As with a lot of technology, the camera takes its mechanical inspiration from the human eye, how often have you walked inside on a bright summers day and needed a second for your eyes to adjust? This is because the iris in your eye will adjust to the light conditions and allow you to see the detail in both shaded and well lit areas.
This is essentially what a standard SLR camera will do when the exposure is set to auto. However, because people (whoever they are) believe that dark shadows are more appealable than blown out highlights, most cameras will slightly reduce the exposure when left to auto.
A note on metering
For the auto exposure setting on your camera to work it needs some way of monitoring the amount of light in the scene, this is known as metering. There are 3 standard metering modes:
Matrix This is the most used by people who dont really understand metering as it takes an average measure of light across the whole scene and will turn out quite good for most shots.
Spot Metering This is when only one spot, usually highlighted in the viewfinder is metered. You can set which spot in the viewfinder is metered.
Multi-Spot metering As above only you can select multiple spots.
So, why would I use manual mode??
For one thing, consistency. If you were shooting many subjects with the same background and the camera meters off the subjects, you will find that in each shot the background is a different shade. Also when photographing people, skin tones can be affected by the colors of their clothes.
There are of course lots of reasons and lots of techniques which will require you to manually set your exposure, but as long as you know the basics you should learn them all that bit quicker!
By: Dave Traynor
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