F Stop / F Number
 
fstop An “f” number indicates a ratio or proportion between the effective diameter of the lens opening (aperature) and the focal length of the lens.  It does not tell us the actual size of the lens opening as measured by a ruler. A setting of f/2 for example, tells us that the lens opening is half the focal length.  A setting of f/8 means the lens opening is only one eighth the focal length, etc. For example if the lens opening is 2.5 cm and the focal length is 5 cm the F number would be 2.5/5 or F/2.

The thing about f numbers that sometimes confuses beginners is that a high f number on the bottom, f/32 for example, indicates a small opening, while a smaller number on the bottom, such as f/4 indicates a large opening.

This confusion ought to disappear if you think of f numbers as fractions in which the numerator always is one and the denominator is the indicated number.  Thus, think of f/32 as 1/32 and f/4 as 1/4.  Obviously, 1/32 is less than 1/4 and indicates a smaller lens opening that admits less light.

How do F-Numbers Effect the Picture Taken?

They change the depth of field for the photo being taken. F numbers that are small such as F/32 make all visible objects appear to be in focus and in the same plane. Larger F numbers such as F/2 will result in photos that can only have one object in focus and all other objects in front of, or behind that appear out of focus. Look at the two examples of pictures below.

This is a picture taken with a smaller opening and an F number of  f/11
Notice all objects are in focus.
 This is a picture taken with a larger opening and an F number of  f/2.4
Notice only the plant is in focus.
large depth of field
 small depth of field


Why Does This Happen?

A smaller aperature, or lens opening will effect not only the amount of light that can enter the lens but also the angles that that light may enter from. A small aperature will limit the rays of light entering the camera to rays that are close to parrallel to each other. These nearly parrallel rays mimic those from objects that are far away. This creates the illusion that all the objects are the same distance away from the lens and therefore are all in focus. The only visual clue of their relative distances are their size or placement in the image.

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