Assignment 14: F-Stops

This activity, is an introduction to an important concept in manual photography, F-Stops. After you have read the following excerpt of an article by Matthew Cole, use the information contained to create a set of notes in a format that you like. You can use outline, Cornell, or a mind-map. Whatever would work for you. make your notes digitally in Microsoft Word. Then, write a one paragraph summary based on your notes. The summary should explain F-Stops and that they describe.

Quick Guide to F-Stops

by Matthew Cole

Photographers set their exposure using a combination of shutter speeds and f/stops to get the correct amount of light on the sensor (or film). The shutter speed regulates how long the sensor is exposed to light coming through the lens. The f/stop regulates how much light is allowed through the lens by varying the size of the hole the light comes through. For any given film speed (ISO) and lighting combination there is one correct amount of light to properly expose the image. This amount of light can be achieved with many different combinations of f/stops and shutter speeds.

Back in the days of film, that was pretty much it for exposure because your ISO (and white balance too) was set by your film choice and you couldn't vary it within the roll. That's changed. Now you can switch your ISO from shot to shot, changing the amount of light needed to correctly expose your photo. This has some interesting implications I'll address further along on this page.

Despite being one of the exposure controls in photography, the f/stop (or aperture, the terms are interchangeable) remains a source of confusion and mystery to many photographers, even to some who use it all the time. The local camera shop has pictures under glass on the counter showing a scene using a range of focal lengths (for a good example of this, see my friend Dave Dahms' Lens Focal Length Chart), photos showing the same scene printed at different sizes and photos showing an action scene shot at different shutter speeds. All that is assumed to be of interest and comprehension to the customers. What they don't have is a set of photos showing depth of field, or a scene shot at a range of exposure combinations where the f/stop's effects are shown. Maybe it just takes too much explanation. Well, too much explanation is what this page is all about, and I'll go over the f/stop and especially its initially-confusing numbering at some length.

Fill That Bucket!

My favorite analogy for exposure is filling a bucket with water. A bucket is of fixed size and needs a certain amount of water to fill it, just like the sensor in your camera, which is of a set sensitivity (the ISO) and needs a certain amount of light to optimally capture an image. To fill a bucket, you can pour a small stream of water for a long time or a fast stream of water for a short time. Either way, you end up with the same amount of water. In photography, the size of the stream of the water is analogous to the f/stop, the length of time you pour is analogous to the shutter speed, and the size of the bucket is analogous to the ISO. Broadly speaking, from the bucket's point of view, it doesn't matter which combination of stream size and length of time you choose as long as the right amount of water ends up coming in. Photography is the same; within limits, your camera is indifferent as to the combination of time and amount of light as long as the right amount of light eventually arrives.

Shutter Speeds

Shutter speeds are a bit easier to understand, so I'll start with those. Both exposure controls run through a sequence of settings which involve doubling and halving the amount of light reaching the film. Shutter speeds are measured in seconds and fractions of a second and so the doubling and halving is fairly self-evident. One quarter second is half as long as one-half second but is twice as long as one-eighth. One second is twice as long as half a second and half as long as 2 seconds. It's pretty easy, and this works through the whole sequence of shutter speeds. On my old film Nikon FE, for instance, the shutter speed sequence is:

8 seconds    4 seconds    2 seconds    1 second    1/2 second    1/4    1/8    1/15    1/30    1/60    1/125    1/250    1/500    1/1000
Each of these settings is clearly half/double the length of time of its immediate neighbors (OK, I know, 1/15th isn't exactly half the time of 1/8th and 1/125th isn't exactly half the time of 1/60th, but it's close). This doubling/halving is thus pretty simple to comprehend for this exposure control.

f/stops are a bit more confusing because the numbers appear so arbitrary. This is the standard sequence of f/stops from f/1.4 to f/22. Although it may not seem intuitive at first, in this sequence the f/1.4 setting lets in the most light while the f/22 setting lets in the least. Also, each of these f/stops has precisely the same halving/doubling relationship as the shutter speed sequence.
1.4    2.0    2.8    4    5.6    8    11    16    22
On the face of it, going from f/4 to f/5.6 doesn't sound like halving the amount of light. What's more, 5.6 is a larger number and sounds like it ought to be more light, not less. Neither does f/4 to f/2.8 sound like doubling the amount of light. In fact, each of the numbers in this sequence is a halving/doubling of the amount of light from its immediate neighbors, just like the shutter speed settings are

Memorizing F-Stops

The best way to memorize the F-Stop sequence is to repeat it as often as you can. This graphic can help you visualize the different sizes and corresponding numbers.