DIFFUSION AND SOFTENING FILTERS
You may have wondered what are these filters for and what is the difference between the two.
Both "diffusion" and "softening" filters are used towards the same objective: to reduce blemishes and wrinkles in portraiture. Make people look better. The soft filter is just a more elaborated diffusion lens.
Before these were mass produced, we placed a stretched fine mesh stocking in front of the lens. What this did was to bend a certain percentage of the light forming the image from its original path and thus defocused it. More recently, these can be obtained as standard optical glass filters, like the Tiffen Softnet series. These function through "selective diffusion." They have a greater effect on small details, such as wrinkles and skin blemishes, than on the rest of the image. The clear spaces in the mesh transmit light unchanged, preserving the overall sharp appearance of the image.
Instead of thinking it through (as we should have all done), it was through plenty of experimentation -and anger from our sisters watching their stockings disappear- that we discovered that the finer the mesh, the more the image area was covered by mesh lines and therefore had a greater effect. Contrast was of course reduced and for high speed B&W film photography it looked very fancy.
Another way to obtain diffusion was to place a glass, in front of the lens, smeared with Vaseline. I built several -very heavy- metal contraptions that attached to the tripod socket of the camera, extending a bracket to the front beyond the lens to hold a window frame, where a flat sheet of glass could be slided from the top. We soon found out that leaving the center clear looked even better, more romantic, by isolating the subject from its surroundings. Eventually all of this came to the ears of manufacturers and now we can buy commercial Center-Spot screw-in diffusion filters.
Hoya, for example, achieves the diffusion through an irregular uneven surface. Cokin's Diffusers 083 or 084 are other alternatives.
A popular type are the concentric ring softeners or Dutto filters; they look like water after you drop a stone into it. Examples of this type are the B+W Soft Focus 1 and 2 and the Marumi Diffuser.
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