Fast versus slow lenses

A fast lens is a lens that provides a large aperture — meaning less than f/3.0. Why is it called fast, then? It is because a larger aperture provides more light for the digital sensor, meaning you can use a faster shutter speed in your images where there’s less light, as in 6-3. Consequently you can shoot faster, not that the lens itself does anything more quickly. It also means you can achieve better narrow depth-of-field images because you have a wide aperture.

 

A stop-action image of men's sabre fencing taken at the 2007 World Fencing Championships in St Petersburg, Russia. A fast lens is a critical tool for taking this type of photograph. Taken with a 1D Mark IIn, EF 70-200mm f/2.8L lens, ISO 800, 1/1000 second at f/2.8.

Fast lenses, especially zooms, are generally more expensive and are of higher optical quality. Lower-quality, slower zoom lenses commonly have a wider, variable aperture that narrows as the focal length increases. For example, Canon’s EF 70-300mm lens ranges from f/4 to f/5.6 depending upon your focal length; at 300mm, the lens physically does not offer an aperture of f/4, and you won’t be able to even manually select that f-stop on the camera.

Variable apertures with settings dependent on focal length are often confusing. If you set a variable aperture lens, such as 70-300mm, at 70mm and set your aperture priority or manual setting to f/4.0, then if you zoom and take the photo, the lens and camera automatically reduce the aperture size — all the way to f/5.6 if you zoom completely — and the result may be an underexposed image. If you don’t know that your lens is going to do that, you may only figure out what happened by examining the metadata later to see that the aperture changed — even when you set it wider!

Faster zoom lenses retain the same aperture width no matter what the focal length may be, and this is one reason these lenses are so expensive: It takes very complex and high-precision optical and mechanical engineering to provide a wide focal range that maintains the same amount of light entering the lens tube.

Category: Science of Lenses

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