Canon EOS Digital SLR Product Range - Eos40D, Eos400D

Exploring the Aperture-priority auto exposure mode on a digital SLR camera (or sophisticated digicams) is a lot of fun. In fact, this mode gives you so much control over the pictures you take that many professional photographers use it for all of their work.

What you need to get started:
1. An advanced digital camera (preferably digital SLR)
2.A tripod (handy but not essential)

Here's what you need to understand:
The aperture in a lens is controlled by an adjustable iris diaphragm and the size of the apertures is calibrated in f-numbers. For a typical interchangeable SLR lens, these numbers usually range from f/2.8 to f/22 in the following series: f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. The largest aperture is f/2.8 and the smallest is f/22 and stepping from one aperture to the next in the sequence either doubles or halves the amount of light entering the camera. Going from f/2.8 to f/4 'stops down' the lens; going from f/22 to f/16 doubles the amount of light entering the lens.
When you're shooting with a camera's manual exposure mode and your pictures are too light, try using a smaller aperture, perhaps changing from f/5.6 to f/8. If they're too dark at f/8, open the aperture to f/5.6 - or even f/4!
But that's not the only effect changing lens apertures can have. Stopping down the lens increases the zone of sharpness in front of and behind the subject. In other words, it makes more of the subject appear sharp. This zone is known as the depth of field in the picture - and it's one of the most powerful tools in a photographer's kit.

1. Select the aperture-priority mode (A) on your digital camera and use the camera's controls to adjust the lens aperture to, say f/4 or f/5.6. If you're shooting with a telephoto lens and the subject is some distance away, you may even be able to stop the lens down to f/8!
2. Take the picture and check the result on the LCD monitor. Use the playback zoom control to enlarge a section of the image where the subject overlaps the background. If the subject looks sharp against a blurred background, your shot is successful. If only part of the subject is sharp, select a smaller aperture and re-shoot. If the background looks too sharp, open the lens aperture by one stop and try again.
3. Use a telephoto lens, stand well back and fill the frame with the subject.

A telephoto lens makes it easy to fill the frame with your subject's face because it gives you a comfortable working distance for shooting. It's also easier to obtain a shallow depth of field with wide lens apertures and blur potentially distracting.

1. Shots in which the depth of field is too shallow, making only a small part of the subject look sharp and leaving some of the details you wanted to record blurred. When this happens, stop the lens down a notch and move a little further back from the subject.
   
2. Incorrect focusing, where the camera's autofocus locks onto a part of the subject that you would prefer to have blurred and leaves a nearer part of the subject unsharp. This often happens with close-ups of clusters of flowers. It's easily avoided if your camera lets you select the AF point. Otherwise, set the camera to manual focus and put the camera on a tripod so you can be sure you focus the lens on the area you wish to have sharp.

Many subjects look best when everything in them is sharply recorded, allowing the viewer's eye to see the subject in its environment and take in all of the detail. This is easy to achieve, especially when you use a wide angle lens; simply stop the lens down to f/11 or a smaller aperture. (If you're shooting with a compact digicam or using the scene modes on a digital SLR, setting the camera to the Landscape mode will achieve the same effect.)

Portraits of people in their environment usually look best when you can see details in the background. Shooting with an aperture of f/8 or smaller allows you to achieve that result.

1. Blurring due to subject movement, especially with telephoto lenses. Using a small lens aperture lets less light into the camera so you (or your camera) must compensate by setting a slower shutter speed. You may need to set a higher ISO value to prevent blurring due to camera shake and subject movement.
2. Distracting elements in the shot, such as rubbish bins, wires, poles, blades of grass, mesh. You may not notice them through the viewfinder, but your camera will certainly record them if the lens aperture is small enough!

Choosing the correct lens aperture is particularly critical for close-up shots because depth of field can be very shallow and you must be able to cover the entire subject while at the same time blur out distracting backgrounds. Here are some tips for achieving these objectives:

1. Fill the frame with the subject. The smaller the area of background, the less you have to worry about it being distracting!
2. Use a tripod. This allows you to take your time with shot composition.
3. Experiment! Always check each shot after you've taken it, using your camera's playback zoom control to enlarge parts of the subject. You can move the enlarged area around the frame with the arrow pad. If the shot isn't quite right; delete it and re-shoot. It costs nothing to take additional pictures with a digital camera!