Most photographers only think of f-stops in terms of depth-of field. When the aperture is open (smaller numbers, such as f2.8) the field-of-focus is relatively narrow. When the aperture is stopped-down (larger numbers, such as f16) the field-of-focus is relatively broad. To get a lot in focus, you stop-down .  To isolate a subject with an out-of-focus foreground (and/or background) you open-up the aperture. Some lenses have scales inscribed on them to indicate the depth-of-field at selected f-stops, but many don't have this handy feature. For these lenses it's necessary to refer to a depth-of-field table or chart.

While depth-of-field is an important criteria to use in determining the appropriate aperture, there are other, perhaps more important considerations when selecting the aperture setting. One of these is lens aberrations. When most lenses are opened up, they are subjected to image-deteriorating conditions such as spherical and chromatic aberrations. Briefly, these mean that rays of light passing through different sections of the lens focus on different planes. The rays that happen to coincide with the film plane will to sharp, but any falling in front of or in back of the film plane will be out-of-focus. Stopping down the lens helps eliminate these aberrations since the rays of light are brought through the lens at the same place on the lens. However, stopping down the lens causes other aberrations to occur. Diffraction is the biggest image-degrader and gets worse as the f-stop gets smaller (larger numbers). The image actually is deteriorated by the edges of the diaphragm blades. When the lens is opened wide, this "edge effect" amount to a very small percentage of the total light that passes through the lens, but when the aperture is very small, the diffracted light is such a large proportion of the total light that the image is fuzzy.

As a result of these image deteriorating factors at both ends of the aperture scale, the optimum quality for most lenses occurs somewhere in the middle of the f-stop scale. The exact point differs with each lens design, but is usually one or two f-stops down from the maximum (wide open). For example, with a f2.8 lens, the best quality is most likely to occur around f4.0 or f5.6. It's fairly easy to determine the optimum f-stop for any lens. Select a fine-grained film and a high contrast subject such as a magazine or newspaper with both large and small print. Place the camera on a tripod, and place the subject at a distance equal to 50 times the focal length of the lens. (With a fixed focus lens, common on many subminiatures, set the subject to the fixed distance). For example, with a 25mm lens, place the newspaper about four feet from the camera. Take pictures at every f-stop setting and make enlargements from each negative. (Make sure you use the best enlarger lens aperture setting for the enlargements.  Check out the DARKROOM for details)  It will be easy to pick out the aperture setting with the least amount of aberration -- it's the one that's the sharpest! Don't be surprised if some of the settings give absolutely horrid results. This test will clearly show you which f-stops to use, which to avoid and which to select when you want the absolute best results.

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