ProMist/Warm ProMist/Black ProMist/Warm Black ProMist
ProMist is a foggy-type diffusion similar to a Fog Filter, except that it softens the image more without the milkier look of a Fog, more of a "misty" look. It was Tiffen's version of a popular plastic filter that appeared in the early 1980's called the Wilson SupraFrost. People liked those except that they were plastic; Tiffen created something similar but in glass and it took off. Schneider makes a similar filter called White Frost.
They added black specs to counteract some of the loss of contrast caused by the misty effect and called that a Black ProMist (they probably got the idea from the Harrison and Harrison Black Dot Texture Screens invented in the early 1980's). This was called Black ProMist and it looks a little more subtle than the same strength regular (aka White) ProMist because it gets less milky. See also the Schneider Black Frost.
They also made a version of both filters with their pale reddish 812 warming filter added, creating the Warm ProMist and Warm Black ProMist.
Black/Warm/Gold Diffusion/FX
Diffusion/FX is sort of the culmination of a lot of research by Ira Tiffen to design a diffusion filter that was "invisible" -- no halation (glowing), no loss of contrast (caused by halation), no artifacts really of any kind... it just made the image softer in definition. Like other diffusions (Mitchells, Soft-FX, Classic Softs) there is a pattern of indentations that cause an area of the image to be thrown out-of-focus over a sharp image that passes through the clear areas. With Mitchell Diffusion, that pattern is a bunch of trapezoids; with Classic Softs, it's a regular gridlike pattern of circles, with Soft-FX and Diffusion/FX, it's an irregular kidney-shaped pattern. Soft-FX was the earlier design and it still causes some halation. Tiffen added a pattern of black dots to improve contrast in Diffusion/FX. I'm not sure the details of the Warm, Gold, etc. The Warm may have an 812 added, the Gold I think uses Gold dots instead of Black dots. Tiffen now makes a Digitial Diffusion/FX that basically has no dots in it, to remove the chance that the pattern will come into focus on a video camera, many of which have a lot more depth of field than 35mm.
Diffusion/FX is perfect for when you need to soften a face in an otherwise unfiltered scenes; but otherwise, I find the lack of artifacts to be a little boring -- when I use diffusion, it's partly to get a little glow and kick from hot spots, etc. So I like Soft-FX and Classic Soft a little better.
Warm Soft/FX
A Tiffen Soft-FX with the 812 warming filter added. Soft-FX softens with a little halation -- not as foggy/glowey as a ProMist but not as artifact-free as Diffusion/FX. Sort of similar to a Schneider Classic Soft except that the halation looks different -- it sort of "smears" whereas as the Classic Soft sort of creates a fuzzy double-edge around a bright object (and a circular blob around points of light on some focal lengths.)
Schneider now makes a combo Classic Soft / Black Frost filter that in some ways has more of a Soft-FX glow.
Black/White Softnet
Not sure -- I think Softnets have a black or white net sandwiched in the filter. Personally, I think the weave they use is a bit crude compared to a true net like from ultrasheer pantyhouse.
Fog/Double Fog
Fog filters have been around for a long time and was a favorite filter of Geoffrey Unsworth ("Cabaret", "Superman", "Bridge Too Far"). Makes the image look foggy, causes halation around lights although it tends to look bluer than the glow from a ProMist. It also softens the image. Fogs sort of lost favor when high-speed stocks were introduced in the early 1980's because the fogginess tended to exaggerate the grain. Less of an issue now that stocks have gotten so fine-grained again. Some people feel that the older Fogs tend to throw the image slightly out-of-focus. (The last major film I remember using a Fog for most of the movie was "Star Trek IV: The Voyage Home", which used a light 1/2 Fog.)
Less "glowy" are Low-Con filters, which still cause halation ("Barry Lyndon" was shot with Low-Cons; Peter Hyams likes them, as can be seen on "2010"; Haskell Wexler alternated between a Fog and Low-Con on parts of "Bound for Glory"). Tiffen made a version with didn't cause halation at all, nor did it soften, just a lifted the blacks and shadow detail -- that was called UltraCons. Schneider or Formatt makes a Low-Con filter that is more like Tiffens UltraCons, not like the traditional Low-Con, which is closer to a Fog but less foggy.
Double-Fogs are actually a Harrison and Harrison invention that combines a Fog with a Low-Con, so it is actually less foggy than a Fog, not more (depending on the strength of course -- a heavy Double-Fog would look pretty foggy compared to a light Fog). It was designed to not soften sharpness as much as the Fog did, so the image is a little milkier than mistier, if that makes sense to you. Vilmos Zsigmond was fond of Double-Fogs, using them on "McCabe and Mrs. Miller."
Some of these filters can age over time, making the effect seem stronger than they were designed to.
