Values indicate the number of effective megapixels (or in some cases also the effective imaging area) for a picture aspect ratio of 3:2 (Canon standard), if not stated otherwise (i.e. Desert Star, Kodak).
Pixel size is one major factor to influence the amount of noise. The more pixels are put onto a sensor, the smaller they get and the more light per surface they have to convert. The less light per surface they need to convert, the lower the necessary noise. Thus a sensor with smaller amount of MPix with a bigger surface area per unit and otherwise comparable stage within technological evolution and features** might probably show much less noise than sensors with a high number of MPix with a small surface area.
** Other important factors for noise are for instance pixel pitch (spacing between the pixels - bigger is better again) and cooling, wherefore professional astro mods usually are deep-cooled.
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