I know, I know: it’s so easy to put everything on automatic and let the camera do the work, and the “M” setting means funkiness and general confusion, but learning to use the “M” will pay rich rewards—I promise—and it’s really not all that hard. In fact, there is probably no single thing you could do to improve your photography more than to learn to use the manual setting if you don’t already know how to do so. Understand this, and suddenly a whole world of possibilities opens up. So, it’s worth a little time and effort to figure out.
With that, let’s take a bold leap into the technical side of photography. Some people love it. I am not one of those people, and you don’t have to be either. For me, it’s a necessary evil, but—alas—necessary nonetheless. You don’t need to grasp the physics, but you do need to understand the basics.
Here’s the deal: in both film and digital photography, basically three things determine how a photograph is exposed—in other words, how much light is recorded by the film or image sensor: (1) the film speed (or ISO setting), (2) the shutter speed, and (3) the aperture.
For purposes of understanding the “M” on your camera, let’s leave No. 1 (film speed) for another day, understanding that most digital cameras set the film speed automatically, but one can set that manually as well, and it affects how much light the camera can/will record and the quality of the final image.
When you turn the dial to “M,” a few things will appear at the bottom or one one side of the view finder or LCD screen: first, something that looks like a ruler that typically has an arrow in the middle and then regular increments to the right or left of that arrow (or above or below), with numbers usually indicating 1 and 2 (on the positive side) and -1 and -2 (on the negative side). Somewhere next to that little ruler you'll see another arrow that tells you how what you see in the viewfinder will be exposed based on the current settings for aperture and shutter speed. If it points right in the middle of the ruler, then, according to the camera's built in light meter--the image is properly exposed. If the arrow points at the positive number 1, then we say the picture is "one stop" over exposed. If it points at the negative 1, it's one stop underexposed, etc.
When first learning to use the manual setting, it's best to tinker with aperture and shutter speed values until you get the little arrow right in the middle of the ruler. Later, you may choose to deliberately over- or under-expose and image, but for starters, just put the arrow in the middle, and everything will be okay. I promise.
Now, none of this will make much sense unless you pull out your camera and look for yourself. So, turn your camera on, turn the dial to “M,” and take a peek. You’ll see all this fun and exciting stuff right there: the little ruler, the little arrow, the positive and negative numbers, and also two values that show aperture and shutter speed, respectively. (You may need to consult your camera manual at this point to figure out which is which, and to distinguish aperture (which may be identified as "f-stop") and shutter speed settings from other settings that may appear in the view finder.)
Whew! Without worrying about the actual numbers for the moment, let’s think about what these two things do: one—the shutter speed—determines how long the shutter is open and the other—the aperture (which is just a fancy word for “opening”) determines the size of the hole through which light enters the camera. If you think about it, both of these affect how much light gets into the camera to expose either the film or the image sensor. A big hole (aperture) = lots of light. A little hole = little light. A slow shutter speed means the shutter is open longer and more light gets in, while a fast shutter speed means he shutter is open for just a split second and less light gets in.
To illustrate, let’s go back to the idea of a pinhole camera—that oatmeal box with the film taped to the inside and a little hole on the opposite side of the box. Because the film is sensitive to light, the oatmeal box must be completely sealed off, completely dark inside. Once the film is in place, the hole must be covered by a piece of cardboard or some similar cover so that no light gets in until the photographer is ready to expose the film. The photographer points the oatmeal box at a subject, and then removes the cover to expose the film for some amount of time: a second, or maybe two or three. The longer the cover is removed, the more light gets in to expose the film, and the less time, the less light gets in. Follow me so far? That’s shutter speed, basically the amount of time that the shutter remains open, even if that “shutter” is just a piece of cardboard.
The size of the hole also affects the amount of light that gets in. A pencil-sized hole will let in a lot of light, even if the cover is removed only briefly. While a pin-sized hole will let in a lot less light, even if the cover remains off for some time. That’s aperture—the size of the hole.
Let’s say that the film our photographer is using requires a certain amount of light to be exposed properly at high noon on a sunny day. Too much light, and the film will turn white when it gets developed (over-exposed); too little, and the film will remain black (under-exposed). The photographer can get to that right amount of light with either (a) a small aperture and a slow shutter speed, or (b) a large aperture and a fast shutter speed. The amount of light reaching the film is exactly the same, but—and here’s the reason any of this matters—shutter speed and the size of the aperture affect the final image in ways having nothing to do with the amount of light they let in.
And that’s why “M” matters. Because, under one set of circumstances, we may want, say, a fast shutter speed, and, in that case, we will likely have to compensate with a large aperture to get the right exposure. Under another set of circumstances, we may want a small aperture, and, in that case, we will likely have to compensate with a slow shutter speed—again, to get the right exposure under that particular set of circumstances.
Why might one want a fast shutter speed? We’ll discuss that in more detail later, but the short answer is that a fast shutter speed freezes action—a desired effect when, say, photographing a kid at a soccer game. A slow shutter speed, on the other hand, captures movement, as in those lovely, silky white waterfalls you see in landscape photos or in the following photo I took the othe other night using--you guessed it--the manual setting.