Diffusion behind the lens - any drawbacks?

George A · December 30, 2023

Hi!

Instead of putting a Black Promist 1/8 (or 1/4) in front of my lens, I am thinking to put a smaller filter (55mm) behind the lens.

The LPL mount has a screw in option so I can place a filter right behind the lens.

Any drawbacks to placing a diffusion filter behind the lens, instead of in front of it? Anything I sould be on the lookut for?

Does the rule of placing a heavier 1/4 filter for wides (14-35mm) and lighter 1/8 filter for medium and longer lenses (50 and up) still apply if I place te filter behind the lens?

Any other advice if I go that route?

I am talking glass filters, not stockings etc.

Thank you so much! I appreciate it.

Evan Walsh · December 30, 2023

The camera would need to be adjusted to account for the glass in the optical path and you would need to install a clear filter when the diffusion was not in use or else the focus of your lenses will be affected.

I worked on a show that did rear diffusion, depending on the scenario or how often you'd expect to change diffusion it can be a little bit of a pain since you have to pull the lens to make changes and take care to not introduce dust/debris. We would run one diffusion during the day and another at night generally of the same strengths so changes were not very often.

The benefit / reason we did rear diffusion was to remove the unwanted filter flares / reflections that can occur with front filtration.

Frank Wylie · December 30, 2023

From what I remember:

1. Any dust or fibers on the lens will substantially impact the image, as it is closer to the film plane.

2. Filter thickness may impact focus at infinity.

3. Spectral highlights may act differently.

4. Easy to forget it's on the lens.

Edited December 30, 2023 by Frank Wylie

David Mullen ASC · December 30, 2023

First of all, make sure the lens has the back-focus set for a glass filter behind it. Second of all, the filters look heavier from behind so generally the 1/8 strength is about as heavy as most people go. Also, very wide-angle lenses are more sensitive to changes in back-focus so keep an eye out for that.