What kind of diffusion?

M

Matthew B

Well-known member
I have a 6' x 6' frame and am looking for a diffusion silk for it. Something like a general purpose diffusion for softening sun and hard light. I was leaning more towards a China silk. What sorts of silks are people using? Any recommendations?
 
John David Miller over at Cinematography.com posted this which I found pretty helpful. I have blatantly quoted his entire post, please correct me if that was wrong:

This is a good question that is often misunderstood.

First off, I feel sorry for any DP who endures a grip crew that cannot skin a frame of diffusion tightly. It is grip 101.

When the sun (a light source) is shinning brightly without any clouds or smog take notice of all the shadows. Get a camera, a cheap one will do, and take some pictures. Get buddy (a subject) and position him so the sun is at his back while you photograph him from the front. Then try it with him facing the sun and then with the sun coming from the side. You can even try 3/4 angles from all sides if you have the time. Just keep track of the suns angle and time of day so you know what you are comparing to. Also find a tree and take a nice wide angle shot that also shows it's sharp, well defined shadow. What you are seeing is "hard light" from the sun with some ambient light reflecting off almost everything. The shadows are sharp because of how far the sun is from us and how small it appears as a source in our sky.

The next step of this little experiment is to repeat everything in overcast conditions. Have your subject wear the same clothes, stand in the same place at the same time. Finish with that nice wide shot of the tree. Now take notice of the shadows or the lack thereof. What you are seeing is "soft light." The shadows are softened because the cloud cover is now the source of light which is closer to us than the sun and is larger from our perspective. This principle can be translated with virtually any source and any subject.

Grab a small object with texture like a rock or a golf ball. Go into a dark room and turn on a single light. Get as far away from the light as possible and hold your rock or ball to the light. Take notice of the sharp shadows and how little the light "wraps" around. Keep your eye on the ball as you walk it closer to the light. Make sure there is nothing in your path to trip over. Keep going until your are holding the ball or rock next to the light. Sure it is now brighter. Notice the shadows or lack thereof. Take notice of how the light "wraps" around. By moving closer you have decreased the distance of the source and increased the size of the source relative to the ball or rock.

The beam coming out of most big studio lights is coming from two sources, the element inside the globe and from the reflector inside the lamps housing. The first order of business is smoothing these two sources out. This applies to groups of lights as well such as Dino's, Kino's, LED's, the goal is to merge everything together to get a single source...causing a single shadow. As the light hits the frame of diffusion it becomes the new source lighting the subject. So the larger and closer your frame is to the subject, the "softer" or more "wrap" the light will have. That is why you will see many DP's bring the diffusion in as close as possible for close ups.

So now we have lots of choices in materials to use as a diffuser. Hampshire Frost, Opal Tough Frost, 251,250,216,129,Full Grid, Half Grid, Half Soft Frost...how do we choose? Size is a good start. Most rolls of diffusion cannot be sewn together to make larger frames of diffusion. Our choices for diffusion that are 6'x6' and larger are:

Full Grid or Grid Cloth-6
Half Grid or Lite Grid-10
1/4 Grid-5
*All of the grid cloths are now made in a "silent" as well that handles noise caused by wind or rain better.
Full Soft Frost or Full Frost or Shower Curtain-3
1/2 Density Soft Frost or 1/2 Shower Curtain or "Howard"-10
Hilite-1
Bleached Muslin-7
UnBleached Muslin-0
Silk-3
Poly Silk-3
China Silk or 1/2 Poly Silk-0
1/4 Silk-0
Black Silk-1
1/4 Black Silk-0
* Many vendors specializing in overheads also have silks and grid cloths dyed with color correction as well.

I may have left some gimmick ones out or forgot some of the nicknames. Now it comes down to preference and how each of these fabrics spread the beam of light and the amount of intensity lost doing so. I have added an arbitrary number after each fabric that rates, 1-10, how often each is used. A zero means I have never used it. Most DP's I work for prefer the Grid Cloths and 1/2 Soft Frost. Using a color correction dyed in is very situational. The choice really comes down to how much intensity can be afforded to lose. Full Grid does a better job of spreading the beam than 1/2 Soft Frost but you lose a lot more intensity.

The same basic principles apply for 4'x frames but now you have more options. You should note that as the size of the source changes so does its "fall off."

See Inverse Law of Squares:
http://photo.tutsplu...rse-square-law/

Opal, 250, 216 are the most used outside of the grids and 1/2 SF for me. How well a diffuser does in high heat is a consideration. Clipping 1/2 SF to the barn doors of a hot light may not be the best idea. Opacity may be what decides which material to apply to windows as a frosted look.

Order a few cuts of some of these and play. Take notice of the shadows and how much intensity is lost.

Please spare me any comments of how you or your DP likes to make 20'x20' frames of 216.
Click to expand...
 
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