Different types of digital camera use different sized sensors. Ignoring specialised studio cameras, the largest are in full-frame digital SLRs, the next largest are in APS-C format SLRs and Four Thirds cameras, while the smallest are in compact cameras.
The physical size of a sensor is now more important than its megapixel rating. The tiny sensors in compact cameras produce much lower quality, even though they may have the same megapixel rating as a digital SLR.
There are two main reasons:
- The image has to be enlarged by much more to produce a same-sized print etc. It’s like the difference between small negatives and big negatives in the days of film.
- The tiny pixels/photosites on small sensors produce much more digital noise, so the makers have to build in strong noise reduction processes. Both the noise and the noise reduction degrade the picture quality.
The bigger the sensor, the better the picture quality, and the gain in quality is pretty much proportional to the sensor size.
Megapixels are no longer particularly relevant for compact cameras because they’ve already reached the point where more isn’t making any difference, and in fact may be making things worse.
You can see the effect above. This compact camera has recorded coarse detail quite well, but the finer textures are just being smoothed over.
Because the sensors in digital SLRs are much larger, there is still something to be gained from higher megapixel counts.
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