There is something that strikes fear straight into the heart of most photographers and it's not a close encounter with a wild bear or a dangerous cliff. It's the manual exposure mode of their cameras.
The manual mode can be overwhelming at first. It typically activates a pair of dials that control a series of settings that move a graph in the viewfinder. Why go through that much bother when the fully automatic mode generally works so well?
The reason is that the camera doesn't know what you're photographing. It can't tell a wildebeest from a waterfall. It uses generic settings that are adequate for the average situation. If you want to freeze dramatic action or capture the motion of water, for example, you'll need to take control of your camera yourself. Besides, who wants an average picture of an incredible scene?
I think people are confused by manual mode because the camera works so much differently from our eyes. When we look at something, we are looking at the light that reaches our eyes in that instant of time. A camera, however, works more like a rain barrel. It collects light over varying amounts of time to produce one image.
To make a photograph of something that is white, the camera needs to collect a certain amount of light. It needs to collect that amount of light whether it's high noon or the middle of the night. Only that specific amount of light will produce white in your photograph. To continue the rain barrel analogy, the idea is to perfectly fill the rain barrel: don't spill a drop and don't leave any room.
When I photograph with people after sunset, some are surprised I can make a picture when "it's so dark." As long as there is any light, you can make a photograph. You just may have to wait a long time for enough light to accumulate on the sensor (or film).
There are two primary settings in manual mode that affect the collection of light: shutter speed and aperture.
Shutter speed affects the amount of time that the shutter remains open to collect light. All things equal, if you're photographing at dusk, you'll need a longer shutter speed than you will if it's high noon on a bright, sunny day. Shutter speeds can be ridiculously fast, like 1/8000th of a second, or several hours long.
The aperture is like the iris of your eye. It expands and shrinks to affect the amount of light that passes through the lens. The most open apertures have the smallest f-stop numbers, like f/2.8. A setting like f/22 lets very little light pass through the lens.
So why don't you always want to use the most open aperture possible? The aperture affects the focus of your image. When the aperture is wide open, the only thing that will be in focus is plane where you focused the camera. Anything in front of or behind that point will generally be out-of-focus. It's known as a very narrow depth of field. You've likely seen this in pictures of birds or flower buds where the background seems to be a solid splash of color.
On the other hand, very small apertures can greatly extend that depth of field, allowing you to capture a field of wildflowers and a mountain and have them both appear to be in focus.
There is also a third consideration. Lenses are typically designed to deliver the most sharpness in the middle of their range, generally f/7, although at that setting you won't get a blurred background or tremendous depth of field.
Perhaps you can now see that these settings all have tradeoffs. When your camera is in automatic mode, it's making decisions about those tradeoffs without any regard for what you're photographing. By using manual mode, you can decide the settings that are right for your particular situation.
Setting the exposure yourself is a two-step process. The first step involves determining whether you want to use a specific aperture or shutter speed. Do you want a nice blurred background? Use the most open aperture available to you, such as f/2.8. Do you want maximum focus depth? Try an aperture of f/11 or f/22. Do you want to blur the motion of the water in a waterfall? Try a shutter speed of ½ second.
The next step is to adjust your other variable so that the camera collects the correct amount of light. If you set the aperture, you will now be adjusting the shutter speed. If you set the shutter speed, you will now be adjusting the aperture.
In the viewfinder you'll see a small graph. When the indicator is at the midpoint of the graph it indicates you are collecting enough light for the average scene. If the indicator is closer to the positive end, it shows your image will be brighter than average. If it's closer to the negative end, it indicates the exposure will be darker than average.
Most of time, you will want the indicator to be in the center. That's typically the correct level of light to make a blue sky blue, or a lawn a nice medium green. Whenever your scene is predominantly full of medium tones, like grass or blue sky, try to adjust the exposure settings so that the indicator is in the middle.
You will need to adjust the exposure if your subject is brighter or darker than those medium tones. If you photographed snow with the exposure indicator in the middle, your image would turn out gray. Again, the camera doesn't know what you're trying to photograph. It's programmed to assume that whatever you're photographing is about as bright as green grass and it calculates an exposure that will produce a medium tone.
Depending on how much brighter or darker than average your scene is, adjust your exposure accordingly. To continue the example of a scene of mostly pure snow, you would likely want the exposure indicator at nearly the positive end. If you're photographing a black sand beach in Hawaii, you would likely want the indicator a couple notches above the negative end.
Selecting faster shutter speeds will reduce the amount of light the camera has time to collect, causing the exposure indicator to move toward the darker end. This is also true of using smaller apertures, such as f/22. The more closed the aperture is (and therefore the larger the f/ number is), the less light passes through the lens at any given moment.
Conversely, using slower shutter speeds increases the amount of light the camera has time to collect and therefore brightens your overall exposure. An open aperture, such as f/2.8, also lets light pass through the lens more quickly, also resulting in a brighter exposure.
Digital cameras have made using manual exposure easier than ever. Once you think you have set the exposure correctly, take a picture and check it out on your screen. If it's too dark, extend the shutter speed or open the aperture a bit more. If it's too light, shorten the shutter speed or close the aperture.
Don't worry if at first it takes several attempts to get a correct exposure. It will get easier and easier. And mastering manual exposure will allow you to create images you can't otherwise.
The image at the top of this post is of a secondary waterfall that forms alongside Snoqualmie Falls in Washington state. When the waterfall is at maximum flow in the spring, some of the mist from the main falls blows into the neighboring wall, forming a secondary falls as it collects and falls into the river below.
If I used a fast shutter speed as the camera would normally suggest, I wouldn't have been able to capture the graceful strands of water. A fast shutter speed would have caught individual drops. If I would have greatly extended the shutter speed, the texture of the cloud of mist would have been blurred so much that all detail would have been lost. I manually set a shutter speed of 1/15 of a second with an aperture of f/7.