When you have a big range of different tones in your picture, from very dark shadow areas to very bright highlight areas, this is known as High Dynamic Range and the camera can sometimes struggle to expose every part of the picture accurately.
The camera will have to compromise and may prioritise one part of the scene, resulting in detail being lost in another. There are a number of ways of dealing with this. Most manufacturers now have a feature that tries to even out high-contrast scenes to preserve more detail in the darkest and lightest areas. Nikon, for example, have their Active D-Lighting system, while Canon have the Auto Lighting Optimiser, Sony call it the D-Range Optimizer and Pentax have Dynamic Range Enlargement.
Part of the problem is that when a camera views the scene it has no idea what part of the scene is most important to you. Therefore your Digital SLR will make what it considers to be an educated guess. This can lead to the photo being either darker of lighter in key areas of the photo than you would like it to be. To get round this or to at least make the educated guess more educated there are a number of things you can try to get the exposure looking just the way you want it to.
These include using exposure compensation, bracketing, spot metering, high dynamic range and manual exposure controls.
Where you have extremes of either very light or very dark tones it can also sometimes cause the camera to get the exposure wrong. A snow scene, for example, may look far too bright to the camera and it will try to reduce the amount of light it lets in and end up under-exposing the scene, making your snow look dirty grey instead of white.
Good cameras will allow the photographer to over-ride the exposure in order to compensate and force the camera to make the shot lighter or darker as needed. This is called Exposure Compensation or Exposure Correction. Entry-level DSLRs may allow you to make the picture up to 2 or maybe 3 'stops' lighter or darker, while more expensive models, allow correction of up to 5 stops.
Many photographers consider exposure compensation to be one of the most important features on a Digital SLR.
Also look out for Auto Exposure Bracketing. Bracketing involves taking several shots of the same scene (typically 3, but can be more) at different exposure settings so that you get a set of pictures that are a little lighter or darker than one another and you can decide later which one looks best. You usually need to go into the menus to set the camera to do this, and can specify exactly how much lighter or darker you want each shot to be.
Digital SLR Basics
Digital SLRs and Digital Cameras Key Differences
Digital SLR or Compact System Cameras
Digital SLR Handling
Help for Beginners
Lenses and Accessories
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Manual Exposure Modes
Help With Tricky Lighting
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Recommended Digital SLRs
Digital SLR Guide Author
This guide was written by Ian Younger
Cameras are set by default to use matrix (also known as evaluative) metering. This means that the camera will test the light levels at various points in the scene you are about to photograph. It will then create an average score from the different points of light and base how bright the exposure is on this score.
An example of where multi point metering can create an undesired result is if you are photographing someone standing with their back to the sun. The camera will take into account the brightness of the sun and decide the entire picture needs to be toned down a lot to stop the shot from being too bright. This will lead to the person you are taking the picture of being too dark.
You can get around this problem by using spot metering. This means you can tell the camera to base lighting levels on part of the person, such as the face, instead of the whole scene. The result will be the person's face is perfectly exposed. The background with the sun will be over exposed (very bright), but the main factor is that the person you are taking the picture of is perfectly lit.
If you prefer not to use exposure compensation you can set the shutter speed and aperture yourself. This can have the same effect as you get with exposure compensation. By using this method you get total control over the overall brightness of the photo. Although this method is available to you, you are likely to find it simpler to dial in exposure compensation.
This feature is not available on all Digital SLRs. It works by the camera firing multiple shots - usually three - and merging them into one. It will take one standard exposure one, brighter exposure and one darker exposure. The brighter exposure will bring out extra detail in the darker areas of the scene. The darker exposure will bring out extra detail in the lighter areas of the scene. It will then merge the three images into one using the extra detail to boost the definition showing in the final image.