What are f/stops?

A couple of weeks ago I ran across an instance where I was talking with someone in a professional capacity and the topic was the exposure of an image. Unfortunately this person was not properly informed as to what an f/stop really was. Many people, especially those new to photography and DSLRs believe that the each increment of aperture on their camera is an f/stop. 99.9% of the time that is not true. Let me shed a little light on what is an f/stop.

First if all, I won’t go into detail about how aperture is calculated, just realize that basically it is the ratio between the focal length of the lens and the diameter of the aperture. (For a more detailed explanation check out this website – A Tedious Explanation of the f/stop). This ratio determines how much light the lens transmits to the sensor or film. You can get bogged down in all kinds of physics about light and lens theory, but in a nutshell that’s all you really need to know.

Here are the full f/stops – f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64.

Contrary to what you might think, the smaller the number, the larger the aperture. For instance, a lens at f/2.8 lets in more light than a lens set at f/16. Here’s an interesting thing about f/stops, when you go down in f/stops, the amount of light reaching the film/sensor drops by half. So, when you change the aperture from f/4 to f/5.6 the amount of light drops my half. By the same token, when you go from f/4 to f/5.6, you double the amount of light that this the film/sensor. Crazy huh?

So now you may be asking, “when I look at my camera, it shows more numbers than appear on your list of f/stops. Why?”. Well, most modern camera/lens combinations allow you tweak your exposure to a greater extent than full f/stops permits. In fact, most cameras allow you select the increment using either 1/3 or 1/2 of an f/stop. Using our example (f/4 to f/5.6) you may see f/4.5 and f/5 (if using 1/3 stops). As you would expect the more increments between full f/stops gives you more precision in regards to your exposure.

Ok, I hope this helps clear up some of the confusion about what f/stops are and how they are used.

Thanks for reading and take some time to explore the rest of my website and blog.

-Ken.