P3D Re: Flash and 3D

[John Ohrt <johrt@xxxxxxxx>]

John W Roberts wrote:
> 
> I think that assumes the umbrella is a mirror-like (specular) reflector
> (and probably also assumes that the surface of the umbrella is flat).
> But all the umbrella reflectors I recall seeing appear to have a grainy
> or diffuse surface, to spread out the light, so that it's more or less
> accurate to model each point on the surface as a separate point source,
> and the entire surface as a diffuse source.
> 
> In principle, the brightness from a point source should drop off with
> the square of the distance (k / d^2), the brightness from an infinitely
> long diffuse line source should be inversely proportional to the distance
> (k/d), and the brightness from a diffuse infinite plane source should
> not vary with distance (k). A diffuse umbrella reflector is not an infinite
> plane, and a some distance it will start showing an inverse square
> dropoff, but the "near field" region where this is not a good approximation
> should extend out a considerable distance (certainly within the range
> of a typical flash).

Unless you are using a much larger umbrella than I envision, the
umbrella surface will not approximate an infinitely dimensioned diffuse
surface, ie. the umbrella's diameter is 30 larger than the illuminated
area of interest.  Inverse square law may be pessamistic, but aside from
small (tabletop) scenes, I think we need some test data :-)

My understanding was that "umbrellas" were used as shadow softeners.

But then I have little experience with them.

Regards,
-- 
John Ohrt *** Toronto, ON, Canada *** mailto:johrt@xxxxxxxx


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