It is tempting to think of sharpness only in terms of focus and depth-of-field but accurate focus is only one factor in image sharpness. What we are really concerned with is 'acceptable' sharpness or circles of confusion small enough to be perceived as sharp by the eye (.01 inch or less in diameter). Anytime you are trying to achieve the maximum sharpness you need to take the following into account:
Camera movement - Any movement of the camera has an effect on the sharpness of the image. While camera shake is obvious with slower shutter speeds even a subtle effect can be seen if the photo is enlarged because the blurs are being enlarged too. Your primary defense against camera movement is your trusty tripod. It's a good idea to use a cable release in conjunction with it so that you don't inadvertently shake the camera while tripping the shutter. If you are in a situation where you can't use a tripod, consider using a monopod. Another trick is to lean your body against something stable, a building, telephone pole, tree, etc. I once shot a waterfall at a quarter second with help from a friendly tree and it looks great at 11X14 (enlarged from 6x4.5cm).
Focus - When focusing always decide what is the most important point in the subject and be certain that you have that point in focus. The point is your center of interest and is the part of the photograph which will naturally attract the viewer's attention. In a portrait it would be the subject's eyes.
Depth-of-field - There are times when you want greater DOF to have more of the image in acceptable focus. You can achieve greater depth-of-field by stopping down (using smaller diaphragm openings). See the explanation of the Circle of Confusion to learn why this works as it does. There are limits to the effectiveness of stopping down to achieve sharpness due to the limits of lens design and the behavior of light.
f/stop - The smallest diaphragm openings produce the greatest relative DOF however the image sharpness is actually less than a moderate opening produces because of diffraction or bending of the light rays. Light rays which closely pass an edge bend toward the edge as they go by much like when you touch the side of the stream of water falling from a tap (try it). The stream (or a portion of it) will bend toward your finger. The rays thus scattered are then out of focus. Diffraction occurs at any lens opening but since the proportion of diffracted rays to non-diffracted rays is greater with very small openings than with larger openings, the image sharpness is slightly less at the smaller openings. The faucet experiment works to show this effect too. Open the tap to the smallest steady stream you can and lightly touch the side of the stream with your finger. Chances are the entire stream will be diverted toward your finger. Now open the tap further. As the volume of the stream increases, less of it touches your finger and more of it falls straight.
Lens quality - Aside from actual differences in the quality of lenses, no lens is equally sharp at all the available diaphragm openings. The maximum sharpness of a lens is obtained in the middle range of the available f/stops. For example, if you have a 50mm lens with a range of f/2 to f/16, your lens is probably sharpest when used at f/5.6 or f/8. This is because of the limits of lens design. Maybe someday we'll figure out how to design a lens that is equally sharp at all openings but we are not there yet. In the meantime lens designers go for the greatest sharpness in the middle range and compromise on the rest. Although not a lens you should consider your filters as part of the lens. Buy the very highest quality you can afford. They are in the light path and your image will be only as good as can be produced by the lowest quality piece of glass (resin, plastic or gelatin) in the light path. It should be obvious that you must keep all your lenses and filters clean.
Film - As important as the focus and transmission of the light rays is, the ability of the film to record the image sharply is what determines whether all your efforts will result in a sharp image. In general, finer grained, slower films are sharper than larger grained, faster films. Some of the newer tabular grained films (Kodak's T-Max films and Ilford's Delta series) decrease the differences between slower and faster films but don't eliminate it.
Processing - If you've never processed your own film you are probably unaware that the processing also enters into the sharpness of the image. Ilford film tech sheets, for example, list the options for processing their B&W emulsions for overall image quality, variations of the ISO, finest grain, maximum sharpness, etc. For maximum sharpness you need to choose an appropriate developer for the film you are using. I like using Ilford Pan F in ID-11 diluted 1 to 3. Kodak's Microdol is an old standby for fine grain and sharpness. A favorite developer for many photographers is Rodinal which can be used in extreme dilution which also helps sharpness. Ilford's Ilfosol -S is similar in is action. Dilution (with corresponding increases in development time) has the bonus effect of extending the tonal range in standard emulsions. Not all developers function well in dilution and tabular emulsions may not respond the same. Check the manufacturer's recommendations. As they are continually updating their products I will not attempt to duplicate that information here.
If you are making enlargements the problem of maintaining sharpness continues into the printing process where you have many of the same concerns. You need accurate focus, a properly aligned and stable enlarger, good lenses (kept clean) and an appropriate paper/processing technique. That will be covered in the lesson on making enlargements.
Return to Lessons Index
"Success is simply a matter of luck. Ask any failure." Anonymous