Diffusion Filter Strength Based on Focal Length

[Ruth Vilas]

Hi everyone,

I came across a cinematographer who adjusts the strength of diffusion filters  based on focal length, whereas I usually stick to the same filter strength throughout the project—assuming I’m using the same lens kit (like 1/8 Black Pro-Mist on Master Primes, for example). If it’s a close-up where I notice more skin imperfections, I might even increase the strength.

Here’s how he uses them:

• Up to 25mm: all filters at 1/2

• 25mm to 50mm: all at 1/4

• 50mm and above: all at 1/8

I didn’t have time to ask what the logic behind this was, so I’m curious to hear your thoughts!

[David Mullen ASC]

I address this with Ira Tiffen in the filter chapter of the 11th Edition of the ASC Manual.

The reason for the contradictory advise comes from the fact that two separate issues are being conflated together.

One: you need less diffusion on longer focal lengths to match the degree of softening from a purely technical standpoint. Some of this is probably due to how a piece of glass affects front focus on longer lenses, but also because you are looking through a smaller portion of the filter so whatever element in the glass is used to blur focus is being enlarged.

Two: the tighter you go, the more diffusion you need because the viewer expects to see more fine detail in wide shots rather than in close-ups of faces, where too much detail can be unattractive, plus tighter shots tend to have less depth of field, so what's in focus naturally looks sharper being framed against a softer background, so can handle more diffusion.

So in some ways, the safest thing is to just use one strength throughout (ignoring other issues other than shot size & focal length that affect perception of sharpness, like lighting contrast, amount of haze on the set, etc.)

 

 

[Mark Dunn]

I can think of a few lighting cameramen who are famous in my book for diffusing closeups (and only closeups) of women long after it became unfashionable- into the 70s, certainly. The cut from a longer shot jars nowadays. Geoffrey Unsworth comes to mind, but Americans did it too.

This chimes in with a theme of David's as I recall- how did people at the time actually see and appreciate what we now see as poor travelling mattes, optical dissolves, and grainy selective enlargements?

[David Mullen ASC]

I think some people noticed optical effects but in general there was less expectation for cinema to be seamless and technically flawless. But I'm sure there were critics back then just as today who didn't "buy" certain effects, did not find them convincing. Also keep in mind that print projection back then hid a certain amount of flaws.

Some cinematographers were better at others at blending levels of diffusion -- I think Harry Stadling was excellent at that, whereas when Russell Metty put on a diffusion filter, it tended to really stick out. Some of it was the attitude at the time towards glamorization of close-ups; it was accepted as a convention, at least in the late 1920s, 1930s through mid-1950s. The widescreen and large format craze of the 1950s started to work against that.

The desire to soften close-ups for the purpose of glamorizing the lead actress hasn't gone away, it's just that today you have more subtle digital tools to blur hairpiece/wig glue lines, reduce bags under eyes, erase blemishes, etc. And in the early days of DI's, it wasn't always subtle either -- I remember "The Island" (2005), where director Michael Bay seemed to have an issue with Scarlett Johansson's mole on her cheek and decided to blur it throughout the movie.

On my show "The Marvelous Mrs. Maisel", we did not shoot close-ups, but in general there was a level of diffusion, usually from a 1/4 Schneider Hollywood Blackmagic. But I had to modulate that -- if we were shooting in hazed sets, the smoke would soften the image so I usually dropped back to a 1/8 strength. And for extreme long shots, I'd go even lighter to a 1/8 Black Frost or no filter at all. 

The thing is that today we have a lot more diffusion filter options and we can see the effect on an HD monitor on set, so it is easier to be more subtle when changing strengths of filters. 

In the 1970s, I think William Fraker was the closest in the U.S. to doing what Geoffrey Unsworth was doing with filters, though Fraker mixed his approach film to film based on the subject matter.  

[Mark Dunn]

29 minutes ago, David Mullen ASC said:

I think some people noticed optical effects

I think one of them was Stanley Kubrick!