In photography, soft focus is a lens flaw, in which the lens forms images that are blurred due to spherical aberration. A soft focus lens deliberately introduces spherical aberration in order to give the appearance of blurring the image while retaining sharp edges; it is not the same as an out-of-focus image, and the effect cannot be achieved simply by defocusing a sharp lens. Soft focus is also the name of the style of photograph produced by such a lens.
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Because soft focus is a technical flaw, many older lenses had soft focus built in as a side effect of their construction. Newer lenses are optimized to minimize optical aberrations, but there are lenses such as the Canon EF 135mm lens f/2.8 with Softfocus[1] and Pentax SMC 28mm f2.8 FA Soft Lens, to name but two, which have adjustable levels of spherical aberration at wide apertures. The effect can be disabled entirely as well, in which case the lens is sharp.
In the mid-1930s, Leitz designed a legendary soft-focus lens, the Thambar 90mm f2.2, for the Leica rangefinder cameras. It was made in small numbers, no more than 3000 units. It is a rare collector's item today. [2]
Nikon produces a series of DC ("Defocus Control") lenses which are sometimes confused with the soft focus effect, but these are not soft focus lenses, as they do not introduce spherical aberration over the whole field.
The soft focus effect is used as an effect for glamour photography, because the effect eliminates blemishes, and in general produces a dream-like image.
The effect of a soft focus lens is sometimes approximated by the use of diffusion filter or other method, such as stretching a nylon stocking over the front of the lens, or smearing petroleum jelly on a clear filter or on the front element of the lens itself.
It can also be approximated with post-processing procedures. Specifically, highlights in an image are blurred.