what is the f-stop

What is the F-Stop setting

An aperture of a lens is the opening which lets light into the camera. The way you measure the size of the aperture is by using what’s known as the ‘f-stop’ setting. This can be adjusted according to the light conditions; made wider to let in more light, or smaller to let in less light. The Aperture also controls how the picture looks, as it effects the depth of field, or how much of the image is in focus.


Viewing this lesson in Virtual Reality

As part of my ‘Basics of Using a Camera‘ workshop, I have filmed this lesson to answer the question of ‘what is the f-stop?’ in a virtual reality 360 degree format which is best viewed using a virtual reality headset like the Oculus Quest. However you may also use a cheaper headset like a google cardboard with a smartphone mounted inside, or failing that you can also simply use a smart phone, or your mouse to click and drag through the video on a screen.

For best viewing make sure you select the highest quality of stream.

Click the play symbol to start

What is the F-Stop

The f-number, or ‘focal-stop’, represents the ratio of focal length divided by diameter of the pupil. The equation looks like this: a = f/D, or aperture = focal length divided by diameter of pupil. The smaller the number, the wider the aperture. Depending on the camera and the focal length of the lens, the f-stop numbers you might find on the lens may be something like this –

The widest aperture on this chart is f/2, the smallest is f/16. The difference between each aperture is known as a ‘stop’, and relative to each ‘stop’ either halves or doubles the amount of light allowed to reach the film or sensor of a camera. This means that for example, changing the f-stop from f/8 to f/11 will halve the amount of light entering the camera, stopping down again to f/16 will halve the amount of light again.

The widest aperture available on any lens is known as the speed of the lens; it is a useful feature of a lens to have a relatively wide maximum aperture as then the camera is more versatile in lower light conditions.

What is Depth of Field

The ‘depth of field’ refers to the amount of an image that is in focus. When much of an image, foreground, middle ground and distant ground is in focus it is said to have a ‘long depth of field’. If some parts of an image are out of focus, for example, the background and the foreground, leaving only the middle ground in focus, then the image is said to have a ‘short depth of field’.

An Aperture controls depth of field as follows:

Examples of a shallow depth of field

Here are some examples of images with a shallow depth of field – or a low f-stop number. You will notice that only the focal point, or the ‘subject’ of the image is in focus, where as the foreground and background are out of focus. This helps a photographer draw attention to the subject of their image.

Examples of long depth of field

Here are some examples of images with a long or ‘deep’ depth of field – or a high f-stop number. Notice that most of the image is in focus, the foreground, midground and background all retain their fidelity. This is useful when attempting to portray a scene like a landscape.

So what is the F-Stop? Conclusion

The F-stop setting on your camera allows you to change the apreture of your lens. This effects the depth of field in your image and also the amount of light entering your camera. The lower the f-stop number, the more light and less depth of field you have, and the higher the f-stop number, the less light and more depth of field you create.