You understand shutter speeds, apertures and ISO settings, and now you know how a meter works, but how does it all combine to give me full creative control over my photographs? In a word: stops. The more you can think in stops, the better photographer you will be.
A “stop” is defined as a doubling or halving of any value. Numerically, if I started at 8 – one “stop” in one direction would be 16, and one “stop” in the other direction would be 4. If I were to count up in stops numerically (not to be confused with aperture setting, which we’ll discuss next) from 8 I would have the following: 8, 16, 32, 64, 128, etc. Wait a minute, That sounds a lot like the shutter speeds we discussed earlier!
Shutter Speeds And Stops
If I were to list shutter speeds from 1 to 1000, they would look like this: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. Shutter speeds are set up in stops – meaning that each shutter speed is twice as fast as the speed before it, and half as fast as the setting following it – 1/60 sec is twice as fast as 1/30 sec, and half as fast as 1/125 sec.
Apertures And Stops
Apertures (sometimes referred to as “f-stops”) are set up in stops as well, although the numerical representation of apertures may be a bit more confusing. If we list apertures starting from f2.0, they would look like this: f2.0, f3.5, f4.0, f5.6, f8, f11, f16, f22, f32, etc. You might think that f8 is one stop from f4, but it’s actually 2 stops. One stop from f8 is f5.6 in one direction, and f11 in the other, meaning that f8 is half the amount of light as f5.6, and twice as much light as f11 (remember, the larger the aperture number, the smaller the opening, resulting in less light).
Combining Shutter Speeds and Apertures – Reciprocity!
Each shutter speed is a stop – and each aperture is a stop – so if we speed the shutter speed up by a stop (say 1/250 to 1/500), we can increase the amount of light through the aperture by a stop (say f-5.6 to f-4) – and we wind up with the same exposure – twice the amount of light with a shorter duration – this is reciprocity (any change in any value equals the same change in another value in the opposite direction). What this means is that for any correct exposure, there are several options, from slow shutter speeds combined with small apertures with lots of depth-of-field to fast shutter speeds combined with large apertures producing very little depth-of-field.
Exposure Compensation – See, I Didn’t Forget
Remember our discussion about metering – how the meter sees everything as “medium tone”? If your scene doesn’t average out to “medium tone” or, worse, doesn’t have any “medium tone” at all (remember that photo of the family snow man?) – you’re going to want a way to override the meter. Most modern cameras have an “exposure compensation” dial or setting allowing you to adjust the exposure depending on the scene. This is also set up in stops – so you can over or under expose (compared to what the meter wants to do) in a very controllable manner.
Let’s go back to the beach with all that bright sand and reflective water. If you trusted your camera’s meter to take a picture of Aunt Peggy, she would most likely turn out way underexposed (remember, the camera wants to make the scene look “medium tone” – which is much darker than the bright reflections and sand). If you set your exposure compensation dial to +2 (2 stops over exposed), it would correct for this underexposure. But, keep in mind, you’ll have to fiddle with it a bit depending on composition and subject (are you facing the sun? Is the sun on my subject’s face? Is she wearing a white bathing suit or a black one?)
Another way to “compensate” your exposure is to use that full-manual mode. We know that the meter wants to make everything “medium” – so meter off of an object (in the same lighting as your subject) that is medium tone, and set your exposure setting for that reading. Don’t have anything “medium”? Then meter off of something of a known tonality and adjust you settings accordingly. For example, the palm of most people’s hands are about 1 stop lighter than “medium” – so if I meter off the palm of my hand, and open up one stop, I have a good meter reading for that lighting. This is pretty convenient being that I can usually find my hand and typically have it with me. One further note – when metering off of something other than your subject, focus for your subject and don’t change the focus when metering. Changing the focus can actually effect the meter reading.