot pixels and I can find several of them across the frame, with some brighter than others.
Why do they appear as little crosses instead of single dots? Are they hot/stuck pixels, and is there any way to fix or reduce them?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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That is what a hot or stuck pixel looks like when it is so "hot" that the fully saturated signal from one pixel figures prominently into the interpolated values of surrounding pixels when the data from the sensor is demosaiced to create colors from the monochromatic luminance values captured by each pixel well on the sensor.
If it is a hot pixel the way to fix it is to allow the sensor to cool down before taking any more photos. If it is a stuck pixel (which stays on every time the camera is used, even when cool) the way to fix it is to either replace the sensor or have the manufacturer map that pixel out of the sensor's readout. Canon will do this (but the fee is several hundred dollars) for any stuck pixels on a sensor, at least among their mid/pro grade bodies. Some, but certainly not all, other manufacturers offer similar pixel mapping.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes — this is consistent with a hot or stuck pixel. The “cross” shape is not the physical shape of the sensor defect; it can appear after demosaicing, when one fully saturated pixel influences the interpolated color values of neighboring pixels.
If it only appears during long exposures or when the camera is warm, it’s likely a hot pixel. Letting the sensor cool down can reduce it.
If the same bright mark appears in every shot, even when the camera is cool, it’s more likely a stuck pixel. In that case, the usual remedies are:
- pixel remapping by the manufacturer/service center
- sensor replacement in severe cases
- cloning/healing it out in post for occasional images
So the cross-shaped appearance is normal for a very bright hot/stuck pixel after image processing, and the fix depends on whether it is temperature/exposure-related or permanent.
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