One of the things it took a long time for me to get to grips with about cameras (and lenses) was crop factor. I wanted to take the time today to explain it as I keep referring to it in my posts without really going into it in great detail.
Crop factor has to do with the size of the sensor in your camera and how it compares to a full frame sensor. All entry level digital single lens reflex cameras have smaller sensors than a full frame sensor. The crop factor is important when considering lenses, because the focal length of the lens is affected by the crop factor, and therefore, so is the field of view.
Let me give you an example: Canon’s entry level camera the Canon 1200D is sold with a kit lens that zooms from 18 to 55mm. This is a pretty standard kit lens, and Nikon, Sony, and Pentax all have their versions with exactly the same focal lengths on the kit lens.
The 18-55mm focal length is a measurement that comes from the days when 35mm film was the standard. Regardless of the sensor that the lens has been designed for, the focal length of the lens is measured as if it were going on a full frame sensor. A full frame sensor is the same size as 35mm film was.
So because the entry level cameras have smaller sensors than 35mm, the apparent focal length of the 18-55mm kit lens is different. For Canon cameras, we multiply by the crop factor for the sensor size: 1.6. So the 18-55mm lens becomes a 28.8-88mm lens. For the other manufacturers, the crop factor is 1.5, and therefore the kit lens has an apparent focal length range of 27-82.5mm.
Why is this important? Well, first remember that focal length affects your depth of field. The main difference, of course, is how much is in the field of view or in the picture. The shorter the focal length, the wider your field of view. Long focal lengths get you closer in.
For reference, the apparent focal length of the human eye on a full frame sensor is about 50mm, which is why 50mm lenses are so popular. They suffer from very little distortion and the images produced by these lenses feel very natural. On a Sony APS-C sized sensor, the equivalent focal length would be 33.3mm, or about 35mm.
What else do we need to remember when it comes to crop factor? Well, the size of your sensor affects your depth of field. Some landscape photographers prefer to use smaller sensors because smaller sensors get more in focus when all other things are kept the same. Portrait photographers like to use cameras with full frame sensors like the Nikon D810 or the Canon 5D Mark III to get just their subject in focus, with everything else going blurry. In fact, they’ll even use bigger sensors like the Pentax 645Z or the Hasselblad systems to get even more out of focus. As a side note, these bigger sensors are called medium format cameras and have different crop factors, but they’re lower than 1.
Also for a given aperture, more will be in focus on a smaller sensor, even though they’ll let in the same amount of light. So f/1.8 on a full frame sensor will have less in focus than f/1.8 on a smaller sensor, even though the shutter speed will be the same for the same ISO. This is exploited by street photographers who like to get a lot in focus while freezing action. They generally prefer smaller sensor which allow them to get fast shutter speeds while having greater depth of field.
Having a big sensor also means bigger, heavier lenses. Street photographers like cameras to be light and discrete, so carrying around a big heavy camera all day makes no sense. They usually opt for something small and light, even opting for prime lenses to help keep weight down.
So to sum up:
Full frame sensor: Bigger cameras, narrower depth of field, bigger, heavier lenses.
Smaller sensors: Smaller cameras, greater depth of field, smaller lenses.
Here is a by no means exhaustive list of the most popular cameras today with their corresponding crop factors: