Image circle

All lenses produce a circular image on the camera sensor or film, and this ‘image circle’ must be at least large enough to cover the full film/sensor area. Different lenses designed for different sensor sizes and formats have different-sized image circles. Lenses designed for APS-C format cameras, for example, have a smaller image circle than lenses for full frame cameras. Some specialised perspective control or tilt-shift lenses have larger image circles to allow for lens movements relative to the camera.

See also: Lens explained

Group (lenses)

Camera lenses used complex configurations of different optical elements, often cemented or fixed together in ‘groups’. Lens groups may be designed to counteract common optical aberrations and you may have autofocus ‘groups’ and zoom groups. Lens elements and groups often move relative to each other in complex ways as the focus and zoom settings are changed.

See also: Lens explained

STM (Canon)

STM stands for stepper motor lenses, a new type of autofocus motor used by Canon in some of its lenses. Stepper motors offer fast, precise and quiet focus adjustments, so these lenses are well suited both to regular stills photography and to video, where autofocus noise can be picked up very easily by the camera’s internal microphone. Canon’s STM lenses work very effectively with cameras using Canon’s Dual Pixel CMOS AF autofocus system.

See also: Autofocus basics

Lenses explained

The lens is a fundamental part of any camera. It’s what creates the image on the camera sensor (or film). Some cameras have a fixed, non-removable lens while others offer interchangeable lenses. Your choice of lens has a major impact on the appearance of your pictures, including the lens’s focal length (angle of view) and its aperture setting (which you may or may not be able to adjust). At a simple level the lens is just the thing on the front of the camera, but on a more advanced level lenses open up a whole world of photographic choices, buying decisions and technical comparisons.

Here’s a selection of further lens-related topics

Elements (lenses)

Camera lenses are made up of not just one single lens but many different lens ‘elements’, sometimes cemented or fixed together in ‘groups’. A comparatively simple prime (non zoom) lens may have 6-7 elements while a complex zoom lens might have 17 or more. The different lens elements are needed to compensate for a variety of common lens aberrations and offer autofocus and zoom capabilities.