A Sensor Perspective

• It took me a while to realise that different digital cameras actually have different sized image sensors in them, even though they may well quote the same megapixel output. But it's true, a 6MP digital SLR can have a sensor which is 10x bigger than your average 'get this free if you join NoPayouta Insurance' digital camera.
• Let's look at this diagram which shows the main sensor sizes found in modern cameras. You can see the largest rectangle represents the sensor size of a full-frame professional digital SLR, while the tiny rectangle in the middle represents the sensor size found in most of the mass consumer produced compact digital cameras found populating supermarket shelves.
• As a rule of thumb, and in the interests of optimizing image quality in any given camera, the bigger the sensor, the bigger the lens has to be. This is because it's no good having larger pixels on a big sensor if the amount of light getting to them is coming through a tiny compact lens - in fact it can make things worse, with more noise destroying image quality.

What Do Sensor Sizes Mean?

• So how do you know what size sensor a camera has before you buy? Take a careful look at the specifications and somewhere it should tell you, or if necessary research it on the web first. The fractions usually quoted look complicated, but you can see from this table that there is a pattern. Pixel density obviously changes dramatically - for example, the 8MP Nikon Coolpix S520 has a sensor size of 1/2.5", which equates to a pixel density of 32MP per cm². Contrast this to something like the 10.2MP Pentax K200D Digital SLR, which has a sensor size approximately APS-C, and a pixel density of just 2.8MP per cm².
The combination of fewer pixels per cm on the sensor, and a bigger lens, means the SLR wins on the quality front every time.

APS-C?

• In case you're wondering, the sensor size 'APS-C' is the most commonly used format on digital SLR cameras, with a size of around 25mm x 16mm, and you can see why images produced from such cameras are often vastly superior to those from point-and-shoot models. The APS-C sensor is three times the size of the best most common 'cheap' sensor, the 2/3" one!