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Diffusion Differences


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#1 Cole Webley

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Posted 28 January 2006 - 01:56 PM

Can anybody explain what the principal differences between black/white pro mists, classic softners, glimmer glass, white/black/etc. nets...

It seems to me that nets and filter differences are pretty obvious, but if I can't get a hold of glimmer glass or a classif softner does a pro mists get the same look?

Any help would be appreciated.

Thanks
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#2 David Mullen ASC

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Posted 28 January 2006 - 02:25 PM

Well, obviously there are differences or else there would be no reason for a filter company to create a new filter. However, some filters made by one company are meant to create a similar look to another company's product. Schneider White Frost is similar to Tiffen ProMist and Wilson SupraFrost. And there are "black" versions of all of those.

Most of these companies make Fog and Low Con filters.

Testing is the best way to know which filter to use, and when in doubt, use the lighter grades. You can see some examples on websites but how heavy a filter to use depends on the degree of enlargement of the image and what format you are shooting on, what lenses, etc. Often books with examples (like "Image Control" by Hirshfeld) have to use pretty heavy versions of these filters for the effect to show up in a 2"x2" photo, but an image projected onto a 50' wide screen needs to use much less filtration.

Diffusion, in principle, is the process of overlaying an unsharp image over a sharp image. It's not the same as soft-focus. A net filter, for example, diffuses by allowing a sharp image to pass through the open holes of the weave but an unsharp image is created by the diffraction of the image passing around the edges of the weave.

But some diffusion filters are more like fog filters, that is, they have "mist" particles that cause bright areas to bleed into dark areas, usually creating halation (glowing) and causing a loss of contrast.

So diffusion filters as an overall group vary in degree from how much they behave like light-scattering "fog" filters versus how much they just soften detail without a foggy look. There's no right or wrong here, just a matter of the look you want.

At the "foggier" end of the scale, filters with "mist" particles, you have Fog filters, Low Cons (which don't soften as much as Fogs, with less blooming around lights), Double-Fogs (a Harrison product that combines Fog + Low Con), UltraCons (which lower contrast without any blooming around lights, just create an even haze like flashing), DigiCons (like UltraCons with darkening black specks to bring down highlights). At this point, UltraCons, DigiCons, etc. often are not considered diffusion because they don't soften too much, just lower contrast.

An odd filter in this group is the Tiffen Smoque filter, which simulates dimensional smoke in the air -- it reminds me of a sharper version of a Double Fog.

A little more softening comes with the mist filters with names like ProMist, Frost, SupraFrost, GlimmerGlass. They soften detail more than just fogging up the image, but they also create halation effects. They often have "black" versions where black specks have been added to restore black levels and contrast a little.

Then the next level of diffusion are filters with patterns in the glass that cause part of the image to be diffracted. Depending on how small and random the pattern is, some halation (glowing) effects still occur but less strongly than with the mist filters. Schneider Classic Softs have a regular grid-like pattern of "lenslets" (looks like half-bubbles in the surface). Tiffen Soft-FX have a smaller more irregular pattern of lenslets. Nets sort of fall into this category since they also cause a little bit of halation.

Finally you have diffusion filters where the pattern in the glass is designed to not cause much halation at all, just soften detail invisibly without any glowiness. Tiffen Diffusion-FX and the old Mitchell diffusers come into this category. These are more for use when you are shooting an unfiltered movie but want to soften a close-up without it looking like a filter was added, just softening the definition. They are practical more than filters that create "looks".

This site has some comparions:

http://www.camerafilters.co.uk/
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#3 Cole Webley

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Posted 28 January 2006 - 02:50 PM

Thanks David. I had asked other young DP's this same question and nobody has been able to give me such a complete answer.
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