What is an f/stop?

This is a frequent question asked on our safaris. The following should help you understand the relationship between f/stops and shutter speeds.

The f/stop determines how far the aperture of the lens will open to let light in. Think of f/stops as fractions 1/4, 1/16. Camera manufacturers only put the bottom number on the lens or f/stop ring. Setting the f/stop at f/4 will result in a large aperture opening; f/16 will be smaller and thus let in less light. Remember, the larger the number, the smaller the opening.

To determine what f/stop to use, you first need to determine your shutter speed. The shutter speed will control how long the lens will stay open and is also depicted as a fraction.
A camera set at 1/60 at f/16 lets the same amount of light in as a camera set at 1/1000 at f/4. A shutter speed of 1/60 of a second is relatively slow compared to 1/1000 of a second

As the shutter speed increases and lets in less light, the f/stop has to open up more, to let in more light. All the exposure settings below will let in the same amount of light.

1/60 f/16 good for greater depth of focus
1/125 f/11
1/250 f/8
1/500 f/5.6
1/1000 f/4 good for faster subjects

Choosing an f/stop and shutter speed will control your depth of field and subject motion.

1/60 at f/16 will give you an ample amount of depth of field. The higher the f/stop number the more depth of field you will have which will result in more of your photo being in focus (ie, foreground and background).

1/1000 at f/4 will create less depth of field (focus) and stop action better creating a blurred background and only the subject being in focus.

This technique is used in most sports magazines where the main subject is sharp because of a fast shutter speed (i.e. 1/1000 or faster) and the background is blurred due to a large aperture (f/2.8 to f/4).

In contrast, a landscape photographer will use a large f/stop number (i.e. f16 or 22 or even f32) to get the maximum depth of focus so the foreground is in focus as well as any distant subjects.

Here is an excellent online tutorial with a short quiz at the end.

http://www.silverlight.co.uk/tutorials/compose_expose/exposure.html