Aperture refers to the width of the shutter opening in a camera when a picture is taken. A wider aperture lets more light in and allows pictures to be taken in dimmer light. However, it also creates shallower depth of focus, so that parts of the picture that are more distant (or closer) than the center of focus will appear out of focus. Depending on what effect you’re looking for, this may or may not be a good thing.

The aperture of any camera (not just digital cameras) is measured in terms of “f stop” or “f number.” The technical meaning here involves a 2-based logarithmic scale so that 1 f number difference doubles or halves the amount of light entering the camera. There’s a mathematical formula for this which we won’t go into, as it’s not terribly important for purposes of taking good pictures. (It’s related to the area of a circle.) The lower the f number, the larger the aperture will be. F 1 is a very wide aperture, while F 8 is very narrow.

A lens comes equipped with a range of apertures which is controlled by device called a diaphragm that functions much like the iris of your eye. Note that this is not a feature of the changeable part of the lens. The diaphragm is part of the camera mechanism behind the lens not of the lens itself. Each lens is sold with a description or rating that specifies the maximum and minimum aperture. This rating is sometimes called the lens speed, as it affects how fast the shutter speed needs to be with that particular lens.

Shutter speed and aperture are inversely related, so that a wide aperture requires a faster shutter speed under any given light conditions. The wide aperture lets in more light, and a faster shutter speed lets in less by reducing the time that the sensors are exposed.

There’s also a consideration when it comes to framing the picture and seeing it through the viewer. A narrow aperture, while it may be appropriate for taking pictures in bright light conditions, isn’t so good for viewing the picture before you take it, because it reduces the light going through the lens to your eye. For that reason, SLR cameras are normally equipped with what’s called “automatic aperture control.” This sets the aperture to the widest possible for the lens while viewing the scene and metering the light, and closes the aperture down to the appropriate level when the shot is taken.

A digital SLR camera can be set to adjust part of its settings automatically in various ways. One type of semi-automatic adjustment is called “aperture priority.” In this type of photography, the photographer manually chooses the aperture and allows the camera to automatically set the shutter speed and ISO sensitivity for the correct exposure. Using this technique allows the photographer to control the type of focus effect desired without having to manually set all three variables. A different look to a photograph is achieved with a high depth of field compared to a shallow depth of field.

By using aperture priority, you can take pictures of the appropriate effects and, within the limitations of the lens you are using, adjust the shutter speed and ISO sensitivity to allow more or less light in according to the f-stop you have selected, f-stop being a measure of aperture, with the larger numbers indicating a smaller aperture. The f-stop is what you select on your camera to set the aperture. (It’s important to remember that a slow shutter speed makes your photography more sensitive to camera motion, though. A very slow shutter speed requires a steady hand or in some cases a tripod to keep your camera motionless. Otherwise the entire picture will be blurry and streaked.)

Sometimes a lens has its best performance when it is not fully opened, i.e. using a tighter aperture than the maximum allowable with that lens. For this reason, it’s best to have a lens appropriate to the type of photography intended, that gives good results with the aperture opening you want to use. Also, there are types of photography in which a high depth of field is desirable even though the subject is very small, for example macro photography. Special lenses are appropriate for this as well.

• Black, Brian. DSLR Photography for Beginners: Take 10 Times Better Pictures in 48 Hours or Less! Best Way to Learn Digital Photography, Master Your DSLR Camera & Improve Your Digital SLR Photography Skills