In this final entry in our series about the Exposure Triangle, we'll look at the Aperture setting, how it can help you control the light going into your camera and how it can drastically impact the way your images appear.

If you've spent any time looking at higher-end lenses, you'll have certainly noticed that the price of a lens goes up exponentially as the aperture value gets larger. The final piece of the exposure puzzle can be a little complicated.

What is aperture value?

An aperture is simply the opening in a lens where light enters the camera. Simple, right? Often referred to as the 'iris' of the camera, this small opening within the lens is usually able to change size, allowing more or less light to pass through at a given time.

This is the third, and some may argue the most important aspect of exposure. In changing your aperture value, you not only change the amount of light that enters the front of your lens, but also the size of the area that will be in focus in your image.

Before we get to that, let's take a closer look.

The vast majority of modern-day lenses consist of an arrangement of thin, metal blades laid out in a vaguely circular pattern. This allows the blades to slide on top of each other and shrink or expand the opening in the center. Other aperture systems will use stamped plates, like those found in the Lomography Petzval lenses.

Interestingly, it's the arrangement of these blades that can affect the way out-of-focus areas look. What you're seeing on the edges of these out-of-focus shapes is actually the outline of the aperture.

Without going too much into the science of optics and properties of light, I'll try to keep it fairly simple. The larger your aperture can open up, the more light you can let in, but this also makes your in-focus areas shallower in their depth of field.

This is why many lenses that have extremely large apertures tend to cost more. Not only do their larger apertures let in more light to give you more versatility in taking low-light photos, they also allow you to achieve that often-desired shallow depth of field that smartphones try to imitate with software.

Depth of field & subject distance

What do I mean by depth of field? In every photo, there is a range of distance from the camera that will be in focus. This range grows with a smaller aperture and shrinks with a larger aperture. In fact, many vintage lenses have marks on top of the lens to signify the areas that will be in focus. As you can see from the photo below, an f/4 setting will capture a smaller range in focus than an f/16 setting.

There are other factors that affect your depth of field, like your film size and the distance from your subject, but that's more of a composition tip than an exposure tip, and I'm doing my best to stay focused here. Tilt-shift lenses can also do all sorts of wild tricks with perspective and focus, but that's a story for another time.

What's an F-stop?

I mentioned earlier that the smaller your aperture, the less light can come through at a given moment. Much like the shutter speed setting's fractions, the higher the number, the smaller the aperture. The minimum aperture on a lens could be f/16, f/22, f/32 or even f/200 as estimated on some pinhole cameras.

The maximum aperture is a bit more complicated, however. It's true that the smaller the number, the larger the aperture size. However, the actual size of the lens opening is not what's measured here. Many zoom lenses have variable apertures that get smaller as you extend the lens toward the telephoto side of things. What gives?

The mechanical opening inside the lens doesn't get smaller, but it does allow less light to come through as a lens gets longer. Think of it like looking through a cardboard tube. The shorter the tube, the more angles of light can enter your eye. If the tube is longer like wrapping paper, only a very small amount of light will actually make it to your eye, even though the size of the opening is the same.

The actual f-stop, as it's called, measures a ratio between the focal length of the lens and the size of the aperture opening. This is why it gets more difficult and expensive to have super-telephoto lenses with f-stops like f/2.8.

The origin of the term F-stop is debatable, so I won't pretend to know the true answer, but it's likely that the 'stop' term came from smaller apertures literally 'stopping' excess light from entering the lens.

What is a T-stop on Cine lenses?

Once again, the world of cinema shows that things can never be too simple. Many cinematic lenses or specialty high-end prime lenses will offer T-stop settings instead of F-stops. What's the difference, and which one is better?

Where an F-stop measures the theoretical amount of light entering the camera based on the focal length of the lens and the width of the actual aperture opening, a T-stop represents an actual, measured value of light transmission. (Hence the 'T' in T-stop.) For many users, they're functionally the same setting, as T-stops are usually within 1/3 of a stop of their respective F-stops.

Most modern-day camera equipment can automatically compensate for this difference, but when shooting a motion picture, time and precision are far more valuable than convenience. This is why cine lenses offer up the tested transmission ratio as opposed to the theoretical f-stop value.

Common F-stops & keeping it simple

Let's say you have a 50mm f/1.4 lens. The maximum aperture of f/1.4 will allow you to let a huge amount of light in. Following that, the next true aperture value is f/2. Your lens or camera may allow you to adjust in half-stop or third-stop values, but for the sake of demonstration, let's stick with whole stops.

Following f/2, you'll see f/2.8. After that, you'll see f/4, f/5.6, f/8, f/11, f/16, and f/22. Each of these steps will allow half the amount of light from the previous one. If you're shooting a subject at f/2.8 with a 1/800 shutter speed, adjusting your aperture to f/4 would reduce the light by half, allowing you to slow your shutter speed by half as well. You'd see similar exposure results from a f/2.8, 1/800 exposure as you would from an f/4, 1/400 exposure.

If this sounds all like a bit too much math, you're in luck. It doesn't always need to be this complicated, especially since most modern-day cameras have specialized settings that allow you to control one aspect of your exposure and let the camera compensate with the other aspects.

Switch over to 'A mode' and you can control your aperture independently while the camera controls the Shutter speed and ISO, where applicable. Alternatively, 'S mode' will allow you to control your shutter speed independently. 'P mode' is sort of a hybrid between AUTO and Manual where you can set a working range for each of your settings and let the camera decide what's best.

What does it all mean?

To simplify, a larger aperture or f-stop will let in more light, but it shrinks the area that will be in focus in your image. A smaller f-stop will allow less light through, and also allow you to get more of the frame in-focus.

In practice, you'll often see landscape photographers shoot with smaller aperture values while portrait photographers will often shoot wide open. The landscape photographer will benefit from getting every detail in focus, while the portrait photographer is able to draw the attention to the subject by letting the background fade into a blur.

You may also get more of the unique characteristics of your specific lens when using larger apertures, like vignetting around the edge of the frame or unique swirly looks in out-of-focus areas. Opening a lens aperture to its maximum value allows light in from more angles than a small aperture would, and this allows the image to be affected more by transmission values in the glass.

Sony FE 85mm f/1.8 - shot at f/1.8, Shutter 1/60, ISO 400
Sony FE 85mm f/1.8 - shot at f/22, Shutter 3.2", ISO 400

Taking greater control of one aspect of the triangle means that the others have to compensate in order to remain properly exposed. Simply changing one will make your image brighter or darker.

f/16, Shutter 1/6, ISO 100

For example, if you wanted to draw more attention to the lavender plant nearest the camera, you may want to open your aperture to f/2.8. This would allow you to blur out the background areas.

f/2.8, Shutter 1/6, ISO 100

Oops. Opening the aperture allows more light in. Too much. Better increase the shutter speed to 1/160 in order to cut down the length of time the light will be hitting the sensor.

f/2.8, Shutter 1/160, ISO 100

Another way to cut the light's intensity without adjusting your shutter speed could be to add an ND filter. These act something like sunglasses for your camera and allow you a lot more flexibility when it comes to manipulating your exposure.

Learn more in our Lens Filters Explained series

By now, you've hopefully learned a bit about ISO, Shutter speeds and Apertures. Armed with these three tools, you'll soon be on your way to becoming a master of your images.

At the very least, you'll know why your images look the way they do. Now get out there and shoot some photos!

Read Part I: How ISO Affects Your Image

Read Part II: How Shutter Speed Affects Your Image