The aperture of a lens controls the amount of light that strikes the camera's sensor. Shutter Speed controls how long the camera shutter is open for. Aperture and Shutter Speed work hand in hand to determine the amount of exposure the image gets. A slow shutter speed of ¼ of a second requires a smaller aperture to give the same amount of exposure as a fast shutter speed of 1/125 of a second.
Aperture
This controls the depth of field (DoF) which is the area in front of and behind the subject that is in focus. For a portrait you want a narrow DoF which serves to emphasise the subject. Conversely a much greater DoF in needed for a landscape shot.
Shutter Speed
This control is used creatively to give a sense of movement or speed to a shot. A fast running stream shot using a slow shutter speed (1/4 of a second) will cause the water to become a white blur so imparting a sense of movement to a still image. A technique known as panning is used to create a sense of speed. This involves holding a fast moving subject in the viewfinder and following it by swing the body. It keeps the subject clear and blurs the background so creating the sense of speed.
How do you know that you have got the best exposure?
OK. Pin your ears back, fasten your safety belt and get ready for some thought shifting stuff. You need to find the Histogram setting for your viewfinder, turn it on and keep it on! The best digital negative does not necessarily look excellent on your LCD screen. It is certainly not the one that you get using the spot metering. The best digital negative is the one that contains the most digital data. The more data captured the more that a final print can be produced that is of greater quality, exhibits less noise, and has more impact. This is what the histogram shows you.
So what is the Histogram? It's a very simple graph. It shows the light value in the scene you've just captured at the exposure values you've captured it at. The far left shows shadows plunged totally into darkness. The far right shows the highlights that have no details, which are totally burned out to white. Between these extremes are all the tonal values from black to white.
How do you determine the best digital negative? The right half of the histogram is able to store more data in it than the left half, a lot more data. And the right quarter of the histogram, much, much more than the other three combined. There isn't much information at all in the dark areas. So you want a histogram that is weighted to the right but which shows information trailing off to the left.
Charles Demain
For your free copy of "The Comprehensive Introduction to Mastering Digital Photography" come on over now to http://www.creativeslrphotography.com/
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