How is an F-Stop Calculated?
While it may appear entirely random at first, calculating an f-stop will provide you with more control over the amount of light you let into your camera when you take a picture. The relationship between the f and the number is inverse. The smaller the number in the denominator, the more light is let into the camera because it is a ratio of light exposure. This is because f-stop is a ratio between the lens of a camera and the focal length setting.
Step 1 - Determine Your Lens Diameter
Your manual should be able to tell you the diameter if it is not marked on the camera itself.
Step 2 - Determine the Lens Focal Length
This too should be included in your manual for the different settings available for your camera.
Step 3 - Divide the Diameter by the Focal Length
Camera measurements tend to be provided in millimeters. For convenience, we will consider a camera with a with a 10 mm and a focal length of 2 mm. The f-stop would be f/5 because the ration is 10/2.
To increase the f-stop you will need to enlarge the focal length. Adjusting it to 5 mm will give you a ratio of 10/5 and an f-sop of f/2.