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F-number - A measure of the light intensity of a lens

Fotografie Objektiv Blendenzahl

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

Blendenvergleich

The relationship between aperture, exposure time and ISO value

In order to properly expose an image, the aperture, exposure time and ISO value must match the given lighting conditions. However, there are different combinations that all lead to the right exposure and thus give the photographer creative freedom. Shutter speed and aperture affect the amount of light that hits the sensor or film. Film sensitivity, given in ISO, determines how strongly the film reacts to light. In a digital camera, film sensitivity is simulated by amplifying the sensor's signal to a greater or lesser extent. Here, too, the specification is traditionally given as an ISO value. Doubling this value means doubling the sensitivity or halving the exposure time.

In this context, it is important to look at the somewhat difficult concept of aperture row. When it comes to exposure times, it quickly becomes clear that twice as long means twice as much light and half the time only half the amount of light. Unfortunately, this is not so easy with the aperture. Here, the factor between adjacent f-numbers is approximately 1.4 (actually, it is the square root of 2, i.e. 1.414)..

An example shows the relationship between the three parameters of exposure time, f-number and sensitivity: In sunny weather, your exposure meter suggests an exposure time of 1/500 second at aperture 8 and ISO 100. You also get the same brightness of the image with 1/1000 second (half exposure time) at aperture 5.6 (next smaller f-number) and unchanged ISO 100; or with 1/1000 second and unchanged aperture 8 at ISO 200 (double sensitivity).

Effect of aperture on depth of field

Changing the f-number can be used to design your images, for example via the depth of field. It indicates which distance range 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. at portrait shots Many photographers choose an open aperture to isolate the subject from the background. Only the face is sharp, the background is blurry and not distracting. On the other hand, if you have a subject that is staggered in the landscape, such as a tree in the foreground and a village in the background, close the aperture so that all planes are in focus. With a high aperture is then usually with the ISO sensitivity or one tripod worked, because at the same time it means lower and lower shutter speeds.

Vergleichsfoto unterschiedliche Blende

Influence of aperture and lens on exposure time

You can make clever use of the connection between aperture and exposure time if you are concerned with particularly short or long exposure times. Take pictures of fast moving cars, trains, children, animals or sports scenes, open the aperture wide to avoid motion blur. If your camera has a Sport scene mode, it will make these settings automatically. In poor light conditions at dusk, a high-speed lens helps to photograph scenes simply by hand without a tripod.n.

Canon Kamera und Objektiv

The photographer understands a fast lens to be a lens that enables the largest possible aperture opening. You will usually find this value together with the focal length. A lens marked 2.8/150 is a telephoto lens with a focal length of 150mm and a maximum aperture of 2.8. With some zoom lenses, the light intensity decreases with increasing focal length. Such lenses, for example, have a designation such as 2.8-4/24-70. The largest aperture decreases from 2.8 in the wide-angle range to 4 in the slight telephoto range.

There is no substitute for light intensity when it comes to flexibility in image design. Unfortunately, it also comes at a price. One f-stop more can more than double the price between two otherwise comparable lenses. For a bright super telephoto on ar full frame camera professionals have to spend a five-digit amount.

The shortest possible exposure time is not always important. Sometimes it is necessary to increase the exposure time by using 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 test series with your own equipment and see what the lens can do with a small or large aperture.

Photo credit:
Lenses with different apertures“ fromKoeppiK – Own work. Licensed underCC BY-SA 3.0 aboutWikimedia Commons.
Aperture comparison: Stefan Englert

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