focal length and filtration

Jody Lipes · June 1, 2004

what is the purpose of adjusting the intensity of diffusion filtration when the focal length of the lens changes? why does a filter appear more or less intense because of the focal length of the lens?

Michael Nash · June 1, 2004

Two factors:

1- the magnification of the diffusion in the filter

2- the magnification of on-camera details you want to diffuse

Diffusion filters work by way of irregularities on the surface of the filter, to spread the light as it travels through/around that irregularity. In something like a Pro-Mist filter, the diffusion consists of a bunch of tiny white (or black) dots on the surface of the filter, kind of like a fine "mist" of spray paint or dust stuck to the glass. Soft FX and Mitchell diffusion filters use a series of bumps or "dents" in the glass surface to disturb the light path. In any case, the longer the focal length the more you magnify these dots or bumps in the filter relative to the frame. So the longer the focal length, the stronger the diffusion appears.

The flip side of that is the magnification of the subject. A wide shot of a person, say head-to-toe, makes small details and wrinkles in their skin pretty small in frame, so you don't need much diffusion to soften them. But when you go in tight on a longer lens, those small wrinkles are suddenly huge on screen, sending you diving for the filter box before the lead actress has a chance to see the monitor...

The trick is deciding on how you want to balance these two competing magnifications. Typically cinematographers increase the strength of the diffusion filter along with increases in focal length, in an attempt to smooth out the difference in detail between wide shots and tight shots. If the added magnification of the diffusion is too strong with an increase in focal length, you can back off the strength of the filter to maintain a more consitent look.

It's not uncommon to use one filter for all the medium focal lengths, and change the strength of the filter only for extreme wide or extreme tight shots. But there are always shots that warrant special circumstances, like a close-up shot with a wide lens. What do you do then? Drop to a weaker filter because you're on a wide lens, or go to a stronger filter because you're close on fine details? Or go extra-heavy with the diffusion because the shorter focal length isn't magnifying the irregularities in the filter as much? In the end you end up making decisions shot-by-shot based on the content of the frame and an attempt to make the diffusion appear consistent throughout the shot in a sequence. Sometimes you even need to use different types of filters to blend the look more seemlessly, as filter strengths and magnifications aren't always a consistent match.

Jody Lipes · June 2, 2004

thanks, that makes a lot of sense. knowing the physics always helps me. but again, good cinematography is just a case of trusting your eye.

Max Jacoby · June 2, 2004

Most DOPs I've worked with increase the strenght of the diffusion filter when they move into close-ups. On 'Girl with a Pearl Earring' Eduardo Serra shot most of the studio scenes with a 1/8 Promist, except for the occasional close-up of Scarlett Johansson, where he went to 1/4 Promist.

The only exception so far was Benoit Delhomme, who stopped using a 1/8 PM when we went on longer lenses (135mm and 180mm UP)

stijnbarbe · June 2, 2004

again testing is the only true answer :)

Michael Nash · June 2, 2004

The only exception so far was Benoit Delhomme, who stopped using a 1/8 PM when we went on longer lenses (135mm and 180mm UP)

That could be because the glass he was using was naturally softer/less contrasty at those focal lengths. Also filters in front of the lens can throw off the focus of telephoto lenses, so you sometimes use gel filters in the gate (so no pro mist there) or the lens may have an internal filterslot. But 135mm isn't really telephoto, just a longer focal length.