Aperture is the most interesting of the exposure settings. Not only does it affect the brightness of the image, but also what parts of the image are in focus.

Inside the lens is an iris, like that in the eye. The equivalent of the pupil is the aperture formed by the blades of the iris. A ring on the lens or a control on the camera allows you to vary the size of the aperture to alter the amount of light hitting the sensor.

Most things in photography are a compromise. The aperture demonstrates this perfectly. A wide aperture lets in plenty of light so you can keep your shutter speed high to take nice, sharp photos. But as the aperture gets wider the amount of the scene that is in focus reduces. This is a particular problem as you get closer to the subject. In extreme cases, the eyes of a person might be sharp but their ears blurred. For portraits this is a popular effect. But for sports or group shots it can be difficult to get everyone in focus, even when you are far away. You will often find yourself balancing shutter speed, aperture and ISO to get the image that you want.

Like shutter speed, aperture is a fraction. But instead of a fraction of one second, it is measured as a fraction of the focal length of the lens (called “f”). So a 25mm aperture in a 50mm lens would be f/2. This is written as a fraction with a stroke between the f and the two, but you just pronounce it “f 2”.

Since these values are related to f and stop the light they are called f-stops.

Typical aperture settings or “f-stops” are 2, 2.8, 4, 5.6, 8, 11 and 16

A lens might have a maximum aperture of f/2 and a minimum of f/16. f/2 is a wide aperture, half the size of the focal length of the lens, and f/16 is a narrow aperture, just 1/16th of the focal length of the lens.

The values appear strange at first glance. There is doubling between 2,4,8, and 16. But then there are these extra numbers between.

This is because we are setting the diameter of the aperture. But the amount of light going through it depends on its area, which varies as the square of the diameter. To increase the area of a hole by two we need to increase the diameter by the square root of two, about 1.4

Don’t worry too much about this. These numbers will become familiar friends over time and you can for now accept that going from 2 to 2.8 halves the amount of light passing through the lens.

A lens with a wide maximum aperture is called a fast lens. These are useful for taking pictures in low light but also help with focusing. Camera focusing systems work best when there is more light. And if you have a DSLR the image through the viewfinder will be brighter with a fast lens.

If you are changing aperture from a narrow to wide setting we say you are opening up the lens. If going from wide to narrow you are stopping down.

We mentioned that aperture affects focus, and will talk more about this in a later episode. But aperture can also affect the overall sharpness of a photo. Most lenses don’t perform at their best when the aperture is wide open. They become sharper when stopped down one or two stops.

If the aperture gets too narrow another effect called diffraction becomes important. Once you get to f/16 this physical effect will start to reduce image sharpness.

For homework see if you can observe the iris opening and closing as you change the aperture setting. You might need to hold down the shutter button halfway to see the effect.

The text of this episode is available on photographywithneil.com

That’s it for now.

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