Why you might want to consider a faster lens even if you don’t think you need the wider aperture – because they can focus faster and more accurately.
What is a Fast Lens?
First of all, what do we mean by the term “fast lens”?
A fast lens is one that has a large maximum aperture, denoted by a smaller f/stop number in its name.
For example an f/2.8 lens is said to be faster then an f/4 lens. f/1.8, f/1.4 etc are called fast lenses. The term probably comes from the fact that using these lenses “wide open” (i.e. at their widest aperture) allows you to shoot with a faster shutter speed.
How Fast Lenses Help to Focus
Something that is often overlooked by photographers is that when you meter a scene, your lens will be wide open, and then when you take the shot the lens “stops down” to whatever setting you chose.
This means that a fast lens with a wider aperture is sending more scene information (in the form of light) than a slower lens to the focus detector.
Since every stop represents double or half the amount of light to the next full stop, lenses with a maximum aperture of f/1.4 send twice as much light to the focus detector as an f/2.0 lens, and f/2.8 is half as much again, f/4.0 half as much again, etc.
Thus using a faster lens helps the focus detector to focus quicker and more accurately.
This could be a factor when choosing a lens, as well as the other advantages to fast lenses: being able to shoot in lower light and being able to narrow the depth of field.
There are of course downsides to choosing faster lenses – they do cost more, and are often bigger and heavier than their slower counterparts.