Learning Exposure the Old-Fashioned Way

Reciprocity

Earlier, you learned that a stop is a measure of light. When you double the amount of light that hits the sensor, we say that you have increased the amount of light exposure by one stop. Conversely, if you halve the light, you decrease the exposure by one stop.

Consider these shutter speeds:

Each one is double (roughly) the previous shutter speed. In other words, there is a one-stop difference in the amount of light exposure generated by each successive speed.

Now look at this list of apertures:

Because most of us aren’t familiar with calculating the area of a circle, the relationship between these numbers isn’t so obvious. But, trust me when I say that each one represents an opening that’s twice as big as the previous one. In other words, there’s a difference of one stop of light exposure between each successive aperture in this list.

When you encounter a situation where you need to balance motion stopping power with depth of field and overall illumination, you can take advantage of the fact that both shutter speed and aperture can be adjusted by the same amount in opposite directions. In other words, the two values have a reciprocal relationship.

For example, let’s say your camera recommends an exposure of 1/500th of a second at f8. Because it can’t make any creative decisions, it has no idea how much motion stopping or depth of field you might want, so it simply tries to recommend a shutter speed and aperture combination that will give you a good level of illumination, and allow for a sharp image when shooting handheld.

If you decide that you want more motion stopping power, and so increase shutter speed from 1/500th to 1/1000th (one stop), you’ll run the risk of darkening your image. But, you can open your aperture from f8 to f5.6 (one stop) to compensate for that one stop of darkening that you introduced with the shutter speed change. (Figure 6.10)

This reciprocal relationship means there are many different shutter speed/aperture combinations that yield the same overall exposure. That is, many combinations of shutter speed and aperture produce the same level of brightness in the final image. However, some combinations might produce an image with more depth of field than others, while others might yield an image that has blurrier motion than others.

Reciprocal ISOs

You’ve already learned about ISO, and how when you increase ISO, your camera’s image sensor becomes more sensitive. If your camera offers the ability to change ISO, and most do these days, then you probably will have a range of settings that goes something like this:

As should be obvious, like aperture and shutter speed, each successive ISO setting is double the previous, meaning there’s a one-stop difference between each ISO. So, if you end up in a situation where your shutter speed and aperture choices have left your scene underexposed by a stop, you can increase your ISO setting by one stop to compensate.

Some cameras offer a wider range of ISOs then what you see here. On some cameras, ISO 50 is an option, while others push the opposite end of the scale all the way up to around 25,000. We’ll learn more of the details of ISO later.

Fractional Stops

If you’re coming from the film world, and you learned to shoot on a manual camera, then you might not recognize all of the shutter speeds and aperture choices on your digital camera. In the old days, shutter speed and aperture controls used the progression of settings that we’ve looked at here, with one stop of exposure difference between each setting. Like your digital camera, the range was usually wider than what I’ve shown.

It is possible, though, to adjust shutter speed and aperture by intervals that are smaller than a whole stop. By default, your camera probably adjusts in 1/3 stop intervals. So, as you adjust the shutter speed control on your camera, you might see a progression that goes like this:

Here, the one-stop increments are in bold. The other values are increases of a third of a stop. All the same reciprocal rules apply when dealing with fractional stops. These fractional values give you a more granular, finer level of aperture control.

On most cameras, apertures also progress in third-stop increments, as do ISOs.

Summing Up

The relationship between shutter speed, aperture, and ISO setting can be confusing to new photographers. Hopefully, the chart below will clarify some of the information presented here.

Remember, if you move any one of these parameters in one direction, you must move one of the others in the opposite direction to maintain equivalent exposure, because all three parameters are reciprocally related. Of course, you can choose to move one parameter or another without moving any others to create an intentional over- or underexposure. Later, we’ll learn why you might want to do this.

In the very old days, before light meters, photographers had to calculate exposure by hand (or rather, by eye). A lot of experience was necessary to properly assess exposure for a complicated scene, but there were a number of tricks that eased the process. Switch your camera to manual mode, and be sure you know how to alter shutter speed and aperture in manual mode, and you can give this type of shooting a try.

In most cases, when shooting in bright sunlight, you can use the “sunny 16 rule” for calculating exposure. Set aperture on f16, and use a shutter speed equal to 1 over your ISO. In other words, if you’re shooting at ISO 100, you would set aperture to f16, and shutter speed to 1/100th of a second. If you’re shooting at ISO 400 you would set aperture to f16, and shutter speed to 1/400th. Those settings should produce an image with correct brightness when shooting in bright sunlight.

The sunny 16 rule gives you a good baseline for calculating an exposure. From this baseline, you can think about how to over- or under-expose to compensate for lighting situations that aren’t completely sunny, or for times when you need to alter an exposure to improve tone.

For example, say it’s a sunny day, but you’re shooting in shade using ISO 100, and you think it might be shady enough to reduce the overall brightness of the scene by one stop. How do you know it’s one stop? After a while, you would know this by experience. When you’re starting out, you have to guess. Because your scene is one stop darker than bright sunlight, you need to overexpose.

Underexposure means you need more light. If you slow your shutter speed down by one stop, you’d get more light, but then you’d be shooting at 1/50th of a second, which might be too slow for handheld shooting. (Remember, because of the sunny 16 rule, at ISO 100 your baseline shutter speed is 1/100th.) So, a better approach might be to open your aperture wider. As you know, lower f numbers mean a wider aperture, so you could switch from f16 to f11, a widening of one stop (consult the list of apertures that you saw in Chapter 6 if you’re unsure as to what a 1-stop aperture change is).

Another option would be to leave shutter speed and aperture where they are, and increase ISO to 200.

Now let’s complicate things further and say that you’re shooting in shade that’s one stop darker than bright sun, but you want shallow depth of field. Because you need to overexpose by one stop, you’ve already decided to widen your aperture, so you’re shooting at 1/100th of a second at f11. But you want an even wider aperture, to ensure shallow depth of field. If you just keep opening the aperture, you’ll continue to brighten the exposure. So, as you widen the aperture, you want to make a corresponding shutter speed adjustment. Changing from f11 to f8 opens the aperture one more stop. To preserve your overall exposure, change your shutter speed from 1/100th to 1/200th. Opening from f8 to f5.6 gets your aperture one stop wider, and so you move your shutter speed to 1/400th. Finally, your lens can go one stop wider, so you open to f4, and make a corresponding shutter speed shift to 1/800th. Now you’ve got a wide aperture for shallow depth of field, and you’ve preserved your one stop of overexposure to compensate for the shade.

Trying a few shots this way in manual mode gets you deep into the interrelationship between shutter speed, aperture, and ISO, and can be good practice. Fortunately, for everyday shooting, you light meter does all of this thinking for you.