[pure-silver] Re: used slide density range?

Rob C a écrit :


My understanding is that optimum subject brightness range for positive film is 
around five stops from black to white.
I have also been lead to believe that films such as Velvia will result in a 
positive with a density range of 12+ stops or around 4.0 logD.
My question is, would a subject of 5 stops range result in a positive of 4.0 
logD. If not, then how much density range will a 5 stop subject use on a 
transparency. I know it will vary from film to film but taking velvia as an 
example what do you think.

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Assuming you're talking of the Velvia 100:
http://www.fujifilm.com/products/professional_films/pdf/Velvia100.pdf
On page 8 you have the sensitometric curve with the relation between exposure and density for the three layers.
From that curve, it looks like the Velvia density range is roughly around 3.5, to achieve it you need around 2 logH, which is a ratio of 100 (roughly 6.5 stops). The straight part being less than that with a density range of 2.7 and a logH of 1.3 (= 20 or 4.3 stops). As the important part of the curve is somewhat straight, then you can assume that if you expose less and inside the straight portion then the obtained density range is proportionally reduced. With 5 stops you're already entering the non-linear parts of the curve and depending where you put your 5 stops, density range can vary quite a lot. Supposing you center the 5 stops (= ratio of 32 = 1.5 logH) on the curve, then you should obtain a density range of roughly 3. 

Claudio Bonavolta
http://www.bonavolta.ch

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