How many megapixels?
Digital compact cameras have grown increasingly powerful over the past few years, to the point where it is now hard to find a camera under eight megapixels, which just a few years ago would have been more powerful than the best professional DSLR. There are now quite a few compact cameras with sensor resolutions of over 14 megapixels, but it seems that some of the main manufacturers may at last be starting to back away from ultra-high resolution compacts. Most of the new ranges announced for Spring 2009 have a maximum resolution of 12.1 megapixels.
Compact camera sensors are very small, and cramming more and more sensors onto them causes a number of problems. The individual photocells and microlenses have to be smaller, and are therefore less good at capturing light, which means reduced dynamic range and colour depth, and increased noise at higher ISO settings. In addition to this the cells are closer together which can cause problems with charge leakage and overheating, which also have a negative impact on image quality. I've covered the problems in more detail in a previous tutorial.
There is also an issue with the resolving power of a compact camera lens relative to the resolving power of the sensor, but we don't really need to get into that here. In short, while adding more megapixels produces larger images, the quality of those images in terms of colour, tone and detail may not be as good as a smaller image from a less powerful sensor of the same physical size.
To decide how many megapixels you need, you need to think about what you want to do with your photos. If all you're going to do is store them on your computer and print some of them at 10 x 15cm (6 x 4in) snapshot size, then really five megapixels is probably more than you need. You can get a perfect photo-quality snapshot print from a camera with a three megapixel sensor. With seven megapixels you can print your photos at A4 size, and with a 12MP camera A3-size high quality prints are possible. You only need the super-powerful sensors of the top digital SLRs if you regularly shoot double-page spreads for National Geographic.
The way to work it out is to look at the maximum image size in pixels. For true photo-quality a print should be made at a resolution of 150 pixels per centimetre, although 100 pixels per centimetre will still produce very good results, so if you divide the pixel size of the camera's image by 150 or 100 you will be able to work out how big the prints can be in centimetres. Here's a table that shows the relationship between megapixels and print size.
* = Approximately A5 paper size
** = Approximately A4 paper size
*** = Approximately A3 paper size
**** = Approximately A2 paper size