The majority of DSLRs have what are known as “crop sensors”, a reference to the film SLRs on which they are based that use 35 mm film. Most DSLRs have sensors that are smaller than a 35 mm frame, partly for cost reasons and partly to reduce the size of the camera. With a lens designed for use on a 35 mm film camera, the sensor captures only the central part of the image.

The diagram to the left shows the circular image that a lens creates. The larger rectangle represents a full size sensor that is the same size as 35mm film and which is just covered by the image circle. The smaller rectangle is a crop sensor and fits well within the image circle.

Shooting a scene with the same lens, first mounted on a 35mm SLR and then on a DSLR with a crop sensor, the film camera will record more of the scene. Cropping the full size image yields the same result as the DSLR, hence the term crop sensor. It will also be seen that a crop sensor has a magnifying effect by reducing the effective angle of view and making objects appear closer.

The area inside the red rectangle shows the view that would be captured if a lens of the same focal length were mounted on a camera with a crop sensor, where only the central part of the image would register.

Since many people are familiar with how lenses function on 35mm cameras, crop sensors are given a crop factor, which is typically around 1.5. This size is also referred to as APS-C, a reference to a film format smaller than 35mm that a consortium of manufacturers introduced in 1995. The crop factor is a multiplier that enables users to gauge how a lens will behave on a crop sensor camera. Hence a 50mm lens on a SLR will be equivalent to 75mm on a DSLR with a 1.5 crop sensor.

The higher the crop factor, the smaller the sensor will be. 1.5 or 1.6 are the most common, although there are a very few cameras that have 1.3 crops sensors. The Four Thirds system has a crop factor of 2 and is unique in that it was designed from the outset for digital cameras only - Four Thirds film cameras simply do not exist. A digital only format has certain advantages since the camera and lens design does not have to take account of a 35mm legacy, but the smaller sensor does require some compromises. The section on ISO has more about this.

A number of manufacturers include in their ranges at least model which has a full frame sensor that is the same size as 35 mm film. Some modern lenses are designed specifically for crop sensors, which precludes use on a film SLR or full frame DSLR. In the future, as digital technology develops, it is likely that this anomaly will disappear over time with the introduction of new camera systems that are not based on those designed for film.

The next section on Aperture and Lenses also looks at the effect of crop sensors on lenses.