What is a “fast” lens?

What does “fast” mean when referring to lenses?

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Many industries use their own vernacular, terms that have little meaning elsewhere, buzzwords, descriptors of tools, techniques or technologies that only mean something to them. Video production is no different.

This writer got into video production in the early 2000s, right around the time digital started making capturing to tape obsolete, or at least greatly diminished. Being instructed to take over video at an office that made magazines, there were no peers to call upon, no fellow shooters or editors to ask for help.

That left a couple of options: books and the internet.

Well, learning how to shoot and edit was relatively straightforward. There were tools, there were techniques and there were right and wrong ways to accomplish tasks. When I didn’t understand what a term or acronym stood for when it came to cameras and shooting, I might Google it, or I might just learn what the button or setting did and leave it at that.

Unfortunately it meant that I, like many self-taught video enthusiasts and pros, are learning video terminology on the fly.

One of the terms that is very often used but isn’t overtly obvious in definition is when referring to a “fast” lens. What does “fast” mean when referring to lenses?

Well, there are a few things on a camera that can be fast, but this term is in reference to the maximum aperture of the lens. The larger the aperture of the camera, the more light that is let through to the camera’s image sensor.

So, a simple way to look at fast and slow lenses are to consider that the fast lens lets in more light and a slow lens lets in less light.

So what exactly does it mean to say maximum aperture? Well, the aperture of a lens is the diameter of the open circle area, or diaphragm, within the lens. This bigger this area is, the more light gets through the lens.

Makes sense, huh?

This lens diameter is expressed to us using an f-number, such as f/1.8 or f/4.0. This f-number refers to a mathematical expression, and while we won’t go into it, it allows us to use lenses of different focal lengths and know that we’ll have the same exposure values.

So here’s how the f-number works: The lower the f-number, the wider the aperture. As we learned earlier, the wider the aperture, the more light that gets to the sensor. The more light that gets to the sensor, the faster the lens. Look for low f-numbers like f/1.2, f/1.4 or f/1.8.

Conversely, the higher the f-number, the smaller the aperture. Smaller aperture means less light getting through the lens to the sensor. These slower aperture lenses will have larger f-numbers, such as f/16 or f/22.

This information is all well and good, but why are other video enthusiasts crowing over the benefits of fast lenses? Well, there are a few good reasons.

The first is low light sensitivity. More light allows the sensor to do it’s job without having to figure out darker areas. More light means not having to crank up the ISO to keep the image bright, and as you’ve probably discovered by now, higher ISO settings result in image noise.

Another benefit is that soft, buttery background we see in pro shots. That out of focus background is a desirable effect, and much easier to achieve with a fast lens.

Wide aperture, fast lenses also allow shooters to use faster shutter speeds, since the light getting to the sensor is greater. This can help cut down on motion blur.

Sidenote: when shooting at maximum aperture, say f/2.8 on a lens that maxes out at that setting, many shooters will refer to that as “shooting wide open”. If you’re ever on a set and a director recommends shooting “wide open” to take advantage of the lighting situation, just set your camera to maximum aperture, and you’ll be all set.