[alt-photo] Re: Your Approach to Making Negs for Platinum Printing? Ideal Negative Contrast and Dmax?

[etienne garbaux]

Francesco wrote:

>I'd like to ask for one more clarification. The step size on a 21-step
>tablet is 1/2 a stop and a 31-step tablet is 1/3 stops. These equate to 1/2
>stop equals 0.15 and 1/3 stop equals 0.10, correct?

They vary -- each manufacturer typically makes a number of different 
step wedges (also called step tablets) for different purposes, each 
with a different number of steps and a different step size.  You 
should get the one you need, with a total range greater than 2.4 (3.0 
is usually sufficient for Pt, although I've seen cases where the 
process ES reached nearly 3.0 -- if it turns out that your process 
does, you will want a wedge with a bit more range).

One stop is log (2), or 0.301 (usually rounded to 0.3).  So yes, 1/2 
stop is 0.15, and 1/3 stop is 0.10 (round figures).  Stouffer is one 
reliable supplier -- Kevin Morris of Stouffer is on the list, last I 
knew.  They make both a 21-step x 0.15 wedge (T2115) and a 31-step x 
0.10 wedge (T3110), each of which has a total range of 3.05.  These 
are the two most often-encountered wedges, and either would be ideal 
for your purpose.  If you need more range (it is possible, as noted 
above), Stouffer makes a 41-step x 0.10 wedge (T4110 -- total range, 
4.05).  http://www.stouffer.net/TransPage.htm

Best regards,

etienne