??? Diffusion of Light in Theory ???

Started Dec 6, 2010 | Discussions thread
ChrisBurch Regular Member • Posts: 436
Re: I have a pain in my neck.

I thought I was going to get in some original comments on this one, but most of my thoughts have already been mentioned...alas, I will give my view anyway.

You can go on for days about lighting theory if you want, but you can simplify all of it by only focusing on 2 things...size and distance of your light. Unfortunately size is a complicated concept, but the distance part is at least rather straight forward. If you include the concept of RELATIVE size, then you really only have to think about everything in terms of size because distance is taken into account.

First of all, forget about the concept of your light source being hard or soft. Hard/soft is only apparent as light falls on a subject, not as it comes out of a light source. Softness is a qualitative measure of how smoothly the bright areas transition into the dark areas. Contrast comes into play because the bigger the difference in your light and dark areas, the harder time you will have in making that transition look smooth. You can therefore think about softness as the ability of your light source to reduce the amount of shadows by itself (i.e. - ignore the use of fill lights and focus on single light source for determining quality). Also, consider anything that puts light on your subject to be a light source.

The sun is a great light source to illustrate quality. It's enormous in actual size compared to everything, and super bright. However, on a clear day though, it's RELATIVE size is quite small -- like the size of a quarter in the sky. Now granted, it's only small "relative" to you because it's so far away, but if it looks small, it is small. A clear day = tiny, bright light = very harsh and crisp shadows.

Then there are cloudy days. We have the exact same light source now spread out to cover the entire sky which makes it quite big compared to you. That means the light from the sky wraps around and fills in what would otherwise be shadows. The difference between the brightest spot on your face isn't so different than the shadow areas, so there is a smoother transition. The light source isn't the sun any more in terms of light quality. The light source is now the entire sky (how it gets lit isn't relevant). The sky (well clouds specifically) is still very far away, but it's much bigger than you. A cloudy day = giant light source = soft, smooth shadows

It's also important to note that the light reflected off of everything helps to fill in your shadows, too -- it's part of your single source because it's ultimately all coming from the sun. That fill light happens even more on the clear day, but it's overwhelmed by the brightness of the direct light. The indirect/reflected light is more significant the closer it is in intensity to your direct light, so reflections from grass or concrete show up a lot more on a cloudy day.

Now let's apply this to a studio. If it's a dark space (black walls or outer space as in your original scenario) your light source is exclusively what is coming out of the flash itself. Since there will be no reflections, it's the exact size of your softbox, the edges of your umbrella, or the size of your direct flash. In that case, all that matters is the actual size of your source. If you're bouncing into an umbrella, you no longer care about the size of the flash, only the size of the umbrella. The same thing is true with a diffused source like a soft box or shoot-through umbrella. Therefore, the bigger the reflector or diffuser, the bigger the source. Diffusion only matters if it makes the source bigger. Others have brought up the distribution of light across diffusers, but just think about that in terms of size. If a softbox has a hot spot and darker edges, simply think of it as being smaller than an evenly lit source. If you're using a beauty dish that's evenly lit, putting another diffuser right on top of it won't do you any good. Putting the diffuser between the light and the subject might turn it into a softbox though, and give you a relatively bigger source. Again...it's all about relative size.

In a white studio, you have reflective light at play. A dispersing light source (umbrella for instance) can now send direct and indirect (reflected) light at your subject. This means your source now includes walls/ceiling/floor, etc -- that makes it relatively larger source. The indirect light is reducing the contrast of the shadows, so the transition between the light and dark spots will be smoother. You will have a softer quality of light if you shoot a subject in a white studio with a shoot-through umbrella than if you shoot with the same umbrella in a black studio. The umbrella will throw lots of light around the white studio that will make it back to the subject and lighten the shadows. Your physical light source is identical, but in the white studio, the walls become part of your source and make your light relatively bigger. Again, diffusion only matters if it makes your source bigger (e.g. - it sends our dispersed light that reflects back onto your subject, or it more evenly disperses light over an area).

In dark spaces, once you have maxed out the physical size of your source (softbox, umbrella, etc) you can't increase relative size unless you move the light closer to the subject. In light spaces, you can still move the light closer, but you also have the option of increasing the size by introducing reflective light. A focused source (snooted flash) will give the same quality in a white or dark space.

So that's my 2 cents worth -- have to admit it shaped as I was typing this up -- never really thought about it to this level of detail before. And yes...I like to talk a lot...size matters in many things.

I really liked the http://www.shortcourses.com/tabletop/index.html info, too. Nice illustrations and breakdowns.

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