softness of light

Started Jun 2, 2008 | Discussions thread
John down under
Veteran MemberPosts: 8,229
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exactly; softness is the degree of diffusion at the subject
In reply to jaronberman, Jun 5, 2008

jaronberman wrote:

At a very basic level then yes, soft light is about directionality
and angle. The "fill flap" on an on-camera flash isn't so much
softening the light as it is diffusing the light. It's making the
spread of illumination more even and more wide. The light is still a
point-source essentially, and as Johng pointed out - the shadow edges
do not soften. They shouldn't, as the light didn't necessarily get
much bigger in relationship to the subject. It got more even, and
hence less ugly...but not a lot bigger. If you'd but a 2'x3' softbox
on the flash, you'd certainly see a difference at the same distance.

Soft light is more or less about how much a light fills its own
shadows. Every point source creates a single shadow. A light bulb
for example. If the filament were infinitely small, it would be a
literal point source. One point glows, casts one shadow. Now line
up an infinite number of those points next to each other to create a
"surface" of light approximately 2'x3'. Every point is STILL casting
a discrete and hard shadow, except now you have an INFINITIE number
of points all radiating in all directions. This is how the front
surface of an idealized softbox works. Every "point" creates a
shadow and concurrently fills-in the shadows of every other point.
If this surface is large compared to the subject at hand, it will
throw soft light.

Now, that must be defined too. "large" is also about angles. If you
draw a triangle between your subject, the center of the light, and
the edge of the light, think about the angle formed between the line
going from the subject to the edge of the light, and the subject and
the center of the light. If this angle is small, the light will act
like a point source. As this angel grows, it will act like a soft
source. Obviously, the length between the center of the light and
its edge hasn't changed, so you see that a softbox or umbrella can
act as either point or large source, depending on how far it is from
the subject. That goes for spot lights too. If you could make it
act "big" (a "big-eye tenner" fresnel spot), it will produce soft
light. A beauty dish when used close to a subject produces soft
light.

Spot on JB. The size of the light source is with respect to how the subject sees the light source and the degree of diffusion at the subject depends on the directionality of the light at the subject.

I've been thinking more about this and the best way I can find to describe the softness of light is the degree of diffusion of the light at the subject, ie the degree to which light within certain bounds of variation of intensity hits any point of the the subject from more directions at once.

While a beauty dish close to a subject can produce soft light, the more directional nature of the beauty dish light when compared to say softbox light means that the beauty dish light close up will still be harder than similar size softbox light from the same distance. As you describe, the result is the sum of all the point sources of light at the subject. A softbox will effectively produce more point sources of light at a more even illumination level at any given point of the subject than a beauty dish will.

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Cheers from John from Adelaide, South Australia
John Harvey Photography http://johnharvey.com.au
Canon 40D, Fuji F100fd

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