Today I'm going to talk about hot/dead pixels, those annoying pixels that show up typically on long exposures (starting from around 1 second), one the unwanted effect under the "digital noise" for long exposures and high ISO; you can see below in the image that blue of the sky is not a even color and it seems grainy, that is the digital noise.


First, some interesting information about the topic, then a very interesting trick to remove many hot pixels in Canon DLSRs, that employ CMOS sensors.

Let's start with some interesting theory.

The hot/dead pixel test is done shooting at a completely dark scene, typically with the lens cap on and disabling any noise reduction (unless you want to hide the problem).

Usually, we should call a pixel "hot" when it is bright instead of being dark, and "dead" when they are abnormally dark in a bright scene - this is much more rare than hot pixels. Others call "dead" pixels those that are very hot, i.e. thay show up also when the conditions are not triggering this defect - see below what the factors are.

Usually the brightness of a pixel is measured in a 0-255 scale, so more than 240 brightness for a pixel where the scene should be dark is the sign that pixel is hot.

Factors that stimulate the exhibition of hot pixels (when all factors are at maximum, you can even see tens of hot pixels):

1) exposure time: the longer the exposure, the more time for the photodiodes (the pixels on the camera sensor) to accumulate noise thus more prone to become hot;

2) hgh ISO: more sensitivity means being more sensitive to unwanted signals;

3) temperature: a warmer camera, thus a warmer sensor, is always noisier than a cooler one;

4) aging, since the sensor normally develops hot pixels during its lifetime.

Noise reduction and Dark frame subtraction technique is typically employed but of course the best is not to have them. Dark frame subtraction means calculates the difference between the photo to be corrected (because it has hot pixels) and a photo of same camera in complete dark: if a pixel is hot, it will show up bright in both photos so, calculating the difference, the bad behaving pixel will become dark. It is used for astrophotography for example, where long exposure is the norm.
Now the trick...

In Canon DSLRs cameras, employing a CMOS sensor, there is a very interesting trick to remove a lot of hot pixels (at least it worked on several cases): taking a very long shoot (at least 30 secs) with the lens cap on very often eliminates them.

Most probably, with no light for a long time when activated to take a shoot (so some current is passing), the bad/stuck pixels something like cool down electrically! Try yourself, it's worthy, I tried myself on my Rebel XTi tried and worked!


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