Full frame sensor
The digital sensor in a camera is an electronic chip that gathers data about the light to which it is exposed. There are a number of different sensor sizes. In the case of a ‘Full frame sensor’ the size is related to a legacy structure. The standard film size for an SLR camera which used film was for 35mm x 24mm full-frame image size for individual image frames on the film’s surface.
Large capacity digital image sensor chips were built to match this 35mm size to allow for compatibility with lenses. The legacy lenses for film SLRs would cast an image circle on the sensor that was equivalent to the size cast on a film in an SLR – Single Lens Reflex camera. Thus, the legacy size for a standard film size was translated into the same size parameters for digital image sensors to ensure compatibility with existing lens ranges.
Today the 35mm digital image sensor has become a standard because it continues the ‘standard’ maintained by the lenses. Lenses are also matched by convention to the 35mm standard. Different focal lengths can be compared between models and makes of lens and camera if used in this way.
A number of film formats other than 35mm film existed. However, as the 35mm standard has been universally adopted for digital stills cameras which we know today as DSLRs (Digital Single Lens Reflex cameras) most manufacturers work to the same standard for lens comparisons. A number of different sized sensors exist other than the full-frame sensor. Most are smaller than the full-frame size (35mm). These smaller sensors are referred to as ‘cropped sensors’. However, in recognition of the standard the smaller sensors are normally given size allocations as a ‘crop factor’ of the full-frame size.
Full discussion on the specification of 35mm film is beyond scope of this website. You can find more on the nature of film in 135 Film.