Aperture: What Is It?

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. Note the difference in the two photos below.

In this picture, the depth of field is shallow, so that only the squirrel monkey and the branch it’s sitting on are in focus. This picture was taken using a wide aperture. It used a relatively fast shutter speed to cut down on the light and correctly expose the photo.

In this picture, on the other hand, the depth of field is extensive, so that the whole landscape appears in focus. This photo was taken with a narrow aperture, and the shutter speed was relatively slow in order to let enough light in for proper exposure. If anything had been moving quickly through the photo it might have appeared blurry as a result, not because of depth of field but because of the object moving across the field of view while the picture was being taken. (We’ll discuss effects of shutter speeds below.)

Which effect is better? It depends on what you are trying to portray. A photo in which only the immediate center of focus is clear draws attention to that subject. This might be appropriate for a portrait or for a close-up of one particular item. It would not be appropriate for a panoramic shot in which the entire scene needs to be in clear view.

Source : DSLR Photography for Beginners By : By Brian Black