What Is Lens Speed?

At least once a week we are asked by one or more of our readers a simple question – what does it mean when people refer to a lens as fast? On this video, photographer Steve Neidorf (from UltimatePhotoGuide) answer this question in very simple terms.

So what does it mean for a lens to be “fast”? the answer is pretty simple actually – a fast lens is a lens with a large aperture. So for example if you lens is a Canon 50mm f/1.8 it has an aperture of 1.8 which states the size of the maximum opening of the lens – the larger the maximum opening – the more light you can bring into the camera (and the sensor).

Look at the size of the opening – a faster lens (f/1.8) has a much larger opening than a slower one (f/22)

Now this can be kind of confusing for a beginner since the smaller the f number the larger the opening (you can see this visually in the 6 images we prepared for you). A lens with larger f number will bring in more light, allowing for more depth of field (or separation of the subject from the background) and more light getting into the sensor allowing you to either shoot with a faster shutter speed (hence the name fast lens) or using lower sensitivity (ISO).

So here you have it – a fast lens is one with a larger maximum aperture (say f/1.2 or f/1.4 or even f/1.8). While a slow lens is one with a smaller maximum aperture (for example a f/3.5-f/5.6 lens). Faster lenses are not necessarily better than slower lens (it really depends on what you want to do with the lens) but many fast lenses are considered to be among the best for suited portraits, sports and other uses.

You can check out many more helpful photography tips on our Photography tips section here on LensVid.

Iddo Genuth

Iddo Genuth is the founder and chief editor of LensVid.com. He has been a technology reporter working for international publications since the late 1990's and covering photography since 2009. Iddo is also a co-founder of a production company specializing in commercial food and product visual content.