Photography Basics: The Aperture

The aperture is a device built into the lens that controls the amount of light reaching the sensor. Large aperture numbers mean small amounts of light, and small aperture numbers mean that a lot of light falls on the sensor. The aperture has a significant influence on the creative tool of depth of field. The larger the f-number, the greater the depth of field and vice versa.

The aperture – what is that?

When taking a photo, there are three main parameters that affect the photo. The shutter speed, the ISO value and the aperture. The aperture is a device built into the lens that controls the amount of light coming in. The measure of the aperture is given as the f-number. It is the ratio of the focal length to the aperture (cross-sectional area through which the light passes) of the lens. Large f-numbers stand for small apertures and small f-numbers for large apertures (you can remember: the larger the f-number, the more “aperture there is”, the smaller the aperture and the less light passes through).

An f-number of 2.8 therefore means a large aperture and correspondingly much light incidence, an f-number of 16 exactly the opposite. The f-number on lenses is always indicated in the form 1:f-stop. So if it says 1:2.8 on a 100 mm lens, it means that you can set a minimum f-number of 2.8 for a 100 mm focal length. Mostly the aperture is built as a so-called iris diaphragm. Circularly arranged blades are shifted against each other in such a way that the incident light beam becomes either smaller (large f-number) or larger (small f-number).

Interplay between aperture and shutter speed

If you arrange the different f-numbers in order of size, you get the f-stop series: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32. Along this series, the amount of incident light is halved with each step. So at an f-number of 5.6, only half as much light hits the sensor as at f-number 4. The value of the shutter speed must therefore be adjusted accordingly: the larger the f-number, the slower the shutter speed (at a constant ISO value). Common usage in photography usually uses the word aperture to refer to the opening of the aperture. “Large aperture” in technical jargon therefore means large aperture opening. This can easily lead to confusion, because a large f-number correctly corresponds to a small aperture.

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The aperture – What can I do with it?

The most important creative tool over which aperture has a great influence is depth of field. Depth of field is a measure of how far the area in focus extends into the photograph. For example, only the foreground may be in focus, while the background is perceived as a blurry, possibly colored area. This option of image composition would be used for a portrait, for example (this is also called “cropping”).

On the other hand, if you want to photograph a beautiful landscape, let’s say a stream, then you want that stream to be in focus as deep into the photo as possible, so you want to achieve the greatest depth of field possible.

What you need to do is to set the aperture correctly! Small f-numbers mean shallow depth of field, large f-numbers mean large depth of field. So for our portrait above you would choose a very small f-number (1 or 1.4), for the course of the stream rather 11 or even 16 if the light conditions allow it; because as I said, the larger the f-number, the longer you have to expose. So a large depth of field usually requires the use of a tripod. The correspondingly longer shutter speeds also lead to motion blur worth seeing, especially with flowing water.

The aperture is one of the three basic parameters with which you can influence the image composition. It controls the amount of light that falls on the sensor and the size of the light beam. It is the most important control when varying the depth of field.

Important parts of the camera: The aperture