Understanding Aperture and F-Stop

A lot of people ask me about where to start in film photography, or what’s my advice for beginners. One of the things that’s most important in photography (film and digital) is understanding the basics of aperture and f-stop. So for all of you who asked me questions on the subject, here’s a little 101:

Aperture is basically the diameter of the lens opening; the aperture is expressed as a fraction of focal length - that’s where the f-stop numbers come from. For instance, f/4  means the aperture equals focal length divided by 4. When we talk about a 1.8 lens or a 3.5 lens that refers to the largest aperture of the lens; so a 1.8 lens would start at f/1.8. An opening of 1.8 is bigger than 3.5, I know that doesn’t seem to make sense, but remember it’s fractions. Ansel Adams explains it better than I ever could:

“The aperture indicates the amount of light that the lens will transmit to the film. Since the aperture is expressed as a fraction of the focal length, all lenses set at f/8 (or any other aperture) transmit the same intensity of light to the film. This amount of light is proportional to the area of the lens aperture (and therefore to the square of the diameter); [an] f/4 lens… is twice the diameter of the f/8 lens, but transmits 4 times as much light.”

If that still doesn’t make sense, it’s much easier to understand if you have a lens in front of you. As you slide the f-stop down, meaning as the number gets bigger, you’ll notice the hole gets smaller.

Let’s look at the “universal” major stops:

f/1     1.4     2     2.8     4     5.6     8     11     16     22     32

In general, photographers know these by heart. If you don’t and you shoot regularly, you should memorize them. When you look at an f-stop, the numbers on either side represent either twice or half of the amount of light. For example, take f/5.6:  f/4 is twice as much light, while f/8 is half as much. Many lenses have numbers in between to represent half and third stops.

The f-stop you choose will have an effect on your depth of field; the way to control this is by using a combination of ISO (film speed) and shutter speed to provide the correct exposure while achieving the depth of field desired. I will explain more on this when I do a post on depth of field, but for now it’s important to note that larger f-stops have a smaller depth of field. Meaning, at f/1.4 you will have to use selective focus and choose either the foreground or background whereas at f/32 you would have much more of the image in focus. This is obviously a very very simple explanation, but I hope it helps some of you trying to understand the workings of film photography.