The size of the aperture in a lens controls the amount of light allowed into the lens.
- Wide aperture (low f-number) allows more light to enter the lens.
- Narrow aperture (high f-number) allows little light into the lens.
A very low f-number (very wide aperture) on a lens means it can use a very fast shutter speed when the aperture is wide open. A Lens with the advantage of wide apertures is referred to as a ‘fast lens’. Typically, a lens that can provide a range of apertures wider than f4 is considered a fast lens.
At long focal lengths the maximum size of the aperture is limited by the size of the optical lenses. To provide very wide apertures and long focal lengths requires larger optical lenses in the lens unit. This substantially increases the price of a lens. Very wide apertures therefore only tend to be built into professional standard lenses.
Lenses in general production of f1.2 are considered the top end of the range of fast lenses. However, in photographic history lenses have been manufactured as wide as f0.7. In general, the expense of lenses this fast is not justified in performance. The depth of field will be extremely shallow at such wide apertures. The need for such a lens is likely to be linked to specialist photographic applications.