A fast lens is one with a large maximum aperture — a low f-number such as f/1.4, f/1.8, or f/2.8 — that lets in a lot of light.
“Fast” has nothing to do with focusing or zooming speed. It’s borrowed from the shutter: more light through the lens means you can use a faster shutter speed in the same light, which is what the word originally described.
The threshold shifts with focal length. At 50mm, f/1.8 is ordinary and f/1.4 is fast; at 400mm, f/4 is fast and f/2.8 is exotic. The table gives the rough conventions, then the rest of this page covers what a fast lens actually buys you and the catches that come with it.
| Focal length | Counts as fast | Typical kit-lens aperture for comparison |
|---|---|---|
| 24–35mm | f/1.4 – f/2 | f/3.5 – f/4 |
| 50mm | f/1.4 (f/1.8 is the affordable classic) | f/4 – f/5.6 |
| 85–135mm | f/1.4 – f/2 | f/4 – f/5.6 |
| 70–200mm zoom | f/2.8 | f/4 – f/6.3 |
| 300–400mm | f/2.8 – f/4 | f/5.6 – f/6.3 |
| 500–600mm | f/4 | f/6.3 – f/8 |
If you’re new to the world of interchangeable lens cameras — DSLR and mirrorless cameras where you can change the lenses — you might have come across mentions of “fast” lenses and wondered what that means. It’s sometimes also known as “fast glass”; “glass” is a way some photographers refer to lenses. But what does it mean for a lens to be fast?
Despite how it sounds, the “fast” part doesn’t refer to any physical movement on the lens. It doesn’t refer to autofocusing or zooming or something physically moving from point A to point B.
Rather, it refers to the maximum aperture of the lens. The gist is that a large maximum aperture allows more light at once. And that means you can use a faster (or shorter duration) shutter speed.
There isn’t a formal threshold on what constitutes a fast lens or a slow lens. It depends largely on the focal length.
At 400mm, a maximum aperture of f/4 would be regarded as fast. But at 50mm, f/4 is not fast at all — the threshold at that focal length is more like f/1.4. There’s also a subjective element: what one photographer might define as fast might not be to another photographer. So it’s a pretty loose term and quite subjective.
The fastest lenses that are commonly available as consumer lenses are those around f/1.2 or f/1.4. Even faster lenses do exist — such as f/1, f/0.95, or even faster — but those are really niche products that tend to be much more specialized (and expensive (though not always)).
For a sense of the extremes: Nikon’s Z 58mm f/0.95 S Noct is a manual-focus lens for its mirrorless cameras, and Leica has made the Noctilux-M 50mm f/0.95 since 2008. At the budget end, brands like 7artisans and TTArtisan make f/0.95 lenses for a fraction of the price. The best-known extreme is the Zeiss Planar 50mm f/0.7, which was made for NASA and which Stanley Kubrick used to film the candlelit scenes in Barry Lyndon.
See what happens when you change the aperture
Here’s a little tool I created that gives a visual sense of what happens when you change the aperture. It mimics holding up a mechanical lens and rotating the aperture dial.
Newer electronic lenses close the aperture when you press the shutter. Older mechanical lenses move when you rotate the lens’s aperture dial.
You can use the aperture “dial” here at the bottom to change the aperture and see its effect on the size of the opening, as well as the out-of-focus blur and starburst pattern that you get at smaller apertures (larger numbers).
You can also visualize the difference in the shape and smoothness of the opening with different numbers of aperture blades and whether they’re straight or rounded (you’ll find this info on each lens’s tech specs table).
The benefits of a fast lens
As you can see easily above, a large aperture like f/1.4 is going to let in a lot more light than a smaller aperture like f/4. And that’s the heart of what we’re talking about here.
There are two key attractions of a fast lens. Both relate to having a large maximum aperture.
- A larger aperture means more flexibility with other parts of the exposure triangle (shutter speed and sensor sensitivity (ISO)). Specifically, it allows for a faster shutter speed and lower ISO (or both) than would otherwise be possible. A faster shutter speed is good for sharper photos and freezing action. A lower ISO is better for less image noise, better color fidelity, and a deeper dynamic range (the spread between highlights and shadows).
- A larger aperture also allows for a shallower depth of field. That’s useful for blurring the background, for instance.
One thing that trips people up is sensor size. For exposure, f/1.4 is f/1.4 on any camera, so you get the same shutter speed and ISO whether the lens is on a full-frame body or a smaller one. Depth of field is different. For the same framing, an f/1.4 lens on a Micro Four Thirds camera gives you roughly the background blur of f/2.8 on full frame, and on APS-C it’s closer to f/2. There’s more on how that works in what 35mm equivalent means.
Good uses for a fast lens
While there are no specific restrictions on how and when you can use a fast lens, there are certain shooting situations that play to their strengths.
If you do a lot of natural-light shooting at night or in low-light situations (eg. a theater or performance), the larger aperture will give you more flexibility on using a faster shutter and lower ISO. A faster shutter will help with camera shake and motion blur (ie. sharper images), and a lower ISO will help with overall image quality (eg. image noise, dynamic range, color fidelity).
A fast lens is also good for isolating your subject from a foreground or background. The effect of this will vary on the precise situation and focal length you’re using, but it can be especially useful for portraits, for example. It’s most commonly used for isolating a subject in the foreground by blurring the background, but you can get some really interesting shots by reversing that or isolating a subject between foreground and background. Basically, a large aperture (and low depth of field) can be useful for any situation where you want to isolate the subject.
You can, of course, use a fast lens at smaller apertures — there’s no rule that says you must always shoot with it wide open (at the largest aperture). So just because you have a 50mm f/1.4 lens, for example, there’s absolutely no reason you can’t shoot at f/8 or f/11.
Things to watch out for with a fast lens
There’s a lot to like about shooting with a fast lens, and I will nearly always reach for one when it’s available. But there’s also a few things to watch out for when shooting with them wide open.
Accurate focus becomes especially critical
Most immediately, you’ll find that you’ll have to be more careful with focus. On a fast portrait lens, for example, you usually want to focus on the eyes. But it’s quite easy to get that a bit wrong and find when you come to edit the photos that the focus point is actually on the nose or ear. So it’s worth taking extra care to make sure that the focus is exactly where you want it. That becomes even more critical as you move into longer focal lengths (for example, you have to be more precise with an 85mm f/1.4 than with a 24mm f/1.4).
On DSLRs, you might also want to calibrate the autofocus for your specific combination of camera and lens (many mid- and high-end DSLRs have an AF fine-tune option for this). Mirrorless cameras mostly don’t need it, because they focus using the image sensor itself rather than a separate autofocus module, so there’s no offset between the two to correct.
Some lenses can be soft wide open
It’s common for lenses to be less sharp at their largest aperture than if you step down a few stops. So an f/1.4 lens, for example, is likely to be sharpest when you start getting into the f/5.6 or f/8 range. But when you shoot at f/1.4, the focus might look a bit soft, especially around the corners and edges of the frame.
That’s not necessarily a bad thing. Indeed, you can put it to very good use creatively. But it’s just something to be aware of, and it’s worth testing a lens wide open before using it for a proper shoot.
This isn’t something specific only to fast lenses. It’s common with most lenses to some degree — it’s just that it can often be more noticeable with the narrow depth of field of fast lenses.
This is one of the core characteristics of individual types of lenses and one of the key indicators that photographers look at when assessing the quality of a lens.
Lens vignetting is more pronounced
Similarly, most lenses show some lens vignetting around the corners and edges of the frame when shooting wide open. That is, the corners and edges will appear a bit darker than the center of the image. Again, it’s not necessarily a bad thing — I actually quite like the look as a way of drawing the eye to the subject. But if you’re shooting something that requires even exposure across the frame — like paintings or architecture, for instance, it’s something to factor in.
The good news is that it’s often easy to fix in post. Most serious image editing apps, like Lightroom or Capture One or Luminar or PureRAW, have tools for correcting or compensating for lens vignetting. And some newer cameras even have built-in algorithms that compensate for it at the time you press the shutter (it’s usually a setting you can toggle on or off).
Not all bokeh is the same
Not every fast lens will have exactly the same quality of the blurred background (the bokeh) — some are more pleasing to the eye than others.
Some have a gorgeous, creamy texture. Others are harsher, and that can show up most prominently in high-contrast backgrounds. It’s just a characteristic of the optics of the lens, and each one is a bit different. It’s yet another thing that differentiates highly sought-after lenses from others.
Fast lenses can be expensive
Because it’s harder and more expensive to make a good-quality optical lens with a larger maximum aperture, fast lenses can be expensive. That’s especially true with longer telephoto lenses, which can get into eye-watering prices.
For example, a long telephoto with a maximum aperture of f/2.8 is likely to be far more expensive than one with a maximum aperture of f/6.3. Similarly, an f/1.2 50mm lens is likely to be pricier than an f/1.8 50mm lens. Of course, there are other factors in the price of the lens, such as optical quality, features like autofocus and stabilization, weather-resistant build, and so on, but the maximum aperture is a big part of it.
Fast lenses also tend to be more highly coveted by photographers, and good ones can command high prices in the used gear market.
Common questions
Is f/2.8 a fast lens?
For a zoom, yes — f/2.8 constant-aperture zooms are the professional standard. For a 50mm prime, f/2.8 is slow; the fast primes start at f/1.8 and f/1.4.
Does a fast lens mean a sharper lens?
Not automatically. Fast lenses are often sharper stopped down a little, but many are soft wide open. Speed buys light and shallow depth of field, not sharpness as such.
Why are fast lenses so expensive?
Large apertures need large, precisely made glass elements, and correcting the aberrations that come with them takes more of them. The f/1.8 versions exist because they skip the last, costliest stop of that effort.
Fast lens or prime lens — same thing?
No, though they overlap: most of the fastest lenses are primes, but a prime can be slow (f/2.8 or f/4 macros and telephotos) and zooms can be fast too (f/2.8 zooms are common, and a few, like Sigma’s 18-35mm f/1.8, go even faster).
Good fast lenses to start with
If you want to see whether fast lenses are a good fit for your shooting preferences, there are many to choose from.
Renting lenses is a great way to try out something new. Good online rental services, like LensProToGo and LensRentals.com, have huge ranges of gear available. And shipping is all part of the service.
And, of course, you can buy lenses, either new or used. As I said earlier, fast lenses can be expensive. But an excellent place to start is with the classic nifty-fifty. Picking up a 50mm f/1.8 or 50mm f/1.4 is relatively affordable but gives you an excellent tool to experiment in various shooting situations. There’s a reason it’s a staple of many photographers’ kit bags.
How to tell if a lens is fast from its name
You can judge whether a lens is fast or not from its name (although that doesn’t tell you whether the lens is any good). Lens manufacturers generally stick to the convention of including the maximum aperture in the lens’s name. In some cases, there’s an aperture range. That’s with some zoom lenses and indicates that the maximum aperture changes depending on the zoom setting.
