This series of articles is excerpted from Rocky Nook’s Enthusiast’s Guide to Exposure by John Greengo covers the controls — shutter, aperture and ISO. The week, John explains what the aperture does.

Aperture is open for viewing and composing

The mechanical device in the lens that changes the size of the aperture comprises several lightweight metal blades arranged in a circular pattern. By default, the unit is fully open, with all blades rotated so that the maximum amount of light can be let through the lens. When you look through the viewfinder, you’re seeing what the image looks like with the aperture unit fully open. If you are using an aperture setting other than the maximum, the unit closes down after you press the shutter release, but before the exposure has begun. The blades move in toward each other, creating a smaller circular opening.

Construction in New York City. ©Kevin Ames

Aperture blades

The number of blades can be important—the more blades there are, the closer to a perfect circle the opening will be shaped. The shape of the opening can be important when considering flare and regions of the photo that are out of focus. The rounder the opening, the more pleasing the look of the resulting shot will be. Blades may also be shaped with a gentle curve so the opening remains round rather than taking on the shape of a polygon. The aperture unit has many predetermined settings available. The list of common, full f-stop settings is: 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32. In this sequence, f/1.0 is the largest and f/32 is the smallest. The numbers follow a pattern that describes how much larger or smaller the lens opening must be in order to double or halve the amount of light passing through it. The f-stops are a number series based on math.

Apertures, their sizes and f number

Remembering aperture values

The key to understanding the series is knowing the square root of 2, which is 1.4. However, it may be easier to remember the f-stop series if you can remember just two numbers: 1 and 1.4. The series follows a pattern in which the next number in the series is equal to double the previous number. To determine what comes after f/1.4, look to the previous f-stop (f/1.0) and double it (f/2.0). To determine the next number, double f/1.4 to get f/2.8. This results in the series 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Additional settings can be determined using this formula for either higher or lower aperture settings.

Variable Aperture

A basic zoom lens supplied with an entry-level camera might have a maximum aperture of f/3.5-5.6. This is a variable aperture lens, which means the maximum opening changes as the zoom lens focal length is adjusted. The most common variety, an 18-55mm, will have a maximum aperture of f/3.5 at the 18mm setting, and f/5.6 at the 55mm setting. You’ll still have the ability to stop the aperture down to smaller settings like f/8, f/11, and f/16 at all zoom settings, but the maximum opening will be different depending on whether you are at the 18mm or 55mm setting.

Aperture Enthusiast's Guide to Exposure Rocky Nook
A sitting area at MoMA, NYC. ©Kevin Ames

Apertures for Depth of Field

When you change the size of the aperture opening, you are not only changing the amount of light being let through the lens, you are also altering the path that light travels. The path the light takes alters the way it strikes the sensor and, as a result, changes how much of the scene will be rendered sharp and in focus. This in-focus area is defined as the depth of field. The lens can only be focused on one point at a certain distance from the camera; but by altering the depth of field, we can vary the amount of space, both in front of and behind this focus point, that also appears in focus. 13.1 Aperture settings for common lenses range from f/1.4 wide open to f/22 closed down.

Opening photo: ©Kevin Ames