In a previous article, lighting was identified as the element which most determines the quality of an image. Working with available light and using internal flash and external light sources were discussed. In addition to the light present, the amount of light on the image sensor, or exposure, is determined by the opening/closing of the aperture and the length of time that the shutter is open. To better understand exposure, this article will discuss aperture, shutter speed, and the concept of “bracketing”.
Aperture is essentially the opening that allows light in through the lens. You will see aperture indicated in “f-stops”. The smaller f-stops represent larger apertures and therefore more light coming in through the lens. To state it more directly, the larger the aperture the brighter the light obtained. It is also important to realize that aperture also influences some of the focusing within a shot, known as depth of field. Technically, a camera can only focus on one item within a shot with items becoming less in focus the further they are from that point. The depth of field is the range or distance of items that are in focus; thus, an image with a narrow depth of field generally has a smaller area in focus. A wide depth of field is used when wanting detail throughout the shot but a narrow depth of field is desirable when a single object or person is the focus of the image. When using a larger aperture, depth of field is minimized while reducing aperture maximizes it.
Shutter speed is generally understood as the length of time the shutter is open. Slower shutter speeds allow for more light and faster shutter speeds less light. Therefore, a night shot often utilizes a slower shutter speed in addition to a larger aperture to maximize the light coming into the lens. Whenever a slow shutter speed is utilized the risk of camera shake is significantly increased and thus, tripods are often necessary to avoid blurring. Fast shutter speeds, such as 1/250, produce very sharp images and can act to freeze action. A slow shutter speed produces less defined images; images that demonstrate movement by blurring action. Just as with aperture size, the shutter speed selected is not “right” or “wrong”, it is merely which will create the type of image wanted.
It is important to understand that when adjusting shutter speed or aperture size it is often necessary to adjust the other to accommodate the resulting change in lighting. For instance, if a faster shutter speed is chosen to freeze action aperture size may need to be increased to obtain adequate lighting for the shot while realizing that there will be a narrower depth of field.
Digital cameras offer the user control over aperture and shutter speed. For the novice, a Fully Automatic Mode in which the camera decides on the shutter speed, aperture, white balance, and focus is most handy. This feature is often sufficient for producing excellent photos but there are limitations. For instance, the camera will not sense when the user is attempting to take a shot which captures faster action. Use of a shutter speed that is too slow in this instance will result in blurring. As described above, it may be advisable to adjust the shutter speed, and possibly the aperture size, for this type of shot.
Many digital cameras also possess a Programmed Mode which allows the user to select the situation that is most appropriate; for instance portrait, sports, or landscape, and the camera makes the decisions from there. As discussed above, adjustments in the shutter speed to allow you to create special effects may be desired but not congruent with what the camera assumes is wanted. Most digital cameras offer a Shutter Priority setting that allows the user to adjust the shutter speed while the camera determines the aperture and other features. In other instances, increased depth of field may be desired and achieved by selecting a smaller aperture. This is possible using the camera’s Aperture Priority setting which will allow aperture alone to be adjusted. The Manual Mode allows both the shutter speed and aperture to be manually controlled. Obviously, this mode allows for the most control of shots but requires a significantly higher level of skill.
A final word about getting the best exposure with a digital camera is about a technique called “bracketing”. Bracketing is used when it is difficult to determine the optimum exposure because the lighting varies within the scene. For instance, taking a photograph of a dog sitting in a snow covered field presents challenges based on the extreme contrast and reflection. Bracketing is simply the practice of taking several shots of the same scene with different exposures. Most digital cameras make this very easy to do as they can automatically vary the exposure, taking a shot at the metered setting and others which are under exposed and over exposed. The user then compares the images, selects the best one, and deletes the rest. During the editing process it is even possible to combine the best parts of multiple shots to get the best composite image.
Although automatic settings produce great images, even a novice can adjust shutter speed and aperture to capture action and create special effects within their photographs. Digital cameras make the process easier and less expensive through progressively manual controls and the option of deleting photographs that simply aren’t desirable.
Christine Peppler
http://www.articlesbase.com/digital-photography-articles/basic-digital-photography-taking-control-of-exposure-87402.html
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