----- Original Message -----
From: "Bob Rapp" <[EMAIL PROTECTED]>
> Reflectivity (not reflections) from the whitest white to the deepest
black
> (not deep shadows) covers a 5 stop range. That is to say in an evenly
> illuminated scene of varied objects, the difference between the
deepest
> black and the whitest white is only 5 stops.
>
Where does this information come from? Sources please! Surely brushed
aluminium (giving a diffuse ie. non-specular highlight) exceeds the
reflectivity of black velvet by more than 5 stops, or 32:1. FWIW I have
just compared the white side of a Kodak grey card to the grey side which
was three stops darker, and then to my wife's black woollen shawl which
was 8 stops darker than the white card under exactly the same light.
Black velvet would be even darker but I'm not going to dig that out at
2am and risk the doghouse :)
> One environment I did a lot of photography with Tri-X where speed and
> negative quality was a must involved indoor basketball. The lighting
was
> 256:1 or 8 stops.
So, if I nevertheless grant you a maximum 5 stop subject reflectance
range, as you claim, and then factor that with this 8 stop lighting
ratio, you would have a subject brightness range of 8,192:1, or 13
stops, providing the subject matter of varying reflectance is randomly
distributed amongst the bright and shadowed areas.
I will explain. Imagine two illumination sources at opposite ends of a
space with NO reflecting surface on any side. One source emits 1 unit
of light and the other emits 8 stops more, or 256 units of light.
Imagine that mid-way between the two lights (suns, stars, lamps,
whatever) are two spheres, one having perfect reflectance (unlikely but
it makes the maths easier) and the other having 32 times less
reflectance, ie. it is 5 stops darker. The two spheres are equidistant
from both lights and do not cast any shadows on each other.
SO - the more dimly lit side of the dark sphere gets 1 unit of light and
reflects 1/32 of it, or 0.03125 units of brightness.
AND - the brightly lit side of the bright sphere gets 256 units of light
and reflects all of it, so has 256 units of brightness.
THUS the subject brightness ratio in this instance is 256:0.03125 which
is the same as 8,192:1.
>It involved toe speed measurements of the film with
> different developers and, once the developer was selected, the
development
> time and agitation method were determined to allow printing on #2
paper. In
> this case, it was Acufine 1:1 for 3.5 minutes and 1 inversion every 30
> seconds. The result was a very easy to print negative such that I
printed in
> batch mode. The projections first and development / washing second.
>
> Bob
With this degree of measurement and trialling of alternative
developments I would have been surprised if you didn't get the result
you sought. Nevertheless, you still appear to regard lighting ratio,
subject reflectance and subject brightness range as interchangeable
concepts, when in fact the first two are independent concepts that
produce the third concept when they meet.
But what has that to do with spotmetering versus incident metering? You
never explained how you could measure the subject brightness range with
an incident light meter. I say you can't, because at some point you
need to measure the reflectance of objects in the field-of-view, and
incident meters can only measure illumination which they convert to an
exposure reading.
Regards,
Anthony Farr
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