Aperture...

What exactly is aperture?
The aperture is a circular hole in the lens that regulates the amount of light that will pass through the shutter onto the film. Aperture is also referred to as an f-stop.

The following is a list of common F-Stops.

Some cameras have F-Stops between these intervals. Take a look at every other number. Notice how almost every preceding number can be divided by two to find the next number? example: 32/2= 16 16/2= 8 8/2= 4 4/2= 2

Lensmakers calculate f-numbers by dividing lens focal length (F) by aperture size (d). The formula is:

f-number= F/d

ex: f-4= 50mm/12.5mm