This week's Photography Fundamentals column answers a question that came to me as an anguished plea in my Twitter feed from a relative newcomer to photography. In short: How can a lens have a speed? Whilst he knew that speed equated to aperture and that a 'fast' lens was one with a large aperture, the terminology felt far from intuitive. How could an opening behind a lens be described in terms of 'fast' or 'slow'?
I'm guessing that if he were perplexed by the use of the the word 'speed' in conjunction with 'lens', there are a few other people out there who find the concept that lenses can be 'fast' puzzling, too. Or illogical. Or something that's accepted terminology. Or something that made sense many years ago but has since become obsolete. This might help.
It's not actually as illogical as it sounds to refer to lens speed. It's more a case of joining the dots.
We know that large apertures are referred to as 'fast'. A lens with a large maximum aperture, let's say ƒ/1.8 for the sake of an example, is described as a 'fast' lens. We know that the larger the aperture, the more light is able to reach the sensor (or film, if you're old school). We know that by the virtue of the exposure triangle, the more light that is able to hit the sensor, the shorter the time the sensor needs to be exposed to capture the image and the faster the shutter speed we can use. The shorter exposure time is a direct result of the larger aperture. The larger aperture allowed for a faster image capture, hence a large aperture is a fast aperture.
If you were wondering, yes, it's for this reason that higher ISOs are referred to as fast ISOs: increased sensitivity allows for faster capture.
A lens can have a 'speed' then, because it refers to how fast it can allow you to capture your images, which I hope isn't nearly as illogical as it first seemed.