Fast lens.

1. Colloquial expression for a lens with a very wide maximum aperture. (small f-stop number)

Such lenses let in a lot of light and can be very large and expensive, especially if they’re zoom lenses. They’re desirable first because they can be used in lower light levels than can slower lenses and second because a fast lens lets more light in when wide open, resulting in a brighter viewfinder image. (since most cameras meter with the lens wide open)

Since it costs more to build a fast lens, as a general rule of thumb a lens with a maximum aperture of 2.8 to 1.8 and faster is likely to be of higher optical quality than a slower lens, though this is by no means a guarantee, particularly with lenses built for cameras with small image areas (eg: digital cameras).

cf. aperture, f stop, slow lens.

2. The term occasionally can refer to the speed at which an autofocussing lens motor operates in the case of camera systems such as Canon EOS, which relies on motors inside the lenses and not in the camera bodies.

cf. EF, EOS.

Entry last updated 2002-04-19. Term 452 of 1487.

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