How to
Get the right exposure. The basics.
1. Aperture: the size of the hole in the lens. Small numbers (e.g. F2.8) let in lots of light; large numbers (e.g. F16) let in hardly any. Each main step (F2.8, 4, 5.6, 8 etc.) lets in half as much light as the previous one.
Also important because: aperture controls depth of field (how much is in acceptable focus either side of the focal point). Small numbers (e.g. F2.8) have reduced depth of field, and large numbers (e.g. F16) have a greater depth of field. The actual size of perceived depth of field varies with lens (wide angles appear to have greater depth of field) and with distance (at life size depth of field is minimal; further away at it can extend from a few yards away to infinity).
2. Shutter speed: how much time the sensor is exposed to the light. Runs from as long as you like down to 1/2000, 1/4000 or 1/8000 of a second, depending on your camera. Each step cuts light reaching the sensor by a half.
Also important because: shutter speed determines blur of moving subjects, and also determines the likelihood of camera shake (bad!). Generally, use a shutter speed 1.5 times the focal length of your lens (e.g. 1/500sec with a 300mm lens) if you are handholding with most DSLRs.
3. Sensor ISO: the sensitivity of the sensor to light. DSLRs have a base level (usually ISO 100 or 200), where quality is maximal. You can increase ISO (100, 200, 400, 800 etc.), and each main step reduces the need for light by half.
Also important because: higher ISOs means increased sensor noise (the blotchiness you see in some images), and above certain points renders images unusable. For most cameras, going above ISO 400/800 is risky.
As you will see, each of these is inter-related, as each is about the use of available light. For example, in low light, you could use a tripod, set the camera to ISO 100, the shutter speed to 1/30 of a second and F4. You could also set the camera to ISO 800, the shutter speed to 1/250 of a second and F4 (or 1/30 at F16). Each decision will affect your image, and what's right depends on the context. The former is good for static subjects, the second for moving wildlife and the latter for macro, for example.
Metering: quite simply, this is where the camera measures the light reflecting back from the subject, and gives you a recommended exposure for the ISO you have chosen. On a bright day in direct sunlight, a short shutter speed and small aperture will be recommended, and vice versa on a dull evening. There are three basic forms of metering.
1. Matrix/evaluative metering. Where the camera breaks up the image into its components and compares what it sees to a database, and recommends the best exposure.
2. Centre-weighted. Calculates an overall average exposure, but gives the central part a much higher weighting. Assumes that the subject is in the middle of the image.
3. Spot. Where you can take a reading from a tiny (around 1-3% of the image) part of the subject. Allows you to choose what's most important to get right.
For most purposes, matrix is fine, centre-weighted works and is predictable, and spot is for when you know what you're doing.
Assumptions.
1. The camera assumes that you want to expose for a midtone (usually an 18% grey card). It's important to realise that it's not grey that matters, but that it's a mid-tone. A blue that reflects 18% of light will be the same mid-tone.
This means that metering overexposes blacks (to make them grey), and under exposes whites (to make them grey). To avoid this, underexpose scenes where the main subject is black, and overexpose images where the main subject is white.
2. Cameras are not good at dealing with backlighting (where there's a strong light behind the subject - e.g. the sun behind a person, a bird flying against a bright sky). Either spot-meter from the subject, or add to the exposure to get the subject right. The alternative is a silhouette, which is fine, as long as that's what you wanted.
The trick with these is to set your camera to manual, and set your shutter speed and aperture based on metering off a suitable mid-tone, such as your hand and add one stop to this measure (variation in human skin tone has little impact on this), gravel or grass, and use this as your setting. This works as long as the light remains constant and that the light falling on your hand is the same as that falling on the subject. Failing that, spot metering works well in these situations.
The histogram. The biggest advance in camera technology for decades - allows you to see the exposure as a graph which runs from black to white. The basic rules are to:
a) not clip the whites (lose the information at the right of the histogram);
b) not to underexpose too much (as otherwise you've got to deal with this in processing, and that usually add noise), and therefore the ideal is to
c) expose to the right of the histogram (to retain as much information as possible), without clipping.
This is less important if you shoot RAW (highly recommended), as that gives some leeway, but be sure not to over-expose jpegs - once lost, the data can not be regained. In other words, expose for the highlights, and let the shadows look out for themselves.
Overall, the ideal is to get your exposure right in camera, and to leave as little to worry about in processing.
The sunny 16 rule
This is an old rule-of-thumb for outdoor exposure that's been around for decades, and is useful for predicting what to do:
At ISO 100 and 1/100 of a second (at different ISOs, just use the inverse of the ISO as the shutter speed) ~
Heavily overcast and shade (no shadows): F5.6
Brighter overcast (very faint shadows): F8
Bright cloud (distinct shadows): F11
Bright sunshine (strong shadows): F16
Bright sunshine for sand and snow (strong shadows): F22
Shooting into the sun (subject in shadow; contre jour): add one or two stops, depending on how strong the backlighting is.
The advantage of knowing what to expect is that you can predict when your camera is having difficulties getting an accurate exposure...