If you're a photographer and you’d like to know more about lenses, then you’ll love our free course, What Every Photographer Should Know About Lenses. In this lesson, you’ll learn what aperture is and how it affects the image, as well as the differences between constant and variable wide-aperture lenses, and what this will look like in your shots.
How Aperture Works in Photography Lenses
What is Aperture?
An aperture is a hole or opening. In photographic lenses, it's the opening through which light travels, and is usually specified as a measured f-number. A lens typically has a set of marked "f-stops" at regular intervals that the f-number can be set to.
A lower f-number is a larger opening and a higher f-number is a smaller opening. The larger opening lets in more light, and the smaller opening lets in less light.
Each f-stop corresponds to a light intensity change factor of two. This means that each time you increase the opening by one f-stop and you double the light, and that each decrease in the size of the opening by one f-stop halves the light.
For example, if a lens is set to f/4 we’ll get a certain amount of light that enters the camera. All else reminaing equal, if we change the aperture to f/2.8 we’ll effectively double the light we’re allowing to reach the image sensor. If we change this again to f/2, we are again doubling the amount of light. The largest f-stop a lens can be set to is called its maximum aperture, and this will vary from lens to lens. Generally, more expensive lenses have a larger maximum aperture.
The most basic thing we need to know about aperture is that it controls the amount of light that is let into the camera. Sometimes, larger apertures are referred to as ‘faster’. For example, if the aperture was adjusted from f/4 to f/2.8 you might say that you chose a ‘faster’ aperture, or if your lens is at a maximum aperture of f/2.8 and you're not getting the correct exposure you might say, ‘I need to get a faster lens'.
We say "fast" because larger apertures allow in more light, which in turn allows you to use faster shutter speeds; another essential creative control. If we’re maxed out on sensor gain, or ISO, we can get a good exposure with a smaller aperture by using a longer exposure time, or shutter speed. The problem is that slower exposure times don't work for a lot of situations because they’ll result in motion blur.
Let's look at an example.
A ‘Fast’ Lens in Action
In the real world, it's hard to get people and other objects to remain constant and when shooting outside. Well, can we set the ISO (gain) higher to compensate. But what if that's still not enough?
In our example, the camera is set to manual exposure with the ISO at 3200, which is pretty high. It's not as high as it can go on this camera, but it's as high as we're going to go for this example because it’s what you might consider the highest ISO on your camera, so we're just going to pretend that 3200 is the highest acceptable ISO on this camera.
The lens is set to f/4.5, which is the most open that this lens can go as it’s an 18-270mm f/3.5 to f/6.3 super zoom lens, set here to 50mm.
The camera is in evaluative metering (meters based on what’s in the middle of a scene rather than off to a specific chosen point like spot metering), which tells us that around 1/13 sec is going to be the right shutter speed for the exposure.
Here is the result, and you can see there’s motion blur, because 1/13 at 50mm is not enough to stop the motion of the camera slider. If this was the highest ISO that you could use on your camera without being ridiculously noisy then it would be a big problem.
This camera can go up to ISO 12,800, but that's very, very noisy, so in this particular case it’s better to use a faster lens, because it's going to give us a better maximum aperture, and so, better shutter speed.
For the next example, the lens has changed to a 50mm f/1.8 prime lens. With the focal length the same but the larger aperture, and now the photo would be two stops overexposed. We can now adjust the shutter speed to make it a little bit faster: 1/50 sec.
This is the result and you can see the motion blur is a lot better. It’s not perfect, there is a tiny bit of blur still but you can see that it’s a lot more usable. If the limit was an ISO of 3200 then the previous lens couldn’t be used inside without a flash. With a flash, you could add more light to the scene and use a faster shutter speed. Without a flash though, just shooting in available light, this would be the best you could get.
F/1.8 is not the fastest 50mm that you can get, you can get f/1.4 and f/1.2, but 50mm lenses that are f/1.2 are over $1,000. You can get a manual-focus-only lenses that have f/.95, which is even faster and that would definitely be fast enough to stop motion, at least the motion in this example, but have a razor this field of focus.
You can see how a very inexpensive (about $100) prime gives us a nice bump up in shutter speed.
What is Considered ‘Fast’ and How Fast is Fast Enough?
What all of this boils down to is that when the light is low, you’ll want a lens with a larger maximum aperture. Depending on the photography you do a wide-aperture lense might be more or less appealing to you. As noise levels on cameras gets better as time goes on and cleaning up noise in post-production gets increasingly efficient, owning fast glass becomes less and less of an issue.
Here are some guidelines:
- A lens with a fast aperture will result in cleaner, sharper images in low light.
- For a zoom lens, f/2.8 is a fast aperture, especially if it's constant throughout the zoom range
- A practical alternative to big, bulky f/2.8 zooms is f/4, which is fast-enough in many situations now with good denioising
- For a prime lens, fast is f/2.8 and above. Like f/2, f/1.8, f/1.4, f/1.2, and even f/0.9.
- There are many affordable prime lenses with f/2.8 or /f4 apertures — don't overlook these options as often they have other qualities that the faster lens designs trade away to gain speed.
Depth of Field and Diffraction
Depth of Field
Aperture has a few side effects, like depth of field and diffraction. Depth of field, sometimes abbreviated as DOF, is the distance between the nearest and the furthest objects in a scene that appear acceptably sharp in an image. Another way to think about this is the area in front and behind the point of focus that appears to be in focus.
Depth of field is a rather complex topic, so if you want more information, you might want to check Advanced Depth of Field and Exposure. For now, we're just going to think about depth of field as the part of our scene that is sharply in focus. Aperture controls depth of field and it works like this:
A larger aperture—or lower number—has a shallower depth of field, and a smaller aperture—a larger number—has a deeper depth of field.
This is important because this is an area that you’ll want to control.
If you’re outside and you need a lot of depth of field, you'll going to use a smaller aperture like f/11, f/16, f/20 maybe even f/22.
If you are shooting a portrait, you might want to use a shorter depth of field to isolate your subject. Maybe something like f/4 or f/2.8.
Most lenses will be able to give you a small enough aperture to satisfy your long depth of field need, so that's not really an issue. If you want super-short depth of field, you'll need a faster lens, and this is what can separate a lot of lenses.
Some zoom lenses have a constant wide aperture. This means that no matter where you are in the lens zoom range, the aperture remains the same.
This is contrasted by lens that have a variable wide aperture. These lenses usually start at a wider aperture on the short end of their zoom range. For example, this Tamron 18-270mm pictured above is f/3.5 to f/6.3. As we go from 18mm to 270mm the aperture gets smaller and this means that at the long end this lens, it’s letting in a lot less light.
An aperture of f/6.3 isn't nearly as wide as f/2.8 but if you're shooting outside in nice, bright sunlight, it would work just fine. The thing to remember is that with variable aperture lenses, you have less control over the depth of field and light levels, as you use higher focal lengths.
Diffraction is the bending of light rays at smaller apertures. This causes the image to become noticeably less sharp, which is sort of an odd thing, because usually lenses become sharper as you use smaller apertures. But diffraction is not an on-off switch, it's a gradual effect.
Diffraction is also limited by sensor size, not lenses. On a full frame sensor, you would be able to use apertures down to f/22 without seeing diffraction start degrade the image.
On an APSC and Micro four-thirds cameras, the limit is around f/11, which is two full stops higher than full frame. This doesn't mean that you can't shoot on f/22 on the smaller sensors, you can, but it will be a softer image. If you want to reduce the light coming into the lens on a crop sensor camera without losing detail, you could use the smallest aperture before you start to see diffraction, and then pop on an ND filter to cut a few more stops.
The main thing to understand about aperture is that it controls the amount of light and the depth of field. In some of our other lessons, you'll see some examples of fast primes and zoom lenses with variable apertures.