What is a Fast Lens?


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Pro Member
Fast lenses are things you'll hear about during your entire photography hobby or career. Amateurs and professionals alike enjoy throwing this term out there to describe a certain type of lens that's usually sought after for reasons I'll explain below. I think you'll notice photographers describe their lenses like they describe their cars. "Dude, I just bought the fastest lens I've ever seen. It's wicked fast!"

So, what the heck is a "fast" lens? Well, to answer that question, we'll need to look at something called "aperture." The aperture of a lens is the mechanical opening inside of the barrel that allows light to pass through to the sensor of the camera. The aperture size is what determines the fastness of the lens. If an aperture is large and allows a lot of light through, the lens is deemed fast. If the aperture is small and it doesn't allow a lot of light through, the lens is deemed slow. Let's look at the reasons why.

Let's say you have a fast lens with an aperture of f/1.2. With this size aperture, the hole that allows the light through is huge. Because all that light is making its way through the lens opening, the shutter speed inside of the camera doesn't need to compensate for any lack of light and can move ultra quickly as a consequence, hence the term "fast." Because of the shutter speed's blinding quickness, people refer to the lens as fast.

Conversely speaking, if an aperture is small, the shutter in the camera will need to compensate for the lack of light with a slower movement. Yep, you guessed it. Lenses with small apertures are referred to as slow.

Would you like to read more about fast lenses? Just click through! Do you have any additional information about these types of camera lenses? If so, please let us know below. Thanks!
What is a Fast Lens? was posted on 11-25-2018 by JGaulard in the General Photography Talk forum. Click to visit our most recent posts or return to the photography forum homepage.