By Gordie Elwell
When you discuss one of your images with a friend, and they say, “You might want to adjust your exposure by a stop or two,” what do they mean? It is often heard in photography circles, referring to a “stop” of exposure. One of our main adjustments, and one corner of the exposure triangle, is called an “f/stop”. You never hear anyone discuss an “ISO stop” or a “shutter speed stop,” yet these other corners of the exposure triangle can be adjusted by a “stop” just as easily as the aperture “f/stop” setting can. So, what exactly is this whole “stop” thing all about, anyway?
Let’s get the definition out of the way right away. A “stop” in photography is a way of changing how much light is recorded by your sensor in a given photograph. One “stop” is either twice as much light, or ½ as much light (depending on which way you are adjusting your exposure). We all remember the exposure triangle… Shutter Speed… Aperture… and ISO. Any one of these (or a combination of them) can be adjusted to adjust your exposure.
Let’s look at the values. Shutter speed and ISO are fairly easy, since their scales are linear… but aperture setting (f/stops ironically) have a different system. Aperture is based on the area of your (circular) lens, so unless you want to carry around a calculator with you, it is best just to memorize these figures. (I am referring to full stops in this article, but in practice you can make adjustments of less than a full stop, generally in 1/3 stop increments). Shutter speed is generally in seconds. Let’s start at 1/2000th of a second. Half that is 1/1000th, then 1/500th, 1/250th … etc. Each change doubles the length of time your shutter is open (and thus doubles the amount of light) as the previous one. Similarly, ISO is a linear scale. Many of us shoot at ISO100. For twice the sensitivity, we move up to ISO 200 . . . then ISO 400, then ISO 800, and so on. Each change doubles the sensor’s sensitivity from the previous setting.
Now for aperture. I will start at f/2.0 (realizing there are wider f/stops below that). After f/2.0, the next full “stop” is f/2.8. Then f/4.0. Then f/5.6, then f/8.0, then f/11, then f/16, then f/22, then f/32 (and more above that). So, your memory challenge is to memorize 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32. So to change your exposure one stop using aperture, you would move from 4.0 to 5.6, for example. This would reduce your light by ½. (Another anomaly for we photographers here… with aperture, the smaller the number, the more light is allowed in) Those are lots of numbers to memorize, but here’s a little trick that makes it easier (for me at least). The aperture numbers double for the second previous value. Notice that f/4.0 is twice f/2.0, the second previous value… with just one stop in between. Notice that f/16 has twice the numerical value as f/8.0, again the second previous value with just one stop in between. So, if you prefer, just remember f/2.0 and f/2.8. Then double the second previous stop value to give you the stream of full f/stops.
To make things easier on we photographers, most cameras have an exposure adjustment meter you can reference when changing your settings. These vary from camera make and models, but generally provide you with the same information, if displayed slightly differently. These “over/under” scales (as I call them) generally look something like this:
Notice that they are scaled by “stops”, displaying what your camera meter is measuring for the scene. The bar (or arrow) will show your exposure based on your camera’s metering of the scene. If you have your camera set with any “auto” exposure settings (like aperture priority, or “auto ISO” for example, this scale will generally show a centered reading). When you start using Manual Mode, or want to override your camera’s best metering judgement, you can change one of your settings and watch this display to determine if you are over or under exposing what your camera’s meter reading of the scene is.
A Scenario. You and a buddy are out shooting some landscape images. Overcast, and towards the end of the day. You take a test shot at ISO200, f/11 and 1/125th of a second. When you look at the screen, you notice that you have quite a dark image. What can you adjust and why? You decide you want to add 3 stops of light, so you think to adjust aperture by 3 stops. f/4.0 will be 3 stops wider. Is this a good plan? Perhaps not if you are counting on that higher f/stop to maximize your depth of field. Well then, what about shutter speed? You can slow it down by 3 stops, or 1/15th of a second. You check your back pocket, and sure enough, no tripod. Not a great option. Well then, how about ISO? You can bump up the ISO to 1600 to get your three stops. You are concerned that this setting is really pushing it for the quality of images at high ISOs for your camera. So, after some head scratching, you decide that you can change each corner of the exposure triangle by one stop, and get a net change of three stops overall. You move the aperture from f/11 to f/8.0; your ISO from 200 to 400; and your shutter speed from 1/125th to 1/60th. Now you have your net three stop change, and minimally affected the characteristics of each of the exposure triangle corners.
As a side note, when out shooting in a very bright environment, you may find that you need to use a “Neutral Density” filter. There are many aspects of ND filters that you need to consider when using one, but just be aware these filters have a ND scale in “stops”. So you can reduce the light coming into your lens by three “stops” for example, when using a “3-stop” ND filter. These usually range from 1 or 2 stops to 10 stops. You would use these, for example, if you were using a tripod and wanted to shoot rushing water or crashing waves. With a “10 stop” filter, your exposure would go from 1/60th of a second to 15 full seconds. This would give you the smooth and silky white water we often see pro photographers display in fine art photography.
So, in summary, when your CCB crowd starts talking about adjusting your exposure by some number of stops, you will be “in the know” now . . . they are talking about each stop adjustment being twice or ½ the amount of light.