Fast Lens

Photopedia

03 April 2008 09:40

Lenses that allow photographers to work with fast shutter speeds are often referred to as fast lenses. The more light that reaches the camera’s sensor, the faster a shutter speed can be used to achieve the correct exposure.

A lens setting of f/2.8 allows the maximum levels of light in as the lenses aperture is open at its widest during exposure. The more light that reaches the cameras sensor the brighter the image the camera can ‘see’. This improves the camera’s auto focus (AF) as the AF system needs to be able to differentiate between the varying elements within the frame to track focus accurately.

Sports photographers are a typical example of professionals who benefit from fast lenses, as they require very high shutter speeds to both freeze the action and help the auto focus accurately track moving subjects. Photographers working in low light situations where flash isn’t an option, such as concert and theatre photographers, benefit too, as fast lenses allows the correct exposure to be achieved without the need to increase the camera’s ISO settings too high, which would reduce the quality of the image.

Top end professional zoom lenses offer the widest aperture setting F/2.8 fixed through it’s entire focal length, meaning shutter speeds are the same at both 70mm & 400mm. Cheaper semi pro zooms don’t offer a fixed aperture and instead push the aperture value the more you zoom in. For example a 70-200mm F/4.5-6.3 lens has a maximum aperture of F/4.5 at 70mm but F/6.3 at 200mm the smaller aperture at 200mm means less light is let into the camera and a slower shutter speed is required.