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Still-life photography, by definition, is concerned with subjects that do not move. This is a key factor in making many images possible. Depth of field is a vital consideration when working close to a subject that may have significant depth, so dropping the shutter speed as far as possible allows small apertures to be used. Depth of field is consequently maximized while still using a reasonably slow ISO setting to reduce noise. The focused distance must also be chosen carefully to ensure that the depth of field envelope (the zone of acceptable sharpness) is used to best advantage. The rule of thumb is to focus at a point one third of the distance from the front to back of the subject, although this is only an approximation. Obviously the camera must be held absolutely motionless during exposures, so a good tripod and a remote release are required to avoid camera movement.

Achieving front-to-back sharpness for the subject is, however, not quite a simple as it might seem. Although the principal of slow shutter speeds and small apertures is sound, depth of field may still prove insufficient at f/22, f/32 or f/64. Small apertures such as these also increase diffraction to a point where images have an unacceptably soft appearance even at the point of focus. The effect is caused when rays of light are bent as they pass through very small apertures and hence travel slightly further than rays that are not bent. Diffraction bent rays become out of phase with others and begin to interfere - so lessening sharpness.

Cameras featuring small sensors, such as point-and-shoot models or those found in mobile phones, render almost everything in acceptably sharp focus even though the overall picture quality may be poor. But cameras with larger sensors, such as those typically used in DSLRs of professional cameras equipped with full-frame sensors, it may be more of a challenge achieving sufficient depth of field. As sensor size increases, depth of field for given apertures decreases when the frame is filled with a subject of the same size and distance. Cameras with larger sensors must be closer to the subject, or fitted with longer focal length lenses, to fill their frames with the same subject. Progressively smaller apertures must therefore be used to maintain depth of field.

With digital cameras it is easy to take test images and examine them to ensure the depth of field is adequate. Where inadequate depth of field is found, try small changes in the focused distance to see if the problem can be overcome. If not, it may be necessary to work a little further from the subject. If this approach fails, then focus stacking can be used to take a series of perhaps six or eight images at slightly different points of focus. If none of the images has sufficient depth of field they may be digitally processed using focus stacking software to combine the in-focus areas of each image. However, this latter approach requires some experience.


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