The difference between full-frame and crop sensors

Some beginners to photography ask about which DSLR camera is best to buy, and one of the considerations is whether they want a camera with a "full-frame" or a "crop" sensor (sometimes called "cropped-frame"). The obvious questions that follow are "What is the difference?" and "Why is that important?".

The difference is actually quite simple, but is understandably confusing when investigating websites and forums on the topic, because there is a lot of technical discussion surrounding terms like 'pixel density', '35mm equivalent', and 'crop factor'.

The sensor is the part of the camera that captures the image. Before digital technology, the 'sensor' was actually one of the single frames on your roll of film. These days, not all sensors are the same size as a film frame, which is why we have these different terms.

So simply, this refers to the size of the sensor in the camera. A full-frame sensor is the same size as a single frame on an old-school roll of film - that is, 35mm. A crop-frame sensor is smaller (how much smaller depends on the brand and model of camera).

Image from www.stevehuffphoto.com

The reason we have smaller, cropped sensors is due to a number of reasons such as cost and fit. A large sensor costs many times more to produce and can only really fit inside larger camera bodies. After all, the mechanics that surround a sensor (such as the shutter curtain) need a bit of space to operate properly.

So when you hear about 'crop factor' this is just how many times larger the full-frame is over the crop. The full-frame might be 1.5x or 1.6x larger. In terms of lenses, this means that a 50mm lens used on a crop sensor will be the same as an 80mm lens on a full-frame (or 35mm equivalent) sensor.

So why is this important?
There are advantages and disadvantages to both types.

The obvious advantage of the full-frame sensor is that more image fits onto the frame. This is great for landscapes or for getting more field-of-view when working in tighter spaces. More field-of-view will also mean less "depth-of-field" in the image. DOF refers to the amount of the photo that is in focus (with out-of-focus foregrounds and backgrounds being outside of the DOF). So basically, on a full-frame you get more blurry foregrounds and more blurry backgrounds - this can be more appealing in many photos. The other advantage is that each individual pixel on a full-frame sensor is slightly bigger which means they have a better sensitivity to light. Therefore, more detail is preserved in shadows and bright highlights (better tonal range). It can be used with more success in low-light situations with less noise. For professional photographers, this is very important.

Image from www.mumbaipav.com

The disadvantage is mainly cost and weight. Cameras with full-frame sensors can be 3x to 10x more expensive than cameras with cropped-frame sensors. So the advantage of full-frame needs to be weighed against budget and purpose.

Crop sensors have the advantage of having a finer resolution (smaller pixels), but this isn't usually too noticeable in images that have not been cropped down further to size. The biggest advantage is that they are better for image quality in zoomed in shots. A 200mm lens on a 1.6x crop sensor is actually equivalent to 320mm. So this would be better for photographing wildlife and sports, provided there is plenty of good light.

The following list shows the more recent cameras of popular brands which have full-frame sensors and which have crop sensors (as of writing of this article):

Crop sensor (common type is called APS-C):
Canon 50D, 60D, 500D, 550D, 600D, 650D, 1000D, 7D
Nikon D300, D60, D90, D5000, D3000, D3100, D3200, D5100, D7000
Sony α A77, A65, A57, A37

Full-frame sensor:
Canon 5D, 6D, 1D
Nikon D3, D4, D600, D700, D800
Sony α A850, A900, A99

Note: outside Australia, Canon cameras may have different names to those shown here, like "Rebel" and "Kiss")