Canon EOS Digital SLR Product Range - Eos40D, Eos400D

Topic One: Aperture

Aperture is the measurement of how wide the eyelet in the lens opens to allow light in. Aperture values are expressed in numbers called 'f stops'. A smaller 'f stop' number means more light is coming into the camera (f/1.4 lets in a lot more light, whereas f/22 is like a pin-hole). Aperture is also important for 'depth of field' (or 'depth of focus') in your shot. When you photograph a subject against a background, minimum depth of field would put the subject in focus, and everything else out of focus. To achieve this, use a low aperture value (eg f/2.8). If you take the same shot with f/22, the subject and background will both be in focus. Its easy if you remember it this way - a higher numerical aperture value means greater depth of field.

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Topic Two: Shutter speed

Shutter speed is the length of time light is allowed to pass into the camera to record the image. A fast shutter speed is 1/2000 of a second (shutter speeds are always measured in fractions of a second). Shoot a moving object with a slow shutter speed, and your picture will blur. If your hands shake as you shoot, you'll also get a blurred image. This is called camera shake. To avoid camera shake, use a lens with a numerical value close to the numerical value of the shutter speed. For example, a 50mm lens can be safely held with a shutter speed of 1/60 sec (there is no 1/50 shutter speed). An alternative is to consider Canon lenses with built-in Image Stabilizer to reduce the image movement that occurs when your hands shake, or use a tripod.

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Topic Three: Sensitivity

You can also change the camera's sensitivity to the light. Although you're shooting digital, the camera follows the old film measurements. Sensitivity generally ranges from ISO 100 to ISO 1600. ISO 100, the standard speed needs twice as much light as IS 200, and so on. Generally, you should try and shoot on as low a speed as possible, as it will give you richer colours and greater precision. But when you shoot greater depth of field (where the aperture will be allowing less light) or use high shutter speeds, a higher sensitivity may be worth selecting.

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Topic Four: Metering and Exposure Compensation

Your camera measures light after it passes through the lens. If there is more light coming from one particular zone, the camera will compensate by adjusting the exposure readings. Occasionally, you may get more creative and override the exposure settings, using the Exposure Compensation function. To activate, press the button on the back of the camera, then turn the input dial left or right to increase or decrease the exposure setting. If you want to take silhouettes against a sunset, for example, you would decrease the exposure setting on your subjects. It takes a bit of practice, and until you get confident use the AEB feature (Auto Exposure Bracketing). This allows you to take three shots - one at the cameras recommended exposure, then one slightly underexposed and one slightly over exposed. You can choose the best one and delete the others.

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Task - Tips to get started

Put your camera onto shutter priority (TV on your camera setting). Take shots of someone walking past your camera using a low shutter speed (try 1/30 for this exercise). Then re-set to a higher shutter speed ((1/125 is about right for a walking subject in daylight) and take the shot again. You will notice your image will be much clearer on the higher shutter speed. Next, try shooting a subject running or jumping in front of camera at 1/125. Then try 1/250. If your image is still blurred, move to a higher shutter speed, say 1/500. Make sure to have your camera absolutely steady as you shoot, and try to shoot your subject in good daylight. By the end of the exercise, you'll have a good idea of the effect of different shutter speeds.

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