Why Are Some Camera Lenses Called “Fast”?

Have you ever heard of a “fast” lens? I have. I hear it all the time, usually when I’m reading reviews for lenses online. I also read about these types of lenses in discussion forums a lot. They’re all over the place.

The problem with photography lingo like this is that no one ever actually tells you what any of these words mean. You have to go look them up yourself. This “fast” word is an example of such lingo. What could this possibly mean?

In today’s post, I’m going to discuss lenses and what makes them fast. I’ll do my best at explaining things in such a way as to help you finally understand what everyone is talking about. By the end of this post, you’ll be the talk of the town, or at least the talk of the photography shop in your town.

What Makes a Lens Fast?

Simply put, fast lenses have large maximum apertures. The aperture is the part of the lens that lets light in. The larger the aperture opening, the more light is allowed in, thus reducing the need for the shutter to be open for very long. If you remember the relationship between aperture and shutter speed, you know that a large aperture allows for a faster shutter speed and a small aperture allows for a slower shutter speed. And that’s what the “fast” means in the photography world.

Examples of fast lenses might be those with maximum aperture sizes of f/1.2, f/1.8 and f/2. These are really fast. When these types of lenses are used in low light situations, they help out tremendously. Think about covering your eyes with your hand while in a dark room and letting only a small crack of light through. You won’t be able to see at all. Now, think about the same situation but with a huge crack of light coming through. You’ll be able to see much better. Everything slows down when you’re only letting a small amount of light through.

Who Uses Fast Lenses

In general, photographers who take pictures in either low light or high movement scenarios use fast lenses. I just gave you the low light example above, so let’s talk about sports and wildlife photography for a moment here.

We all know that, in general, low ISO values are better when it comes to photography because the lower the ISO value, the less grain there will be in the resulting photographs, in general. I say “in general” here because there’s more than ISO when it comes to noise and grain, but we’ll leave this here for the time being. So, if we keep the ISO value relatively low in our camera, we’re left with aperture and shutter speed to deal with. Now, let’s say you’re out there taking pictures of your son at his soccer game one afternoon. He’s wild out on the field and he’s running all over the place. If you kept your ISO value low and increased your shutter speed to 1/500 of the second because you want all your shots to be crisp and clear, what do you think your camera is going to do with the aperture? That’s right, it’s going to try to open it all the way. The question is, what if you have a slow lens, or more specifically, a lens that doesn’t have a large maximum aperture? I’ll tell you what’s going to happen. Your camera is going to take the shots, but your photos are going to come out dark because of the lack of light getting through to your camera’s sensor. Something has got to give in situations like this. In your situation, because you’re bound by your camera’s restrictions, you either have to slow down your shutter speed, which you don’t want to do because you’ll have photos full of motion blur, or increase your ISO value to make your camera’s sensor more sensitive to light. I’m betting you’ll turn your ISO value to “Auto” and you’ll let your camera do its thing. That’s probably the smartest move.

Now let’s pretend that you have a fast lens. A lens with a huge maximum aperture of f/1.2. In this case, you’ll most likely be able to keep both the low ISO value and the fast shutter speed because of all the light the lens is letting through. Just remember, large apertures cause shallower depths of field. If you use one of these types of lenses, you’re going to get some blur in the foreground and background of your shots. This isn’t a bad thing. It’s actually a good thing because it can effectively isolate your subject.

Oh yeah, one last point. Have you ever tried to use auto focus in dimly lit scenes? Has all the hunting of the focus driven you nearly mad? I’d say that’s some slow focusing under those conditions. Fast lenses can make focusing faster too. Keep that in mind. They let more light through, which the focus mechanism just loves.

I hope I effectively explained what fast lenses are when it comes to photography. If you have any questions regarding this post, please let me know down below. You can also ask any related question you wish in the camera lens discussion forum. Thanks for reading!