What Makes A Lens Fast?

When I first heard the term “fast lens,” I was totally confused. How can something that doesn’t move be considered fast? Is there something I’m missing here? If you’re feeling the same way as I did, know that understanding the meaning behind this photography term is a lot less complicated than it seems.

A fast lens is any lens with an aperture of F/4 or wider. Since a wider aperture allows more light into the camera, you can use a faster shutter speed for your photos. Hence where the term “fast lens” comes from!

To help you better understand why fast lenses are so desirable, let’s break down the aperture and the advantage to wider aperture settings.

The Advantage To Using A Fast Lens

There are a few different reasons why photographers prefer to use these types of lenses. Since a wider aperture allows more light into the camera, you gain a few advantages.

– They Perform Better In Low Light

If you’re taking photos just after sunset, at night, or in a dimly lit room, having a fast lens will make all the difference. Although it might not seem like much, gaining an extra stop of light gives you way more flexibility with your settings. Since a wider aperture creates a physically wider hole for light to pass through, your camera can function far better than with a smaller aperture.

A perfect example of this is with astrophotography. When you take pictures of the stars, there’s little to no light around you. Yet somehow, your camera can make the night look like day. That’s because, with a wide aperture, your camera can take in even the smallest amount of available light. Couple that with a slow shutter speed, and no darkness can stop you!

– They Create A Shallower Depth Of Field

One of the main purposes of an aperture is to control the depth of field in your photos. The depth of field is how much can be in focus at once and controls how much background blur there is. Especially when shooting portraits, a shallow depth of field is a super desirable look for most photographers. It can help to soften the background and draw more attention to your subject.

You can capture a shallow depth of field with any aperture from F/5.6 and wider (F/4, F/2.8, F/1.4, etc). As you make the aperture wider, the depth of field narrows and creates more background blur. Using a fast lens, you can get extremely blurred backgrounds and get more creative with shallow depths of field.

– You Can Shoot More Handheld

Like I mentioned earlier, using a fast lens allows you to let more light into the camera, thus letting you use a faster shutter speed. When it comes to shooting handheld, a faster shutter speed is necessary to avoid having motion blur in your photos.

As a general rule of thumb, it’s best not to shoot photos any slower than 1/60 second. Any slower than this and the natural movements in your hand can translate into slightly blurred photos. In certain situations, you might find yourself pushing the limits of 1/60, but with a fast lens, you can open your aperture wider to let more light in. With that bit of extra light, you can increase your shutter speed and continue shooting handheld without worries.

If you were to use a slower lens, you’d be stuck having to increase your ISO or bring out the tripod. Especially if you’re shooting an event or portraits where you don’t have time to set up a tripod, a faster lens can really save the day.

What’s The Downside To Using A Fast Lens?

So far, using a fast lens seems pretty darn awesome. Why isn’t everyone doing it, am I right? Well, it turns out there are a few downsides to using this kind of lens. After reading these, you might reconsider buying one right away.

– They Are Much More Expensive

The main downside to buying a fast lens is the fact that they’re way more expensive. Just take the Canon 24-70mm lens, for example. This lens comes in an F/2.8 and an F/4 version, but the price difference is $800!

For the F/2.8 version, this lens costs $1,699 USD, while the F/4 version costs $899 USD.

Now you might be thinking this is some anomaly, but I assure you it’s not. You pay a premium for faster lenses because you’re really getting a better and more capable lens.

With that said, you need to decide if that $800 difference (in this case) is worth it to you. For most beginner or hobbyist photographers, the answer’s likely no. You could literally buy a second lens with the extra money you saved.

However, for anyone more serious about photography, the difference is worth every penny. You might just need to take a little more time to save!

– Autofocus Doesn’t Always Work As Well With Very Shallow Depth Of Fields

Once you start getting into extremely wide apertures like F/1.8 or F/1.2, the depth of field becomes very shallow. So shallow, in fact, that you could have one eye in focus and one eye out of focus when taking a portrait. Since there’s such a narrow focus plane, it makes the work of your autofocus system a lot harder. For most cameras on the market, they’ll struggle to get reliable focus at these wider apertures. That leaves it up to you to use manual focus and make sure it’s done right.

Of course, you can use a faster lens with a smaller aperture setting, but this is worth keeping in mind if you love shooting wide open.

– Ultra-Fast Lenses Tend To Be Primes

Although a fast lens is considered as F/4 or wider, most photographers can’t get enough of it. Everyone wants the fastest lens possible, whether they need it or not. Perhaps it’s for the versatility, or maybe it’s for the street cred. I suppose we’ll never know.

With an aperture of F/1.8, F/1.4, or F/1.2, it becomes increasingly difficult to find a zoom lens that’s this fast. Most photographers like using zoom lenses for versatility without the need to change lenses. However, zoom lenses are limited in terms of their widest aperture.

For the most part, ultra-fast lenses are reserved for prime lenses. Prime lenses have a fixed focal length but work exceptionally well for photos of people or the night sky. If you love your zoom lenses but want an ultra-fast lens, I’m sorry to say it, but you’ll be stuck with F/2.8.

What Does A Fast Lens Look Like?

A fast lens looks the exact same as any other lens on the surface. It’s the aperture where things look a lot different.

The aperture is a set of ‘leafs’ that slide together to create a smaller or larger hole for light to pass through in your lens. When a lens is set to its widest aperture, this is where you can start to see the difference between a fast and slow lens.

You likely won’t even see the aperture blades on a fast lens since it’s trying to let in as much light as possible. With a slower lens such as F/5.6, you’ll see how the aperture is more closed up.

This is easiest to see when you use a lens with a de-clicked aperture since you can manually open and close the aperture with a ring on the lens.