Diffuser size vs distance - any formula available?

AnnieMacD wrote in post #17925794
Fantastic illustration - thanks so much for doing this so we don't have to!

+1

digital paradise wrote in post #17925718
Basically you are re-directing night, not bending it. Like playing pool with light.

i prefer to think of it as the light that comes off the last diffustion panel that isn't going more or less straight ahead, goes away. As in, gets absorbed by the fabric. I'm sure some of it is re-directed, but lots of it is just absorbed and all that is left is the light going straight.

At least this is the way I think of it with fabric grids in softbox modifiers.

Left Handed Brisket wrote in post #17925904
i prefer to think of it as the light that comes off the last diffustion panel that isn't going more or less straight ahead, goes away. As in, gets absorbed by the fabric. I'm sure some of it is re-directed, but lots of it is just absorbed and all that is left is the light going straight.

At least this is the way I think of it with fabric grids in softbox modifiers.

Yeah that makes perfect sense.

I forgot to say that there is so much more to this than the illustrations I posted. Those are just very basic examples and then you build on it with things like grids, umbrellas vs soft boxes, additional lighting, etc.

digital paradise wrote in post #17925919
build on it with things like grids, umbrellas vs soft boxes, additional lighting, etc.

this reminded me. i didn't want it to go unmentioned, or rather under appreciated, that windpig also mentioned feathering the light. The combination of the grid and feather, relative to the subject, help restrict the direction of the light even more.

ftr, include me in the thinking that atmospheric conditions are what make the sun less of a point source. not that i totally discount the fact that it really isn't a point source. Make a speedlight the same apparent size of the sun, and it will be a harder source, unless you are in outer space.

Yeah I was in another thread about feathering light which was really interesting. There is so much you can do. I like getting into these conversations based on my personal experience where I had no clue about what light did. The ones who sell gizmo diffusers that claim to defy physics made some money on me. After my courses and with guidance from members here I gave most of them away. Made my flash world much simpler.

Left Handed Brisket wrote in post #17926102
ftr, include me in the thinking that atmospheric conditions are what make the sun less of a point source. not that i totally discount the fact that it really isn't a point source. Make a speedlight the same apparent size of the sun, and it will be a harder source, unless you are in outer space.

Certainly. An opaque object in the sun does not cast a black shadow. If there is a blue sky, then the shadow will be blue, because it will be illuminated by the blue sky. And if there is haziness, then the shadow will be brightened by the light from the haze.

... which is to say it is complicated. But if you want a formula, and if we can simplify, then the formula would be in terms of solid angle subtended at the subject. The greater the solid angle (portion of a sphere), the softer the light.

Left Handed Brisket wrote in post #17926102
ftr, include me in the thinking that atmospheric conditions are what make the sun less of a point source. not that i totally discount the fact that it really isn't a point source. Make a speedlight the same apparent size of the sun, and it will be a harder source, unless you are in outer space.

Insightful comment! In discussing shadows, we must distinguish between hardness and contrast. For an ideal point source, the shadow transition is immediate (no penumbra), distance makes no difference, and we would label that edge as "hard". Otherwise, the edge has a penumbra and becomes progressively softer as subject distance from background increases, until the umbra formally disappears. Demo with distance decreasing:

xseven wrote in post #17923156
Is there a formula one can use to calculate at what distance a modifier with area "A" will be no different from a point source of light?
And the reverse (even though this one is more subjective): at what distance a modifier with area "A" will provide a "soft" light?

It depends on the degree to which the modified scatters the light when it leaves the surface/panel. Assuming that the amount of light making contact with the surface is uniform and there are no hot spots, then the modified will create a cone of light that, assuming a 45° scatter angle, will coverage at a distance equal to the surface's radius. So if you're using a 7ft umbrella with diffuser panel (e.g. PLM), you'll need to get your subject within 3.5ft of the surface.

Another option is to use multiple sources of collimated light and placing them in an arc around your subject (key, fill, side). The benefit is that you can control each light individually and thus control the pattern of light better. And if it's a perfectly collimated light, there is no limit to the distance you can have the lights away from the subject. As long as the sources are slightly larger than the subject that you wish to light.

I decided to run a test series to illustrate what happens with a Fresnel source along with a snoot, positioned 14' from the target wall.
My light-blocking object was a light stand with accessory clamp at the top, positioned at 11" from the target wall at the closest and stepped outward to 31" from the target wall as the farthest position, at about 4" incremental distances in the series.

I love three pages threads with 2.5 pages that are only "incidentally" (see what I did there?) related to the OP, it almost seems for future reference this thread should be renamed.

To the op yes there is a formula, perhaps if you really want I could calculate it, but just from a understanding principles type of thing, relative size equivalence is what matters. Imagine and umbrella outside between you and then sun as you move it away from you it will appear smaller and smaller until it is the same size as the sun. At the distance qualities of the light will be identical to the sun. Say you have a point source a small distance away and put he umbrella behind it you have to move it away far enough so that when you move the point in and out of position it appears identical in size.

It's kind of like how you can make someones head appear the same size as your thumb.

travisvwright wrote in post #17926869
I love three pages threads with 2.5 pages that are only "incidentally" (see what I did there?) related to the OP, it almost seems for future reference this thread should be renamed.

That is because the answer was in the very first response! 2X the dimension of the softbox is where it stops being purely a 'large source' and transitions to 'small source'. 3X is 'good enough' for it to be mostly a 'large' source.

To show the changes as the distance from light source to subject increases from 2X long dimension thru 5X long dimension, here is a series taken with a small softbox (13cm x 20cm) at 40cm(2x) , 60cm(3x), 80cm(4x) and 100(5x) cm from subject, and the target wall is 75cm behind the subject.

AnnieMacD wrote in post #17925794
Fantastic illustration - thanks so much for doing this so we don't have to!

You're still going to want to test and experiment for yourself, which will give you practical experience for real-world shooting and save you valuable time during setup.

Ulysses01 wrote in post #17931475
You're still going to want to test and experiment for yourself, which will give you practical experience for real-world shooting and save you valuable time during setup.

True, I need to buy a mannequin as nobody will volunteer to sit for ages while I faff about with lights and grids. This was a great intro though.