No, “sensor cleaning” doesn’t actually clean the sensor. In front of the actual sensor are a couple of filters. These handle things like UV/IR blocking and there’s also a low-pass filter (handles anti-aliasing to reduce moire in digital images).
Any dust that would accumulate will accumulate on the front-most filter. The camera uses a piezoelectric effect to vibrate the filter and shake the dust free. The idea is that you’d point the camera at the floor (with lens removed) and manually invoke a sensor cleaning cycle to (hopefully) shake loose whatever dust is sticking to the filter. They refer to this as “sensor” cleaning but it’s really “filter” cleaning.
Dust is easily spotted in images when you shoot at very high f-stops (e.g. f/16) although it can show up at lower f-stops but it will be harder to notice. If you ever suspect dust, just shoot a photo at a very high f-stop of something with no contrast such as a plain blue sky or a plain white wall or ceiling (no need to focus).
Stuck pixels or hot pixels would be on the sensor itself. This can also easily be confused for “noise” (and I suppose it is a kind of noise). Loads of things contribute to noise... heat, quantum effects, etc. but the biggest contributor is usually something called “read noise”. It’s usually random but there is pattern noise as well. Noise is inherent in every digital sensor. At the camera’s base ISO, noise typically isn’t noticeable. It’s when you have to apply “gain” (amplify the values read off the sensor) that you start to notice it.
Basically, noise is a result of inadequate exposure... with “exposure” being controlled via shutter speed and aperture but NOT by ISO (ISO is not technically part of “exposure” despite the huge numbers of articles that make claims to the contrary. An easy way to recognize this is that when you boost ISO, the ISO gain isn’t actually applied until AFTER the shutter closes and the image is captured.). But at that point, you’ve also already captured whatever noise you’re going to get as well. So when you apply ISO gain, you boost both the signal you want and the noise you don’t want. If you shot at base ISO and capture a good exposure, you already have all the signal you want and there’s no need to amplify anything... so you don’t boost the noise.
If you shoot in low-light and compensate by using high ISO, you’re going to notice more “noise” in the image ... this is normal.
Tim Campbell
5D III, 5D IV, 60Da
