F-stop is the focal length divided by the diameter of the lens. For example, a 200mm f/4 lens will be 50mm wide. Get out your ruler and measure it. 200mm/50mm = f/4. That is why f-stop is typically written as F/4, meaning "focal-length over 4" or "focal-length divided by four".
Where do those numbers come from?
Lenses are marked with a series of f-stops, each one lets in half as much light as the previous one. The light-gathering ability of a lens is determined by its area, and f-stops are determined by diameter. Area is related to diameter squared. The progression of f-stops, 1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32, are powers of the square root of 2.
For a further explanation of f-stops, try this.
Taking the nature photographs like those in my books about Rocky Mountain National Park, Wildflowers and Wildlife require an assortment of lenses depending on the subject. Typically normal to wide angle lenses are used for landscape photos and very long telephotos are used for wildlife. The photographs below show the views of a single scene taken using these lenses.
These eight photos were taken from the same place with different lenses. The subject is Longs Peak from Upper Beaver Meadows in Rocky Mountain National Park.
"I want to take a bluebird picture. What lens should I use?"
It is a symptom of bird photography that your lenses never seem to be
long enough. This is because birds are so small.
When photographing large animals, a 400mm gives you decent image size
from a reasonable distance. But remember, an elk is six or seven feet tall.
A bird is barely six inches long, so when your subject is twelve times smaller,
you have to be twelve times closer.
As you can see, even a relatively huge 600mm lens at the relatively close
distance of 20 feet doesn't give the huge magnification you might expect.
The field of view is about twelve inches.
It takes a big lens to photograph a small bird.
© Copyright David Dahms