The two cameras are a Canon A550 7.1MP compact digital with a sensor size a typically small 4x6mm and a Pentax K100D DSLR 6MP with a sensor size 24x15mm — some 15 times larger, even though it is 1MP less than the Canon compact.
Both cameas were aimed at a star field in the night sky set at maximmum MP (7.1 and 6 respectively) and sensitivity 800 ISO.
How do they perform? Would you expect the Canon at 7.1MP to be better than the Pentax 6MP — not so!
Full-frame Canon Powershot (compact). You can easily see the noisy (grainy) sky backgound. (Click on image to enlarge).
Full-frame Pentax K100D DSLR, Background noise is much less and this is 1MP LESS than the Canon compact. (Click on image to enlarge).
Full res crop from the Canon compact shows plenty of noise. Even though the sensor is 7.1MP, the pixel cells are 15 times smaller and subject to noise.
Full-res crop of same starfield area with the Pentax DSLR. Pixel cells are fifteen times larger so they have a higher photon efficiency and therefore less noise. In addition resolution is far superior showing fainter stars. Also the lens on a DSLR typically has a physically larger apperture which admits far more photons per pixel, further reducing noise.
So, a DSLR at 6MP beats hands down a compact 7.1MP. It's the larger sensor size and large physical lens aperture = more photons = less noise. That is, it wins on the basis of the laws of optics, even before electronic processing comes into play.
Further comparisons in daylight forthcoming.
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