What is a "fast lens"?

"Fast lenses" are highly prized, but what is a fast lens – and why is it is something that most photographers and videographers value, no matter what type of shooting they’re into?

Lens "speed" is a term that describes the maximum aperture diameter, and refers to how wide the aperture can open, measured by the f-stop number. Basically, the smaller the f-stop, the larger the aperture opening So what is a fast lens? One where the aperture opens wide, such as f/1.8 or f/1.2.

Lenses that can open wider usually cost more, and for good reason – they offer better performance. To understand why an optic with a wider aperture is attractive, you need to take the exposure triangle into account. A wider opening lets a lot more light hit the camera sensor, which is a game-changer in low-light situations to keep shots well-exposed.

Plus, if you're shooting fast-moving subjects, like cars in a race, a quick shutter speed is crucial. Since a fast shutter only lets in light for a short time, a wider aperture helps balance that out by allowing more light to hit the sensor, leading to a balanced exposure.

For portrait photographers, going with a lens that has a wide aperture is really important. A larger opening (think low f-stop number) creates a shallow depth of field and pleasing “bokeh.”

This effect blurs the foreground and background, making the subject pop and drawing the viewer's attention right where you want it, while distracting elements fade into the background.

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Check out the fastest lenses for cameras, along with the best lenses for portraits – with apertures as "fast" as f/1.2. To learn about more camera terms, take a look at our pieces on what is aperture on a camera and what is exposure in photography.