The Basics
The amount of light that is captured by the film or CCD sensor is called exposure. Recording mediums vary in their sensitivity to light, but to record an image of normal exposure, all need a specific amount of light at a given sensitivity (cf. the ISO numbers of conventional film). Too little light will lead to underexposure, while too much will yield a lighter than normal or overexposed image.

The required exposure value (EV) (value given to a single exposure) is determined by the combination of the sensitivity or ISO speed of the film or CCD, the aperture of the lens and the shutter speed used.
A perfect exposure of a scene shows details in both highlights and shadows.

Exposure - the pitfalls
Given the sophistication of today's digicams you would think that it's almost impossible to take a bad exposure. However, there are still situations where even the best metering system will struggle. Realizing when to override the camera's setting is the key to successful images. Typically white overcast skies, snow scenes, or dark subjects against dark backgrounds will fool your light meter.
When shooting into the light such as with sunsets or backlit portraits, or landscapes shot into the sun, the brightness of the background will cause the subject to record as a silhouette.

This can produce stunning results, but if you want your main subject or the foreground to be correctly exposed, you must increase exposure by 1 – 3 stops depending upon the background's brightness. Another method is to aim your camera down to exclude the sky and meter from the foreground, or in the case of sunsets take a substitute reading from a midtone area of sky and bracket exposures. For a backlit portrait you could also try to use fill-in flash which will make for a nice rimlighting effect.
A dark background with a small brightly lit subject can be problematic too. A typical example would be a theatre scene with a subject lit by spotlights against a dark background. For a correct exposure take a meter reading from a midtone area such as the stage floor not lit by direct spotlight and bracket exposures by 1 stop either way.

Auto bracketing
Exposure - Auto bracketing - Digital camera and photography tutorialsAuto bracketing is an easy way to make sure that you get an accurate exposure in tricky lighting conditions. The camera automatically takes a sequence of 2 to 5 exposures with settings at, over and under the suggested exposure with a diffference of anywhere between 0.3 and 2.0 EV. Of course it is not necessary to bracket every shot you take. The trick is to recognize when your camera's meter might be fooled into a wrong exposure. This will happen in lighting situations where dark tones or light tones dominate the scene, or where the dynamic range is too large for the camera to record.

Take night time buildings lit by floodlight for instance. The contrast range between the spotlights and the dark sky will be quite high.This would be a perfect situation for a bracketing sequence. Take a couple of shots under and over what the meter suggests and see in which image the lit buildings and the surroundings are recorded best .
Not every camera has the possibility for auto bracketing, but if it has a manual mode
it is easy to bracket exposures yourself. Set the meter's exposure on your camera and change shutter speeds or aperture by 1/2 to 1 stop for each image in the sequence.
Taking 2 images under and 2 over the suggested exposure is usually enough to ensure one perfect exposure of a scene.

Exposure compensation
Exposure compensationAs we have already seen, light meters cannot see color. They render every scene as 18% middle gray and adapt the exposure accordingly. This will be OK for the majority of scenes, it is only when very dark or bright parts dominate a scene that the meter will be fooled by trying to render it as middle gray.
Most digicams will allow you to compensate the exposure by 1 to 2 EV plus or minus in 1/3 or 1/2 stop increments.

Typically white overcast skies, snow or beach scenes will need compensating by +1 to +2 stops (EV) as the (overoptimistic) meter reading will lead to underexposure.
The opposite holds true for dominating dark scenes like dark foliage or the proverbial black cat in a dark alley. The meter will overexpose in trying to render the dark scene as middle gray, so you should decrease the exposure by -1 to -2 stops. As a rule you should use + exposure compensation when the subject is bright and - when it is dark.
Since correct exposure is only “correct” in the eye of the photographer, EV compensation can also be used as a creative tool. If for instance you feel the mood of an image is enhanced by darkening a stormy sky with –1/2 EV or creating a high key portrait by compensating with +1/2 EV please do so. Remember that the best way to learn what exposure compensation can do is to experiment and try out different settings.

Dynamic range
Dynamic range is the amount of detail your camera can record between highlights and shadows in an image. Scenes that have bright sunlight and deep shadows are said to have a high dynamic range. Some cameras let you decrease contrast in these situations to bring dynamic range within the camera's limits.
No camera and not even the human eye, can record the nearly infinite dynamic range that exists in real life, so compromises will have to be made.

Whether your camera can record the whole dynamic range present in a scene or not, depends on the sensitivity of the CCD and the built in processor. As a comparison conventional film has a dynamic range of about 5 f-stops while black and white film can record about 9 stops. Modern CCDs have a higher dynamic range than film.
If dynamic range is outside the camera's limits, it will automatically compromise or the user himself can decide which part of the scene is most important and should be recorded in detail. He can set an exposure by metering for the highlights or the shadows, so that area is shown accurately in the final picture. This can be done by using exposure lock on your camera. An alternative would be to use fill-in flash to lighten shadows and reduce contrast.

Exposure Values (EV)
Exposure Values (EV)Exposure value refers to the amount of light for a given exposure. It is denoted as a single number. Its value is subsequently used to calculate the correct combination of shutter speed and aperture at a given ISO speed of the sensor.
Starting point is a shutter speed of 1 second at f1, which is denoted as EV 0. Further up the scale a single exposure of EV13 could be ISO 100, f8 - 1/125s, or f11 - 1/60s.
It doesn't make any difference which combination you choose, as all variations at the same EV will ensure a correct exposure.

Remember though, you are always balancing camera or subject movement against depth of field. This is because a change in aperture will require a change in shutter speed for an equal amount of light to reach the sensor or CCD. If you set the shutter speed one stop slower and aperture one stop smaller, exposure doesn't change, but you increase depth of field slightly and the possibility of blur.

Most digicams will calculate all these values for you automatically and select a combination of aperture and shutter speed to give a correct exposure.
Some more advanced digicams will let you choose the right combination manually, offering you the choice to overexpose or underexpose in a given situation.

ISO sensitivity
ISO sensitivityISO is the international standard used to denote film speed. It has been carried over to digital imaging and all manufacturers now provide the ISO equivalent for the sensors they employ in their camera.
It works on the principle that doubling the sensitivity of the film or sensor results in a doubling of the number. A 200 ISO sensor is twice as sensitive as an ISO 100 sensor.
The ISO rating in a digicam limits the exposure range of the camera because it acts as the base from which all exposure combinations are made.

The smaller the ISO number, the slower the sensor. This means that an ISO 100 sensor requires either a slower shutter speed or a wider aperture than the ISO 200 sensor. Most consumer cameras have an ISO rating of 100, but some high end professional models offer electronic enhancement of the sensor's sensitivity of up to 1600 ISO.
This can be compared to “pushing” of conventional film. When film is pushed, it often causes grain to be clearly visible. With pushing image sensors, pixelation or color shifts may increase and become visible as noise in an image.
Although with manufacturing processes getting more advanced all the time, quality will improve and before long we may have sensors with higher sensitivity with less or no visible loss in quality.

Manual shutter setting
Exposure - Manual shutter setting - Digital camera and photography tutorialsManually selecting shutter speeds allows you to control the amount of subject (or camera) movement that is recorded in the picture.
If you want to freeze movement or emphasize it, selecting the right shutter speed is dependent on the speed of the subject, the direction it is moving in relation to the camera and how big it is in the frame.
As a guide, a person jogging head on to the camera can be recorded with a shutter speed of 1/60, while a formula 1 car traveling across the frame would need 1/2000 to 1/1000 to freeze it.

If you want to emphasize movement, you could try panning with your subject.
This involves tripping the shutter while tracking the subject with your camera.
The effect will be a relatively sharp subject against a blurred background. Try experimenting with slower shutter speeds until you get the hang of it and take
plenty of images.
You don't always want to stop all subject movement. Flowing water for instance can be recorded by a fast shutter speed to make it look frozen or glass-like, while selecting a slower shutter speed will give it a soft almost ethereal quality.
One shutter speed worth mentioning is B (bulb) This keeps the shutter open for as long as you keep your finger on the release button and is used for very long exposures.

Metering modes
Metering modesLightmeters in modern (digital) cameras are intelligent enough to automatically determine the correct exposure
for most shots so the photographer can concentrate on composing the image. They all work on the principle of measuring reflected light from the subject. The three most common types are discussed here. Center-weighted metering asesses the whole image area but assigns the most importance to the central 40 to 60%. This system is accurate in average lighting conditions, but can be fooled by excessively dark or light areas.

Spot metering only evaluates a small area in the center of the viewfinder so you can meter for a specific detail of the scene (e.g. when photographing a subject against a
bright or dark background).
Matrix-metering divides the image area into many small segments and analyses the readings for the best possible exposure. It is remarkably accurate in most situations.
Automatic exposure modes work fine for most scenes, but the meter can only see scenes as neutral gray and compensates for any deviations accordingly. So you should know when to override the meter's results. If the subject contains more than average dark or light tones you should take a substitute reading of an average gray area, lock that exposure and then recompose the image and release the shutter. Green grass or medium blue sky are good examples of average gray.