What Is A F stop – The Short Version

One of the most mystifying and confounding aspects of photography is the F stop. What is it? What does it mean? How does it work? If you’re new to photography it’s easy to see why this is a difficult concept to deal with. I want to make the newbies feel a little better and mention the fact that there are seasoned photographers out there, and very good at what they do I might add, but have a very limited understanding of F stop if any at all.

Since the digital camera revolution, your camera does most of the thinking for you so you can take great photos without worrying about it too much. However, think of the phenomenal photos you could create with a better understanding of what F stop is. Well, I’m going to try and make it a little easier to wrap your head around.

First, lets break down the term “F stop”. The “F” stands for the focal length which is the distance, in millimeters, between the optical center of the camera lens and the your image receptor (your film or sensor) when your subject is in focus. The use of the word “stop” in photography is confusing to say the least. What most people don’t keep in mind is that the word stop can be used with multiple meanings. In photography or optical terminology the word “stop” can be a physical object.

For our discussion, a stop (the aperture) is part of the optics that controls the amount of light moving through the lens. Its made of thin pieces of plastic that move in and out to form an opening and closing circle in the center. A good analogy is the pupil in your eye. It opens and closes letting in more or less light as needed. Put together, F stop is a ratio between the focal length and the diameter of the aperture of your lens. The F stop scale on a lens usually runs like this:

1.0       1.4      2.0       2.8     4     5.6      8       11       16      22

The short version is this. Each setting lets in double or half of the amount of light as the setting next to it. The lower the number the more light that is allowed in through a larger aperture diameter.

We can prove this by doing a little math here. If you have a 35mm lens (diameter of aperture) and you set F stop 2.0 (35mm, our focal length, divided by 2.0…the size of our aperture) the diameter of your aperture is 17.5mm.

Now move up the scale. Take the same 35mm lens and bump up the F stop to 4 and see what you get. 35mm divided by 4 will give us a aperture diameter of 8.75mm which is obviously smaller than 17.5mm from the previous stop. So the F stop of 2 gives you a larger aperture diameter than an F stop of 4. Therefor the F stop of 2 lets more light into the camera.

The other setting to be used in conjunction with your F stop is your shutter speed. Your shutter speeds function similar to your F stop settings. Each shutter setting is in seconds or fractions of a second and is half or double the one next to it. So, you control the length of the exposure with the shutter speed and the amount of light coming in to hit your image receptor with the F stop.

I hope this has helped some of you folks that have been baffled by this whole F stop thing.  Understanding and learning how to use your F stop settings together with your shutter speeds will help you produce your most stunning work ever!