Classifying Diffusion Filters in Concrete Terms

[Michael Glantzis]

Hi All,

I've been finding myself watching a lot of filter test videos lately, and was wondering if anyone had a more precise method to classify the differences between the types of diffusion filters. Something like Tiffen's "Triangle of Diffusion" interests me, however I find it troublesome that this model presents halation, contrast reduction, and resolution reduction as mutually exclusive. Each of these qualities is a scale in and of itself, and adding to one does not necessarily take away from another.

Additionally, I've noticed some qualities of diffusion filters that are not represented on the triangle of diffusion, or are sub-categories of those that are. These qualities include:

• Darkening of the Highlights
• Brightening of the Shadows
• Changes in Saturation
• Color of Halation
• Color Rendering
• Warming/Cooling
• Atmospheric Effects

And I'm sure there are more distinguishing qualities that I'm not including. 

I was wondering if any information is available that describes the behavior of filters in a more comprehensive way. Of course, the artistic impression of a filter is ultimately going to be subjective, but I do think it is extremely helpful when developing an artistic intuition to understand precisely how a piece of optics affects an image. Ultimately, when I'm telling my colleague why I prefer Glimmerglass over Black Pro Mist, I want to talk less about Glimmerglass's "dream like quality", and be able to point to specific, concrete, variables that are changing within the image. 

While I've been spending some time evaluating these filters myself, I'm fairly new to the field, and would love to hear the approaches of people who have been doing it longer than me. Excited to hear what people think.

[David Mullen ASC]

Another important factor is the changes in strength over a series. Modern filters tend to be better spaced in strength with more subtle jumps, whereas a 1/2, 1, and 2 Tiffen Soft/FX are pretty distinct between them. So it depends on how much you plan on switching between filter strengths to modulate the effect.

But you'd have to come up with your own nomenclature I'm afraid to describe what you are seeing.

[David Mullen ASC]

Filter designs for diffusion tend to show up in the bokeh or when you stop way down and get too much depth of field, so it might be interesting to shoot a test of a tiny bright slit of light against black, to look at halation, on a macro lens stopped way down and then pan past the slit of light, then zoom into the image and see the interaction between the light and the filter structure. It would be misleading in terms of the look the filter was designed to create in normal situations but it may help you understand how the filter is interacting with light rays.

Diffusion filters tend to be "mist" designs or use something to diffract focus around certain points (patterns in glass, dimples or bumps, threads of a net, etc). Or they combine both types. But keep in mind that particles designed to "mist" and glow are also diffracting focus so they act to soften detail, just not as heavily as a filter specifically designed to blur detail.

[Michael Glantzis]

Thanks for the response. I agree that would be an interesting experiment. Perhaps some day when I have access to more filters I'll give it a go haha.

In the meantime, I suppose I'll have to stick to what's available. We're really fortunate to have so much testing footage available to us, and I'm sure I'll develop more of an intuition around these things as I get more experience shooting.

Once again, thanks for the response. Definitely interested to see if anyone else has thoughts on this.