Back to the basics
A camera has basically three independent adjustments, the combination of which determines the success of a picture or its loss (independently from other parameters, besides the camera) :
The distance (or focus) adjusts the camera-to-subject distance for a
focused image. It is measured in feet (or meters).
These basic definitions , probably known by most of you, being done we now come to secondary parameters which stem from these three basic ones. Two of these parameters are important to understand :
Exposure is an abstract notion which defines the quantity of light that reaches the film (intensity of light x exposure time). For a film of a given sensitivity, the film should always receive the same average quantity of light, whatever the photographed subject is. According to the above definitions, it follows that either the aperture, or the shutter speed, can be adjusted to get the same result. And that is how we can get the same exposure with a shutter speed of 1/100th second and an f:16 aperture as with a shutter speed of 1/200th second (exposure time / 2) and an f:11 aperture (aperture x2). So, there are many exposure time/aperture combinations that provide the same exposure. In standard photography, the general rule is to have the smallest aperture when the scene is very bright and the larger aperture when the scene is very dark, so as to keep exposure time around 1/100th second. Of course, there are many exceptions to this rule and photographing models is part of these exceptions.
Depth of field defines the distance range over which the image is in focus. This distance range varies according to the distance setting and the aperture. It's the presence of a lens which causes the loss of focus for very short (or very long) distances and nothing will do, except using one of the ancestors of the camera, from the beginning of photography, which didn't have any lens. But it is possible to minimize this phenomenon, without being able to fully suppress it.
Rules #1 and #2 show that we are not really in the best conditions when photographing models since distances are usually very short and we want to use a large aperture since the lighting is dim. The only answer to this problem is to use the smallest possible aperture (usually f:22, or even f:27 or f:32 with some lenses). This results in very long exposure times (several seconds or even several dozens seconds) and the need to use a tripod so as to keep the camera still.
As an example, the photos at left shows the same scene photographed, under the same conditions, with two different aperture settings. The top photo was taken with an aperture of f:5.6 while the bottom photo was taken with an aperture of f:22. In both cases, the focus was made on the combine car. These examples clearly show the effect of aperture on depth of field. Note that this scene is only 20 inch deep and that the depth of field effect would be more important on a deeper scene.
In the Download section, you'll find an Excel sheet that allows you to calculate the depth of field, depending on your camera settings.
As for the focusing distance, there is no miracle solution since close-up photography requires to bring the camera closer to the scene. Try, however, to stay at reasonable distances whenever possible.
Some (older) lenses have a depth of field scale which is very convenient to determine the depth of field, depending on aperture and distance settings. The photos at right show two examples of depth of field for an aperture of f:22 and two different distance settings. In the top photo, the lens is set for a short distance and the depth of field is only 1.5 to 1.7 ft. In the bottom photo the lens is set for a long distance and the depth of field goes from 6 ft to infinity. This illustrates well the influence of distance setting on depth of field.
By strictly following these rules, you'll get the best possible depth of field, according to the situation. But everything will not be perfect and you'll still have some out-of-focus zones in the close foreground and/or the far background. Try to focus slightly in front of the main subject (to take into account the 1/3 - 2/3 depth of field rule) in order to get the deepest in-focus zone. If, notwithstanding, you are not satisfied with your pictures, you'll need to go a step further and build your own pinhole lens (which is not commercially available), as I explain in a subsequent page.
However, the difficulties do not stop here ! As we just saw, model photography is synonymous with long exposure times, even very long ones, depending upon the film used. Films don't react well to these long exposure times and here comes what's called the "reciprocal" law. We saw that when the aperture is decreased in a certain ratio, the exposure time should be increased in the same ratio. This is true when exposure time stays within reasonable limits (less than a couple of seconds) but becomes inaccurate when exposure time increases. In that case, it becomes necessary to "over-compensate" by increasing the exposure time. It is difficult to predict the result and the best method is to take the same picture several times, with different exposure times.
Now, let's see how to put all this theory into practice.