In our last SLR Lounge Shooting Guide article, we covered the basics of shutter speed and how it is responsible for either conveying motion or freezing motion in an image. It is also one of the three factors that control exposure.

The second component of the exposure triangle is aperture. This article is part 1 of our aperture guide that will talk about aperture and its role in photography. We will start off by explaining what aperture is, what f-stop is, and how aperture controls exposure.

What is Aperture

The simplest explanation for aperture is simply an opening inside the lens in which light can shine through. The size of the aperture’s opening is controlled by a set of overlapping metal blades called a diaphragm. This set of adjustable blades determines how much light hits the sensor at any given time. Just think of this opening like the valve handle of a water faucet. The wider you open the faucet, the more water pours out, and the wider the aperture opening, the more light pours into the sensor.

Aperture-Diaphragm by SLR Lounge

Most modern lenses allow you to control the size of the aperture opening from the camera itself, but some modern lenses and the majority of vintage lenses have an aperture ring on the lens itself that allow you to control the diaphragm directly.

Measuring Aperture with F-Stop

Measuring aperture can be confusing at first. This is because aperture is measured by f-numbers or f-stops, which is the ratio of the lens’ focal length divided by the effective aperture diameter.

So if you take a 200mm lens and divide it by a 50mm aperture diameter opening, you end up with an f-stop of 4, or f/4. Now, for a 100mm lens, you only need an aperture diameter opening of 25mm to get to f/4.

So what this means is that unlike shutter speed, which is measured in absolute value of time, an f-stop is relative to the length of the lens and the diameter of the aperture opening itself.

The main thing that you do have to remember about the f-stop is that they are inverse values, which means that just like a fraction, the larger the number is, the smaller the diameter opening. A way to visualize this is to think of slices of a pizza pie. A quarter (1/4) slice of pie is larger than an eighth (1/8) slice of a pie. So an aperture opening with a lower value like f/4 is a larger than an aperture opening with a higher value like f/8. In other words, a large or open aperture has a low f-stop number, while a small or closed aperture has a high f-stop number.

Open-Closed-Aperture by SLR Lounge

Got that? Ok, because it is going to get a little more confusing from here. Going back to the slice of pie analogy, we all know that a 1/8 slice of pizza is half as big as a ¼ slice of pizza, and a 1/16 slice is going to be half as big as a 1/8 slice of pizza. With f-stops, the halving/doubling relationship doesn’t quite work in the same way because in order to double or half the amount of light coming in, you actually multiply by a factor of √2 (approximately 1.41). Doubling or halving the light is called one full stop. So, going from f/4, if you want to cut the light coming in by half or go one full stop down, you multiple f/4 by 1.41, which gets you f/5.6. If you cut that light in half again, you get f/8. So unlike the pizza slices where a 1/8 pizza slice is half the size of a ¼ pizza slice, an f/8 aperture lets in a quarter of the light compared to an f/4 aperture.

Now, to go the other way when you want to double the amount of light coming in, or go one full stop up, you divide by 1.41. So starting back at f/4, to go one full stop up, you divide by 1.41 to get f/2.8.

So here is a diagram that shows you the list of full f-stops from f/1 to f/22:

Full F-stops by SLR Lounge

In between the full f-stops are ½ stops and 1/3 stops. These fractional f-stops gives you more control over how much light you allow in. Here is the same diagram with the ½ stops and 1/3 stops added.

F-stops in full, 1/2, and 1/3 by SLR Lounge

Lens Speed

You may have noticed that lenses are typically labeled with an f-stop number. This is what is known as the lens speed or the maximum aperture available for that lens. A prime lens with a large aperture like f/1.8 or lower is considered a “fast lens” because it can gather a lot of light quicker than a “slow lens” that has a maximum aperture of f/4.

Some lenses, like the Voigtlander Nokton 25mm or the Leica Noctilux 50mm have a maximum aperture of f/0.95! Now that is blazing fast! Of course, you pay the high price for these lenses, with the Voigtlander Nokton retailing at $1199 and the Leica Noctilux retailing at a whopping $10,995.

Zoom lenses come in two varieties, variable maximum aperture and constant maximum aperture. Typically, it is the high-end zoom lenses like the Canon EF 24-70mm f/2.8L USM Standard Zoom Lens that have a constant max aperture, while the cheaper zoom lenses like the kit lens that comes with a DSLR like Canon EF-S 18-55mm f/3.5-5.6 IS II SLR Lens have a variable max aperture. The reason that the higher end lens with the constant max aperture is far more expensive is because the lens manufacturer has to figure out how to maintain the same max aperture without actually having to increase the diaphragm diameter.

Aperture and Exposure

So what is the point of all these f-stops? One of the reasons that you want adjustable aperture is to control exposure. If you remember from our Explaining Exposure article, we talked about how exposure is a three-way balancing act between shutter speed, aperture, and ISO speed. When one or more factors increases or decreases one way, the other factor must swing the other way in order to maintain the same exposure.

So if you slow down the shutter speed by one stop, you are doubling the amount of light that is hitting the sensor. In order to have the same exposure as before, you have to close down the aperture by one stop. Conversely, if speed up the shutter speed by one stop, then you have to open up the aperture by one stop. The same principle applies with ISO.

You can use aperture to correct for under and overexposure, as well. If your image is too dark, try opening up your aperture to let more light in. If your image is too bright, you can stop down the aperture to limit the amount of light hitting the sensor.

An easy way to remember the exposure balance is the water faucet analogy again. If you have a lot of water pouring out (large aperture), you only need a short amount of time to fill a cup (short shutter speed). If you allow less water to pour out (small aperture), it will take you longer to fill the same cup (long shutter speed).


So to recap, aperture is the opening at the end of the lens that lets light to hit the sensor. The size of this hole is determined by an adjustable set of overlapping metal blades called the diaphragm. The bigger the opening, the more light comes through at any given time.

The way aperture is measured is by f-stops, which is the ratio between the focal length of the lens and the actual diameter diaphragm opening. To double or half the amount of light coming in, you multiply or divide by a factor of √2 (approximately 1.41).

Not only can we adjust for full stops on our lenses, but we can also adjust the aperture by 1/2 stops and 1/3 stops.

Lenses are rated by their maximum aperture or lens speed. Some lenses are very fast, which means that they can open up to f/1.4 or faster to let a lot more light in.
Finally, one of the primary functions of the aperture is to control the exposure in combination with the shutter speed and ISO.

In our next article, we will go more in-depth with aperture and talk about how it influences depth of field and focus. Until next time!

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