A soft focus filter is any one of a variety of photography and film-making filters that give a scene a blurry look.
Why, you ask, can't one just defocus the scene with the focus ring ? Yes, but the effect looks different because of bokeh and because the defocusing effect depends on the subject-lens distance; while a soft focus filter will blur the scene uniformly.

There are many different types of soft focus filters: some have a pattern (rings or cross-hatching) etched on the glass, others use gauzy material.
On the other hand, you can make your own: just stretch a piece of nylon stocking or very fine mosquito netting in front of your lens - experiment with different weights to get various effects. Or you can sacrifice a UV filter, and coat it in vaseline. I mean, you are just trying to degrade the performance of a perfectly good optical system, so be creative.

Marriage photographers use this stuff a lot, because (among other things) it hides skin blemishes and wrinkles. If you really love the effect, there are lenses with a feature called DC (Defocus Control), that works like a tunable soft filter.

Some additional info at: soft/fx and fog enhancing filter

One of the uses of the soft focus filter that many of us are familiar with is in old Star Trek episodes. After watching a number of these I referred to it disparagingly as the "chick filter", because any attractive woman on the show invariably had her close-up shots soft-focused. It's all hard-edged Captain Kirk, then cut to a close-up of Lieutenant Uhura, who appears to be standing in a cloud of talcum powder...

I was recently shocked and amused to find that on the board of faculty pictures at my new university department, the two female professors' photos were the only ones in soft-focus! This despite the fact that many a warty, hairy male prof could have well benefited from it...

In addition to soft focus filters, there are also soft focus lenses out there, that work very differently and (according to many photographers, myself included) with better results.

I only have experience with the Canon 135mm f2.8 Soft Focus lens, but all of those lenses should work aproximately the same, so this w/u should apply to them all. I know for a fact that Minolta also makes soft focus lenses, but I’m not sure about Nikon.

How to operate it

The Soft Focus on the Canon 135mm lens has three settings; off, 1 and 2. When set to off, the lens performs just like any other bright 135mm lens. When the setting is on 1 or 2, the soft focus is incurred.

The soft focus function only works on apertures wider than 5.6. The soft focus effect changes depending on the aperture as well; More soft focus on f/16 than on f/8, for example.

What it looks like

When you turn the control ring to 1 or 2, a soft, diffuse glow is added to the picture. This means that small irregularities (for example in skin and other textures) is blurred, while the overall sharpness (focus) of the picture is kept very well. "Dreamy" is a word that describes the effect best.

How it works

By turning the soft focus ring, a controlled spherical aberration is introduced to the lens by turning an aspherical lens element. As a matter of fact, the lens does not add spherical aberration as such, but it rather removes its protection from the aberration, by turning the lens element in such a manner that the light passes through the lens unaltered.

How to mimick it

There is a way to mimick the effect, actually, through double exposures; Do a light metering first, using a large aperture (such as f2.8). Before you take the picture, turn the focus as close as you can get it, throwing your subject way out of focus. Set your camera to manual, and expose half of the shutter time you need (i.e if you need a 1/60 second exposure, use a 1/120 second exposure instead). Now, focus exactly on your target, and do the rest of the exposure.

If you want more blur, then vary the exposure times accordingly - longer blurred and less time non-blurred. Notice that the "sharp" picture adds definition to your final picture, and without the focused picture, your final result is going to look like crap.

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