Manual Exposure

Recently on photo.net I read yet another discussion forum thread on the question of manual versus auto exposure modes. Usually these threads are a newbie asking what mode to use, and the answers from seasoned pros vary from "who cares, use whatever you like" to "I always use manual because that's what I learned in the Stone Age" to "unless you use manual mode you are a mere snapshooter, not a photographer." (Those are paraphrases.)

The vast, vast majority of people in these discussions are missing the point, which is this: Modern cameras have meters. And except for people who really did learn photography in the Stone Age and are very comfortable with their skills, people use them. We rely on these electronic devices to measure light, because the perceptions of light levels we're fed by our eyes and brains are just not accurate enough to set a digital camera by. The only difference between manual exposure and non-manual exposure is the interface by which we choose to communicate our desires to the camera.

I agree somewhat with the sentiment that a "real photographer" is distinguished from a "mere shapshooter" partially by the fact that she or he pays attention to the technical aspects of what is going on rather than just aiming the camera and pressing the shutter button. I take issue, though, with the way people apply this notion. Stone Age photographers think understanding what's going on means knowing what aperture and shutter speed you want and telling the camera to use them. Digital Age photographers understand, or should understand, that it really means knowing as much as you can about how your camera works, and using that knowledge to make it do what you want in an efficient and reliable way.

In my efforts at learning digital photography, I have discovered two things: first, all exposure modes are equivalent; and second, all exposure modes are not the same. After elaborating on the former at some length, I will explain the latter.

In the digital era, cameras have three canonical exposure parameters: aperture, shutter speed, and ISO. And pretty much every camera that has a Manual mode (in which the photographer dials in all three of these) also has a Shutter priority mode (labeled S or Tv on the mode dial), an Aperture priority mode (A or Av) and a Program mode (P). The choices you get besides these will differ between cameras, so never mind them. There are two big things that many people seem to miss: First, in every single one of these modes, including Manual, you are probably relying on the camera's meter to almost exactly the same extent. Second, on many if not most digital SLRs, in every single one of these modes you have independent control over all three of the canonical exposure parameters.

I'll elaborate that second one first. In all four modes (M,S,A, and P) you get to set the ISO. And in each of the four modes (with the possible exception of P on some cameras) you have two other inputs that control the exposure. So you have three parameters that need setting, and you have three degrees of freedom in your control of the camera. This means that within certain limits, you can always get the ISO, shutter speed and aperture you want.

In M mode, you set aperture and shutter speed.

In S mode, you set shutter speed and exposure compensation.

In A mode, you set aperture and exposure compensation.

In P mode (on many or most camera bodies), you set exposure compensation and program shift.

So suppose you somehow know that you want to take your next shot at ISO 400, 1/250s at f/5.6. Set your camera to ISO 400, and then you have four choices. Go into M mode and set the shutter speed to 1/250 and the aperture to f/5.6; or go into S mode and set the shutter speed to 1/250 and adjust the exposure compensation until the camera chooses f/5.6 for the aperture; or go into A mode, set the aperture to f/5.6 and adjust the exposure compensation until the camera chooses 1/250 for the shutter speed. In P mode it's a bit harder to describe how to get what you want, but trust me, if you move the exposure compensation and program shift controls around enough you'll eventually be able to get the camera to choose both 1/250 and f/5.6 as long as this is not too radically different from what the camera would have chosen in Auto.

Therefore, if you happen to know what aperture and shutter speed you want, you can get them in any of the four modes. Of course, human beings almost never know what aperture and shutter speed they really want, because we're bad at measuring light levels by eye. So under most normal circumstances you're going to let the camera's light meter measure the amount of light in the scene for you.

Assuming you trust the meter, what do you do? If you're in P mode, with the exposure compensation set to zero, the camera will measure the amount of light coming through the lens and choose an aperture and a shutter speed for you. If you're a casual snapshooter or are in a hurry, you can just press the button. On the other hand, if you take the time to look at the parameters the camera has chosen and don't approve of them, you can use program shift to override them. But notice: if you keep the ISO the same, then for any given aperture there will be at most one shutter speed that gives the correct exposure, and vice versa. If you want a particular f-stop, you watch the aperture as you turn the program shift dial; when the f-stop you want is displayed, you take the picture and the camera uses the right shutter speed. Similarly, if you want a particular shutter speed, you watch the shutter speed display as you turn the dial, and when you see the number you want, you take the picture. What if you choose the shutter speed you want and then notice the camera has chosen an aperture you don't like? Well, tough luck -- that's the aperture you need to use in order to let in the right amount of light for the brightness of the scene as measured by the meter (which, for now, we assume you trust), unless you change the ISO. So you may end up looking for a compromise, using a slower shutter speed than you thought you wanted in order to get enough depth of field, or a larger aperture than you thought you wanted in order to freeze motion, or whatever.

What happens in A or S mode? The exact same thing! If you're in A mode, you adjust the aperture directly, and as you do so, the camera follows you by changing the shutter speed in the opposite direction to keep the overall exposure in line with the brightness of the scene. So if there's a particular f-stop you want, you adjust the aperture until you see it, and if the corresponding shutter speed is okay with you, you take the picture. Conversely, if there's a particular shutter speed you want, you can just watch the shutter speed display while you adjust the aperture, and when you see the speed you want, if the aperture you had to set in order to get it is okay with you, you take the picture. If you set one parameter and the camera chooses a value for the other that you don't like, you look for a compromise. In other words, the aperture dial in A mode does the exact same thing as the program shift dial in P mode. The same is true of the shutter speed dial in S mode.

Now for M mode. M mode is "manual" in the sense that after you've set one parameter (aperture or shutter speed) it's also your responsibility to set the other one. Naturally, you use the meter to do this. Maybe you think you know what f-stop you want, so you dial it in using the aperture control; now you must set the shutter speed to get the right exposure. Looking at the "meter" in the viewfinder, you turn the shutter speed dial until the needle is centered, pointing to zero. When that happens, you know you've set a combination of parameters that will provide the amount of light the camera's metering circuits think is appropriate. Since we're still assuming you believe the meter's analysis of the scene, all you have to do is make sure the shutter speed you've selected is acceptable, then shoot. Similarly, if you start out thinking you know what shutter speed you want, you dial that in first, adjust the aperture until the meter reads zero, check that the aperture you've arrived at is okay with you, and shoot. If, after setting one parameter, you find that the value of the other one that centers the needle is too high or too low, you still have to look for a compromise just as in the other modes, except now you are responsible for changing both settings to find a pair that works for you and centers the needle.

It would seem, then, that a major difference between manual and non-manual modes is that in non-manual modes the camera won't let you choose a combination of aperture and shutter speed that disagrees with the meter, but in manual mode you can do anything. That's not quite true, though.

Suppose you look at the scene and say to yourself, "I know from experience that my camera's meter will think this scene is brighter than I think it is." Maybe you're taking a picture of somebody who's backlit by the sun, and you think the camera will pay more attention to the bright sky than to the face that's in shadow. Or maybe you're taking a picture of a polar bear on a glacier, and you think the camera will mistake the white subject and white background under an overcast sky for a gray subject and gray background under full sun. In either case, you are going to want to let more light into the camera than the meter will tell you to. It's not hard to see how to do this in M mode: everything is the same as before, except that rather than finding a combination of settings that centers the needle at zero, you will look for a combination that puts the needle somewhere above zero. If you're taking a picture of a black bear in a cave, you'll be placing the needle below zero. Naturally, it is up to you to decide how far above or below the camera's recommendation you want to place the exposure.

This may sound like the kind of elite hackery that can only be done in manual mode, but of course it is not. Making this kind of adjustment in A, S and P modes is the entire point of exposure compensation. All you need to do is guess how far off you think the meter's recommendation will be. For instance, if you're in A mode and you think the meter will tend to underexpose by, say, one stop, you set the exposure compensation to +1, then adjust the aperture as before, watching the shutter speed, until you find an acceptable combination. S and P are analogous.

Thus we've returned to a point I claimed earlier: no matter what mode you're in, you need to know whether you think the meter is misunderstanding the scene, in what direction, and by how much. You can do this by guessing, or (in the Digital Age) you can do it by taking a test shot and reading the histogram. Regardless, using the auto modes does not excuse you from thinking about metering, nor does manual mode force you to think about it significantly more. They are the same.

Well, almost. The exposure compensation on most cameras is only settable from -2 to +2, or from -3 to +3, depending on configuration. So non-manual modes do confine you to working within 2 or 3 stops of the meter's preferred exposure level, while manual mode does not. But honestly, would the camera maker only give you a meter that reads from -3 to +3 if they thought you would be working outside that range very often?

So much for why all exposure modes are equivalent. How are they different, and what should you use?

My implicit assumption when arguing that all modes are equivalent was that you are only taking one photograph at a time: you set the exposure parameters once, click the shutter once, and start over from the beginning. Real life is not like this: you adjust some settings, shoot a few frames, adjust some more, shoot some more, and all the while you must pay attention to what is going on around you. The more you need to concentrate on composition, lighting, timing and focus, the less attention you will be able to give to exposure. This is where taking advantage of your modern, highly automated camera can help you.

The difference between M, A, S and P modes is how control over the image is divided between you, the camera, and The World. The only real reason this is important is that things you control will remain fixed from one shot to the next until you take the time to change them.

In all modes, The World controls the amount of light available for making a picture. In non-manual modes, the camera has enough control over the exposure that it can react to changes caused by The World, keeping the image consistent without your having to do anything. In manual mode, the camera stays out of it and if The World suddenly gets brighter or dimmer, so will the image you record.

If you believe that The World is unlikely to change from one shot to the next, then any difference in levels as measured by the meter is probably misleading. Thus, you should choose M mode so the camera does not mess up the images by compensating for "changes" that aren't really there.

If you believe that The World is likely to change very fast -- if there are clouds moving past the sun, or you're moving from sun to shade, or you're walking around a room with big windows at one end and dark corners at the other -- then you may want the camera to react to these changes so you don't have to. Thus, you should choose some mode other than M.

Which auto mode to use when using an auto mode? That depends on which exposure parameters you want to hold fixed until you change them. If you're shooting a waterfall on a partly cloudy day, you might use S mode so that when the sun goes behind a cloud, the camera will react by changing the aperture rather than messing up your carefully chosen shutter speed. If you're shooting a landscape, or an extreme close-up, you might want to use A mode so the camera's automatic adjustments won't affect your depth of field. If you're running around after your 2-year-old, you may want to use P mode, giving the camera permission to speed up the shutter when you're in the sun but also to open the aperture when you're in the shade.

None of this makes you any less of a photographer. Anyone who says it does is stuck in the Stone Age. What makes you a photographer is that you know what you want, and you get it.