Digital BW, The Print

The notion of "middle grey" really becomes, in many ways, a bit of a moot
point with today's ability to edit an image.  In the old days, one had to
(ideally) make a decision with regard to middle grey (or at least ONE point
on the exposure scale in which case it's handy to make it near the middle)
at exposure because there was very limited ability to adjust that colour at
a later date and very limited ability to manage tonal transformations from
exposed film to print - most image values "fell where they fell".  This
discipline of fixing one point with confidence through to print is obviously
the core of any system for systematically correct exposures in print (and,
notably, the Zone System).

But today we can "shift exposure" (speaking in broad terms) very easily.  So
in the end the focus can, if need be or if recommended, fall back towards
the image editing (or RAW processing) stage to rendering an image to a
pleasing state overall.  With digital capture, exposing for middle grey may
actually lead to a disadvantageous situation due to the linear nature of the
sensor - you may well be much better off intentionally over-exposing the
part of the image you envision ending up as whatever you decide "middle
grey" to be and pulling it back in the RAW converter.

As a result, the issue comes down to the broader one of how you map your
pleasing image file (as portrayed on screen) to a pleasing image on paper -
how each and EVERY grey gets maps to the new dynamic range.  The tools
available to a pure analogue photographer to manage the tonal range
transformation from processed film to print were, in a relative sense, very
limited (choice of paper grade and dodging and burning etc).  Today you can
do wonders with things such as PS curves and colour management.

So when you start asking questions with regard to the numbers attached to
the same colour in different workspaces (or start with just one colour:
whatever you decide "middle grey" looks like) you are really beginning to
dip your toe into the relationship between file pixel values, how those file
pixel values are interpreted into colours depending on the encoding (colour
space) we attach to them, and what the printer does when it receives those
file pixel values.  (Remember your printer doesn't know what a colour is.
It just fires certain jets in a certain order depending on the 8 bit number
it gets hit with.)  That relationship is part of the topic we call "colour
management".  The colour you decide to call "middle grey" has a number
attached to it which differs according to colour space.   Send any of those
numbers to the printer and you'll likely generate yet a different shade of
grey.  The question should be: when you send off towards your printer the
number that in your workspace looks "middle grey" to you, how can you ensure
that the printer receives not necessarily that same number but a number
which will cause it to print the "right" grey?  That same question needs to
be answered for all possible shades of grey you see in your image - not just
"middle grey".


> From: petexp2 <kafoozalem@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Mon, 31 Oct 2005 19:47:38 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] Re: Mid grey : 50%K or 128RGB?
> 
> Thank you Paul for such an informative answer. I really did think it
> would be simpler but you have helped me answer it to my own
> satifaction. In fact playing around with a 21 step wedge in Photoshop
> I note that the when you convert it from gamma 2.2 grayscale to RGB
> it has a midtone value of 128. Alternatively when you convert it from
> dot gain 20% grayscale to RGB the mid point value is 148 RGB. I will
> certainly favour dot gain 20% as a starting point for making prints
> and bear in mind what you say about the compressed tonality of a
> matte paper.
> 
> Pete.
>