Soft focus, or the art of softening image details goes way beyond the times of photography, one of its most proficient users was a certain little known Leonardo da Vinci who painted an obscure picture called Mona Lisa using this technique, which he did not call soft focus but sfumato (lit. smoked, meaning dissolved in smoke).
Mona Lisa by L. da Vinci (source: Wikipedia)
With the perfecting of photography at the time of the Calotype photographers ran more and more into the situation that people are, after all, vain. While it was quite easy to not paint wrinkles on a face (and in fact less work not to) it was quite something else on a photo. If there are wrinkles it will show.
To compensate that photographers started experimenting and by the mid 1850s had discovered that if you put your subject at either the front or the back end of the DOF extremes it retains its contour sharpness but blurs finer details, such as wrinkles.
Badly photographed owl (focused, soft foused, out of focus) by Alf B. Meier
Now, not many photographers, as we constantly see, are capable or willing to calculate the depth of field of their cameras, much less to use a ruler to measure the actual distance to the subject. Inventiveness has led to many ways to emulate the camera setting, from putting a nylon stocking over the lens to use a clear filter with Vaseline (TM) smeared on it. For practical purposes one can buy soft focus filters from well known manufacturers like Heliopan or Cokin that come in various graduations
Soft focus filter (without, with) Source: Heliopan
If we compare the image results we come to the conclusion that the better results are achieved by the focusing mode, as with the filter the blur is not exclusively on the center of the subject but also blurs the contours, additionally a soft filter shifts the color intensity.
When to soft focus? There are no rules, except doing away with wrinkles. Soft focus is an artistic (and often misunderstood) decision. From the average portrait to the almost genial (and controversial) images of David Hamilton it has its justification. Sometimes, with a little blur you can convert an ordinary (and boring) picture into something...lets say ...more artistic.
Olympos (combination of soft focus and structured filter blur) by Alf B. Meier
