The speed of a lens relates to its maximum aperture. The larger the aperture (smaller the F number) at which it can operate, the faster the lens.
If we refer to our article on aperture and the diagram below, we can see that the aperture of lens primarily affects the amount of light it allows in. A larger aperture (smaller F number) will allow more light in to the camera by means of widening the opening inside the lens.
But How Does This Make It Fast?
Imagine photographing a particular scene where a perfect exposure is obtained with the following camera settings – F/4, 1/100s, ISO 100. If we were to use a lens with a maximum aperture of F/2.8, such as a Canon EF 70-200m F/2.8 or Sigma 24-70mm F/2.8, we could open up the lens from F/4 to F/2.8. This increase in aperture from F/4 to F/2.8 allows in twice as much light (an increase of one stop of exposure). The result is that you could then double your shutter speed (a decrease of one stop of exposure) and still obtain a perfect exposure. Therefore, your new settings would be F/2.8, 1/200s, ISO 100.
Essentially, as you open up your lens’ aperture to allow in more light, you can shoot with a faster shutter speed and still obtain the same exposure.
Why Are Fast Lenses Useful?
Fast lenses are useful (quite often vital) in situations where there is low light. In particular when hand-holding your camera, you may find that obtaining a good exposure in poor ambient light results in having to use a slow shutter speed. This can cause your images to blur. Using a faster lens, however, means you could open up the lens to a wider aperture, thus allowing in more light and allowing you to select a faster shutter speed. A faster lens also reduces the need to bump up your ISO to where image noise becomes an issue.
Fast lenses generally cover any which can operate from F/4 right up to F/1.2. The faster the lens, generally the more expensive it is. A rare exception tends to be the Nikkor and Canon 50mm F/1.8 lenses which can be bought for as little as £80 / $70. There’s a reason why these are the biggest-selling lens!
Remember, a wider aperture will reduce your depth-of-field, so shooting at between F/1.2 and F/2.8 suddenly becomes a new challenge in itself. Don’t forget to read through our article on aperture to fully understand its role in exposure and depth-of-field.
Author – Oliver Pohlmann