[Alt-photo] Christopher James - terminology question

John Isner john.isner at gmail.com
Thu Oct 26 13:16:40 UTC 2017


I am new to alternative process, having come from a digital photography
background (no darkroom experience).  This is my second post to the mailing
list.

I'm struggling to understand two terms that Christopher James repeatedly
mentions in his book, but never explicitly defines: contrast and tonal
range.  For example, on page 185 in the cyanotype chapter, he says that
using a particular developer chemistry will give "a significant increase in
the range of values (2 to 4 stops) but a decrease in contrast."

My understanding of contrast and tonal range is based on the histogram in
digital photography:   An image with a large tonal range has a wide
histogram, that is, large "distance" between the darkest and lightest tone,
while a high contrast image has relatively few midtones.  To my mind, the
alt process equivalent of the histogram is a printed sample of a step wedge
with 256 brightness levels from black to white.  When James says a
"decrease in contrast," I interpret this to mean that the tones bunch up at
the dark and the light ends of the step wedge, with very few tones in
between.  That makes sense to me.  However when he says "a significant
increase in range of values (2 to 4 stops) I'm confused.  Since you can't
get any whiter than paper white, an "increase in range of values" could
only mean one thing: the darkest tone gets darker!   I suspect this is NOT
what he means.

Can someone help me understand this?
Thanks,
John


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