When deciding on what camera body to use or purchase, the debate of a full frame or crop sensor is a common one. Know that there is not really a better one to choose, they are merely different.
Firstly, what is a sensor? It is what has replaced film in modern cameras; the sensor is the device which captures the light coming through the lens, and uses it to form an image. A full-frame sensor is the same size as a piece of 35mm film which was, and still is, the most widely-used type of film in analog cameras. This converts to a 24mm x 36mm sensor size in digital cameras. Anything that is smaller than a 35mm frame is called a crop sensor.
The physical size of the sensor inside the camera is also going to affect the effective focal length of the lens. It will not actually change the focal length physically but it changes what you can see.
If a lens is on a crop sensor body we do not see the full frame area of the lens. You only see the inside portion, with the edges cut off (see image above). The major noticeable difference is therefore your field of view; with a crop sensor, the image appears more zoomed in.
For example, with the Canon 7D (which uses a crop sensor) you will need to multiply the focal length by 1.6. So, a 50mm standard lens will become an 80mm. This can be an advantage when it comes to telephoto lenses, as you’ll gain free millimeters, but the flip side is that wide-angle lenses will become standard lenses.
The question therefore is, which sensor type is better for you?
Full Frame cameras let you use your lens at its normal focal length. They also perform extremely well when shooting with a higher ISO, in low light. When shooting landscapes a full frame camera may be the better option as the image quality is better than with a crop sensor and more of the scene will be in the shot.
For nature and wildlife, a crop sensor may actually be an advantage, as the crop can make you seem closer to your subject, without you having to splash out on an expensive lens with greater focal length. Because crop sensors are smaller, the saving on space and weight within the camera body often allows for higher burst rates, which is one of the reasons that a Canon 7D mk II can shoot at 10 frames per second, while the 5D mk III can only manage 6 fps.
When cropping an image during post processing, you are going to start losing quality faster with a crop sensor than with a full frame, however.
Here are some examples that show the differences in the respective fields of view between a Canon 7D mk II (Crop sensor) and a 5D mk III (Full frame).
Both cameras had on Canon 100-400mm mk II lenses. Corresponding images were taken as close to simultaneously as possible.
From the images above we can see that there is a marked difference between the effects of a crop sensor or a full frame one.
Remember though, the difference we are seeing here is purely in field of view. Zoom capabilities are certainly not the only thing when choosing between a camera with a full frame and one with crop sensor. Size, price, FPS and a number of other factors all play a role, so in this brief example of how the visual outcomes between the two differ, we’re only just scratching the surface.