Aperture – Measure of a lens's light-gathering ability

Strictly speaking, the aperture is nothing more than a mostly mechanical device within the lens. Normally, the aperture is fully open so that, for example, a bright viewfinder image is produced in a SLR camera. The photographer sets how far the aperture should close at the moment of exposure. The set value is the f-number, which is usually referred to as the aperture. A large f-number (e.g. 22) corresponds to a very small opening, while a small f-number (e.g. 2.8) stands for a wide opening.

The relationship between aperture, shutter speed, and ISO value

To expose an image correctly, aperture, shutter speed, and ISO value must match the given lighting conditions. However, there are various combinations that all lead to correct exposure and thus give the photographer creative freedom. Shutter speed and aperture influence the amount of light that hits the sensor or film. The film sensitivity, specified in ISO, determines how strongly the film reacts to light. In a digital camera, film sensitivity is simulated by amplifying the sensor's signal more or less. The value is also traditionally given as an ISO value here. Doubling this value means doubling the sensitivity or halving the shutter speed.

In this context, it is important to look at the somewhat difficult concept of the aperture series. With shutter speeds, it is quickly clear that twice as long a time means twice as much light, and half the time means only half the amount of light. With aperture, unfortunately, it is not that simple. Here, the factor between adjacent f-numbers is approximately 1.4 (strictly speaking, it is the square root of 2, i.e. 1.414…).

An example illustrates the relationship between the three parameters: shutter speed, f-number, and sensitivity: Your light meter suggests a shutter speed of 1/500 second at f/8 and ISO 100 in sunny weather. You will get the same image brightness with 1/1000 second (half shutter speed) at f/5.6 (next smaller f-number) and unchanged ISO 100; or with 1/1000 second and unchanged f/8 at ISO 200 (double sensitivity).

Influence of aperture on depth of field

You can use the change in f-number to compose your images, for example, through depth of field. This indicates which range of distances in your photo is perceived as sharp by the viewer. The smaller the f-number (i.e. the larger the aperture opening), the shallower the depth of field. For portrait shots, many photographers choose a wide aperture to separate the subject from the background. Only the face is sharp, the background blurs and does not distract. If, on the other hand, you have a staggered motif in the landscape, for example, a tree in the foreground and a village behind it, you close the aperture so that all image planes are sharp. With a high aperture, you usually work with ISO sensitivity or a tripod, as this always means shorter shutter speeds at the same time.

Influence of aperture and lens on shutter speed

You cleverly use the relationship between aperture and shutter speed when you are aiming for particularly short or long times. If you photograph fast-moving cars, trains, children, animals, or sports scenes, open the aperture wide to avoid motion blur. If your camera has a "Sports" scene mode, it will automatically make these settings. In poor lighting conditions at dusk, a fast lens helps to photograph scenes handheld without a tripod.

A fast lens, to a photographer, means a lens that allows the largest possible aperture opening. You usually find this value along with the focal length. A lens with the designation 2.8/150 is a telephoto lens with a 150 mm focal length and a maximum aperture of 2.8. With some zoom lenses, the aperture decreases with increasing focal length. Such lenses might have a designation like 2.8-4/24-70. This means the largest aperture decreases from 2.8 in the wide-angle range to 4 in the slight telephoto range.

Aperture speed is irreplaceable for flexibility in image composition. Unfortunately, it also comes at a price – one stop more can more than double the price between two otherwise comparable lenses. Professionals have to spend a five-figure sum for a fast super telephoto lens on a full-frame camera.

It is not always about the shortest possible shutter speed. Sometimes it is necessary to extend the shutter speed by a small aperture opening, for example, to depict flowing water or other movements. However, you cannot make an aperture opening arbitrarily small, because the quality decreases significantly due to so-called diffraction effects. It is best to start a small series of tests with your own equipment and see what the lens can achieve with a small or large aperture.

Image credits:
"Lenses with different apetures" by KoeppiK – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
Aperture comparison: Stefan Englert