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Old October 7th, 2006, 12:32 PM   #1
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Diffusion

I'm still in the learning stage. I have been experimenting with the Rosco diffusion sample kit of 15 gels, using an egg as a subject. I see almost no difference between the various gels. Is it that subtle? Am I doing something wrong? I am using a Home Depot light with homemade barndoors, 250 watt.
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Old October 7th, 2006, 03:20 PM   #2
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Probably is that subtle...

You are applying essentially the same lighting technique to a round, white, featureless object. Different materials have different diffusion qualities, but if all other variables remain constant it may be difficult if not impossible to distinguish a difference, especially on a low contrast subject. More important than Hamburg frost vs. tuff spun is changing the apparent size of the light source. Play around with light>diffusion distances and diffusion>subject distances. Also, try bounced lighting on a variety of surfaces. These should give you more dramatic feedback. Also, consider a subject with more detail/texture.

EDIT:here's a list of lighting links (mostly still-related, but still helpful).
http://forums.dpreview.com/forums/re...ssage=16476605
This one has a nice side-by-side comparison feature:
http://www.bron.ch/vt_pd_lg_sc_en/index.php
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Old October 7th, 2006, 04:01 PM   #3
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I had a nice shadow on one side of the egg, of course. So I was looking at the edges of the shadow and also the way the light wrapped around the egg itself.

Thanks for the excellent links. Just what I needed.
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Old October 7th, 2006, 05:12 PM   #4
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I'd agree with Steve. Putting different diffusion types on the doors is not going to show much variation... especialy on an egg. It might be more educational to practice on something with a "nose". Maybe you can find a mannequin head at a thrift store or something. Seriously. The nose shadow is one of the big things you have to deal with when lighting faces, and using it will give you better results with your tests.

Testing bounce surfaces was a good suggestion. Try bouncing into a white wall, a 4x4 sheet of foam core, and a 4x4 or 4x8 sheet of styrofoam (with the plastic peeled off). There's quite a difference. It'll also show you what a large soft source does to a face when compared to a 1' square source (i assume the gel pack is 1' squares?).

If you can get a hold of some brighter units, like a 650 or 1k, you can also play with contrast, ket-to-fill, backlight, etc...
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Old October 7th, 2006, 05:12 PM   #5
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If you are serious about a career as a DoP, starting with a single, simple object may be the way to learn it right. I skipped a couple of steps and went straight to faces, because I'm lazy and figured my income was directly connected to how good people looked in front of my camera. There's a good forum for cinematographers that I used to have bookmarked at work. If I can find it Tuesday I'll forward it. You could probably find a dozen experts that will swear by one type of diffusion over another and be able to back it up with ancedotes from actual movies.

If you can train yourself to identify diffusion based on the shadow of a round object, you're way ahead of the pack. At the very least, you should win a lot of bets with your crew.
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Old October 7th, 2006, 05:13 PM   #6
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Rich, the key to diffuse lighting is not so much the strength of the gel, but the size of the diffuser. If you want to change the properties of the shadow, you must change the size of the diffuser. You can simulate this by moving your existing light to different distances from the subject. To get more of a wrap-around, you need a proportionally large diffuser. To wrap around an egg, a 12-inch diffuser places several inches away will work. To wrap a shadow around a car and to prevent sharp highlights, a 20'x20' diffuser might be needed and placed only a few feet from the car.
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Old October 7th, 2006, 05:25 PM   #7
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Quote:
Originally Posted by Matt Irwin
Seriously. The nose shadow is one of the big things you have to deal with when lighting faces.
Very true...in fact many facial lighting techniques are named for the shape of the shadow created under the nose.
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Old October 8th, 2006, 01:24 AM   #8
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I do have such a styrofoam head that I have painted in flesh tones and use for such tests. But it has so many variables I couldn't detect anything in particular with the different gels, so I tried the egg, figuring that the differences could be more easily discerned.

I think i used Tough Rolex, Tough Spun, Tough Silk, Light Tough Frost and Tough White Diffusion.

Thanks guys, you are more than helpful.
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Old October 8th, 2006, 01:33 AM   #9
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From what i've read, the distance between the source and the gel is very important, (the farther the better). My gels were about 12 in x 15 in in size and attached to the barndoors with c-47s.

So it also is affected by composition of the destination, the contrast, ket-to-fill, backlight, etc...?
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Old October 8th, 2006, 02:23 AM   #10
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Quote:
From what i've read, the distance between the source and the gel is very important, (the farther the better). My gels were about 12 in x 15 in in size and attached to the barndoors with c-47s.
The distance between the diffusion and the source can affect the softness, as does the distance between the diffusion and the subject.
If I wanted a soft soft cross-key from a hard source, I could hit a 4'x4' frame of diffusion from a few feet back (this can also be done with a large chimera), making sure the diffusion is as close to the subject as possible. I could also double diffuse (light through 2 layers of diffusion with space between them), or bounce-diffuse (bounce the light and then diffuse the bounced light-- like a booklight), or something else...

There's of course nothing wrong with putting diffusion on the doors-- it's done all the time. It just produces a harder shadow. You might want that, you might not.

(BTW, if you do decide to try making larger diffusers and you're on a budget, you could get away with a white translucent shower curtain. I've done this on extreme-low-budget shoots before and it works fine. Considering rolls of gel run at around $120-160...)
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Old October 13th, 2006, 09:14 PM   #11
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Light to subject distance and the size of the source are very important. A large source close to the subject, say a 2'x3' softbox, three feet away from the subject will be very soft and the light will wrap around the subject. If you back up six feet from the subject, the subject will become constrastier and a little harder. If you back that light up twelve feet away, it will be harder and contrastier still.

If you have a willing human subject who can pose for you while you light them, that would be helpful, so that you can see the effects not just on the face, but on the whole body, especially if they are standing.

Using the diffusion on a diffusion frame might be helpful, that way you can use the whole sheet. Move the light back a foot, or a few feet and close the barn doors down so that there isn't any spill outside of the frame.

If you have some beadboard (white styrofoam with a silver side), or a sheet you can use that as your fill.

At some point you may want to invest in some fresnel and open face lights that are in the 200W - 650W range. Usually, open face are best for bounce, or for softboxes. Fresnels can be used with, or without diffusion. Play and learn. Learn the rules and then forget all that sh*t and light (paraphrasing Bird's quote).

Like anything else, you have to take the time to learn it. Luckily, it's fun and you can see what you like and what works best for you. Be sure to take notes.
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Old October 14th, 2006, 06:29 PM   #12
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When you use diffusion gel, the surface of the gel becomes the source now instead of the light behind the gel, and the SIZE of that surface relative to the subject determines the softness.

The only reason why different diffusion gels in a large frame -- let's say a typical 4'x4' frame with gel on it -- have different softening effects is that in theory, if the light EVENLY filled the diffusion frame from edge to edge with no hot spot in the center, then it wouldn't matter what the diffusion material was, but in reality, different materials spread the light behind them more or less evenly and efficiently, and some even impart a texture to the soft light because some specular (hard) light will leak through the material (especially with fabric diffusers like silks, muslin). So the reason why the effect of switching from a lighter gel like Opal to a heavier gel like 216 makes the light softer is that Opal is light enough to cause a hot spot in the center of the frame that becomes more of the "source" rather than the whole 4'x4' frame area.

And it's also why you can't see much difference when putting the gels on the barndoors of a light, because you are only creating something like a 1'x1' surface area of gel, and whether it is filled evenly or has a hot spot in the center is not going to have a radical effect on softness since you are ultimately limited by the 1'x1' size of the diffusion area.

I said the size of the diffusion frame affects softness relative to the distance to the subject because from the subject's point of view, a 4'x4' frame up close versus a 20'x20' frame of diffusion farther away may be the same size in their field of view and produce the same sharpness of shadow. The only difference then is in terms of fall-off: when you have larger diffusion frames farther away, a subject is not going to increase or decrease in brightness radically by stepping a little closer or farther to the frame of diffusion, whereas if it is a 4'x4' frame only a few feet away, just taking one step towards the frame will cause a visible increase in brightness. It's the same reason why having real soft overcast daylight coming through a window is different than covering a window with diffusion gel (like tracing paper) and lighting it from the outside: in real life with overcast light coming through the window, you may only get a stop or two brighter when stepping up closer to the window, whereas if the window was covered in diffusion, you can get several stops brighter just stepping up to the window. Again, the reason is that the fall-off in intensity is more gradual with a bigger light that is farther away. This is true with hard lights as well.
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Old October 14th, 2006, 08:09 PM   #13
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What he said.

Heh, I was thinking of you as I wrote my reply.
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Old October 18th, 2006, 12:16 PM   #14
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Quote:
Originally Posted by Mark Sasahara
What he said.
Isn't it always?! :)

Btw David, I'm really looking forward to "The Astronaut Farmer" - I loved "Northfork", and your cinematography was simply beautiful. Congrats on your ASC membership, too - nice work fella!...
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