What is an F-Stop?

Photography has a language all of its own and can leave those just starting out with lots of questions. One such question that people seem to be embarrassed to ask is “what is an f-stop”? Whilst there is quite a technical answer to this, it’s probably not an answer that will help you as a photographer. What I think people are really asking is for someone to explain what all these settings mean and how can they be used when taking a picture.

F-Stops and Apertures

If you want to know what is an f-stop you first need to know what an aperture is. Put simply it’s an opening in the lens which allows light to pass through to the cameras sensor. In some simple cameras the aperture is fixed but in more complex cameras it’s possible to make the aperture larger or smaller. When the aperture is large it allows more light to pass and reach the cameras sensor than if a small aperture were used.

The f-stop is the measurement of how much light is allowed to pass through the lens and is a standard unit of measure. A lens set to f/8.0 on one camera will allow the same amount of light to pass as another camera and lens which is also set to f/8.0.

The point that sometimes confuses people is that the smaller the f-stop number, the wider the aperture. A lens with an f-stop of f/1.4 will allow much more light to pass than if the f-stop were f/16.0. You can see this in the diagram below.

How aperture controls the amount of light in photography

How aperture controls the amount of light in photography

Controlling Exposure

Being able to control the amount of light reaching the camera sensor is one of the tools we have to control the exposure of the image. Controlling the exposure is all about ensuring the image is not too light or too dark. But the aperture is not the only control we have at our disposal. The other is the shutter speed and it’s the relationship between the aperture and shutter speed that controls the amount of light reaching the camera sensor.

One benefit of being able to use a large aperture is that more light can reach the sensor and so a faster shutter speed is possible. Faster shutter speeds are often desirable as they help reduce the chance of camera shake ruining the photograph.

You will sometimes hear a lens being referred to as a fast lens. Such lenses tend to have a large maximum aperture size such as f/1.8 or f/1.2. The reason these are referred to as being fast lenses is that the aperture can be opened wider so allowing faster shutter speeds.

Creative Camera Control

Aperture size is not just about controlling exposure though. There is a second purpose of the aperture which is to control something called the depth of field. Depth of field is the area in the image that appears to be in focus. When we use a large aperture the depth of field is much less than if we had used a small aperture (there are other variables that affect depth of field but for this tutorial we will ignore these).

At times it can be desirable to limit the depth of field as it helps focus the viewer’s attention on the subject of the picture. Equally there are times when having a large depth of field is desirable. You can see a couple of examples below.

Shallow depth of field used to focus attention

Shallow depth of field used to focus attention

Large depth of field used to achieve front to back sharpness

Large depth of field used to achieve front to back sharpness

How you chose to use depth of field is one of the creative decisions you as a photographer will need to make in each of your photographs. It’s the aperture that allows you to control this and it’s the f-stop that allows you to control the aperture.

For a more detailed explanation of the aperture, shutter speed, exposure and creative photography, you might like to consider my book “Beginning Photography the Right Way”.


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