One of the questions you may ask yourself when deciding to buy a digital SLR for the first time is ‘Full Frame or cropped sensor?’ Even if it isn’t, it should be. The obvious next question is: what’s the difference? Well, it’s more complicated than you might think.
A full frame (‘FF’ hereafter) camera has a sensor that is the same size as a frame of 35mm film (36mm x 24mm); so-called ‘cropped sensor’ (‘CS’) cameras have smaller sensors, though there is no single standard, for example the sensor in a Canon EOS 550D measures 22.3mm x 14.9mm whereas in a Canon EOS 1D mk IV it’s 27.9mm x 18.6mm. Generally, FF cameras cost considerably more than CS models, however you might be able to pick up a second hand Canon EOS 5D or Nikon D700 for a reasonable price.
Many people talk about the ‘magnifying effect’ of a CS body. I find this term misleading; any magnification is done by the lens, and a 50mm lens is a 50mm lens on any camera. The image projected onto the sensor plane is identical, but the smaller sensor means only a part of the image is captured compared to a FF camera. To physically print the image would require more magnification but this is applied after the photograph is taken, and so is not technically a function of the sensor.
This also means that to get the equivalent field of view in a CS camera to a 50mm lens on a FF model, you would need to user a wider lens, eg 35mm, though the exact focal length would depend on the physical dimensions of the sensor. Using a lens with a shorter focal length means you get more depth of field, so this is one of the first things you need to think about when choosing sensor size; does your style of photography require narrower or wider DoF? Below 35mm though, it must be said, DoF is generally so wide that it makes little difference.
Cropping the image also means it is easier to fill the frame with a subject, especially useful in wildlife photography where small animals are shot using telephoto lenses of 300mm or more. This is where it is beneficial to use a smaller sensor; while it is also possible to crop a FF photo in post processing to achieve the same effect, you end up wasting pixels and losing detail as the resulting resolution is so much lower.
Of course the converse applies to wide angle lenses. Much of the drama of a good landscape comes from using lenses of 20mm or less, and when the extremes of a photograph are cropped out, this added drama can be lost. Unlike with telephoto lenses however, you cannot reverse the effect in PP.
The one final consideration is image quality. Generally, except for the most expensive lenses, this deteriorates towards the edge of the picture, so by using a cropped frame sensor you are eliminating the less well defined areas of your image meaning you don’t necessarily need to spend as much on good quality glass. On the other hand (though this is less true with the latest developments in sensor technology), the high ISO performance of CS cameras is not as strong as that in FF cameras. This is because the pixels are more densely packed in a CS camera, and so are smaller and more sensitive to non-optical factors, such as electronic interference caused by the camera’s own circuits.
It is not my intention to say what is right or wrong, as I don’t believe there is such a distinction. But if you are fully aware of the consequences of your choice of sensor size, hopefully it makes it easier for you to make the decision that works for you.