One set of terms that you’re certain to come across when researching your next DSLR purchase are Crop Sensor and Full Frame.
35mm film is the standard by which all cameras are measured. Cameras that have sensors which replicate the 35mm film style are referred to as being “Full Frame.” Not all cameras replicate 35mm film. Most cameras have sensors smaller than 35mm. Those cameras are referred to as “Crop Sensor.” To keep you from having to buy different lenses for different style cameras, professional lenses are made to work on either style. However, they will produce different results. Crop sensor cameras produce images that seem magnified.
The illustration below shows the relationship between a crop sensor camera and a full frame camera. Your lens is round, but your camera’s sensor is rectangular. If you can remember the film days, you remember that you loaded a roll of film into the camera and then dragged it across the back of the camera. Each frame of film was a rectangle, just like your camera’s sensor.
As you can see below, the 35mm (or full frame) rectangle stretches across the entire circle, such that its corners touch the edges of the circle. It is the largest rectangle that can fit in the circle, hence the name Full Frame. The smaller rectangle represents the size of a crop sensor.
When light comes through the lens, it actually comes through the sensor opening. Imagine two separate cameras, one with a full frame sensor and one with a crop sensor, both taking the picture below, using the same exact lens and the same settings. In the diagram below you can see that the full frame sensor camera would capture much more of the image than the crop sensor camera would.
But just how much more? It depends on the camera. In the Canon world it’s 1.6. So what does that mean?
It means that if you put a 105mm lens on a crop sensor camera, the image you get would be the equivalent of a 168mm lens on a full frame camera, or 1.6 larger. This can actually be considered a feature for some people. Besides my full frame Canon 5D Mark IV, I also carry a crop sensor Canon 7D. If I’m shooting sports and need a much larger zoom lens than I have, I attach my lens to the 7D, this giving me more zoom. My 200mm lens behaves like a 320mm lens on a crop sensor camera.
The trade off however is when you are trying to shoot wide angle. Using my 17mm lens on the full frame camera allows me to actually shoot at 17mm. If I only had a crop sensor camera, that 17mm lens would be shooting at the equivalent of 27mm. I’d have to use a 10mm lens in order to shoot at 17mm.
Confusing for sure.
The bottom line is, a full frame sensor captures as much of the image as is possible for a given lens, while a crop sensor captures less of the image. It’s neither good nor bad. It just is what it is. Full Frame cameras are inherently more expensive and are considered to be “professional.” Many photographers carry a full frame and a crop sensor and use them as necessary for the shot they need to take.