What Does it Mean for a Camera Lens to Be 'Fast'?

What does that mean? What exactly is a fast lens, and why would you want one?

What Is a Fast Lens?

The term "fast lens" refers to the maximum possible aperture setting of a lens. The aperture size determines how big or small of a hole lets light into your camera. This number is usually labeled with an F on your camera's settings menu. The smaller the number, the larger the hole - and therefore the more light is allowed in (i.e. F1.8 or F2.8). The larger the number, the smaller the hole - and therefore less light is allowed in (i.e. F11 of F22).

A lens that allows you to set a large aperture (a small number, i.e. F1.8 or F2.8) is considered a fast lens. This is because the large aperture also allows you to set a high shutter speed while still allowing enough light into the camera to properly expose your image. The term "fast lens" comes from this ability to use a high (fast) shutter speed.

Ok. What Is It Good for?

There are three big advantages to fast lenses.

Action/Sports Photography. While this is not the only reason to own a fast lens, it is the most obvious. By using a wide open aperture, you can use higher shutter speeds. This in turn allows you to capture action shots with much greater clarity.

In ideal lighting conditions (i.e. broad daylight), a fast lens isn't always necessary. But cloudy days, night games, or indoor sports will all require a faster lens to obtain great shots. A common "fast lens" for sports photography is a 70-200 f2.8 lens. Example: Canon lens or Nikon lens.

Indoor, Low Lighting Conditions. Another use for fast lenses is simply to capture lots of light. This can be useful for indoor event photography, such as wedding receptions or conferences.

In this case, you don't necessarily need a high shutter speed, but the large aperture lets you turn down the ISO rating to a reasonable level. A great lens for this purpose might be a "nifty 50" (a 50mm, f1.8 prime lens) or a wide-angle zoom lens (i.e. a 17-55mm f2.8). Example: Canon lens or Nikon lens.

Portraits. You don't necessarily need a "fast" lens to allow in more light for portraits. These are usually taken with lighting equipment, so light is not an issue. However, one of the side effects of using a larger aperture is that you have a small depth of field. In other words, everything behind the subject in your photo is blurred out.

This effect, called "bokeh", is often used to blur the background of a portrait. For this, you will want a wide open aperture setting - f1.8 or f2.8. Prime lenses are great for this purpose (i.e. a 85mm f1.8 prime), but a fast zoom lens (i.e. a 70-200mm f2.8) can also be used with great effect. Example: Canon lens or Nikon lens.

Great Tool to Have

Whatever you're doing, a fast lens is a great tool to have in your photography toolkit. You may not shoot sports, indoor events, and portraits...

But chances are that you'll do one of the above at some point in time. If you do, then you'll want a fast lens that allows you to open the aperture wide - either to let in more light or shorten the depth of field and blur out the background.