Exposure And The Right Aperture
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Photographers should aim for small steps in their journey into advanced photographic techniques to avoid getting lost in a maze of complex exposure settings.
Aperture is a good place to start experimenting and gaining experience with basic exposure settings. The Av or aperture priority is the first step away from programmed scene modes and is a semi automatic mode. This is where photographers select the lens aperture best suited to their creative vision, while the camera sets the shutter speed, and commonly now the ISO sensitivity as well.
Aperture is more than controlling the amount of light entering the camera, as the quality of the resultant image varies with aperture. Image quality usually drops off approaching maximum aperture, and improves as the size of the aperture reduces. Then as the aperture reduces further, image quality deteriorates as smaller apertures suffer from the increased influence of diffraction around the edges of the aperture blades.
The exposure settings on a camera interact and producing different emphasis and characteristic effects on the image. Compromises are essential with conflicting requirements of aperture, shutter speed, and ISO. This leaves scope for creative decisions on the relative importance of these factors to the photographer’s view of the image, as there is no right answer to exposure decisions.
While aperture does vary the amount of light, the variation in depth of field with changes in aperture setting is more important than light control.
The Right Aperture
There is no such thing as the right aperture as the choice relies on a number of creative and practical considerations. For a good general-purpose starting point, try f8 for the best optical quality with reasonable depth of field. The general rule of thumb is lenses start performing better at two f stops down from their maximum or wide-open setting. However, f8 should be right in the aperture sweet spot for all DSLR lenses.
A good lens has a larger sweet spot in the middle of its range of apertures, allowing a greater variation of depth of field without compromising image quality.
Diffraction
Diffraction of some light rays passing through the lens aperture always occurs. However, as the aperture reduces, the small amount of diffraction becomes significant due to the decreasing proportion of un-diffracted light passing through the center of the lens.
Landscape photographers particularly ignore diffraction effects. The greater depth of field increases overall sharpness of the image and counterbalances a small loss of quality from diffraction at small apertures.
Vignetting
Vignetting is a common problem in wide-angle lenses and it becomes more noticeable at wide apertures. Vignetting is the darkening in the corners of the images. It may also be from shadow from filters on the front of the lens, such as a UV or polarizing filters.
Shutter Speed
While smaller apertures provide some advantages, there is a price to pay. Faster shutter speeds reduce the impact of camera shake on image quality and this requires larger apertures, reducing depth of field and optical performance of the lens.
ISO sensitivity
One way of achieving a fast shutter speed without a large aperture is to increase the ISO sensitivity if the image recording device. The downsides are grain in film images or noise I digital pictures.
The compromise is a large aperture to allow more light for a fast shutter speed and to keep the ISO down for good digital noise performance. Using the Av mode photographers can experiment with different apertures in a variety of settings without being completely distracted from compositional issues as the digital camera helps with useable exposure settings.
Example Photographs
The first two illustrate a change in depth of field at f8 and with the lens aperture wide open. Examine the bench seat in the foreground to see the difference in sharpness. In the second set of examples, the Sigma 18-125 lens displays a characteristic tendency for serious vignetting at wide apertures. The second shot at f8 shows much less darkening in the corners, or vignetting.
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