Originally Posted by Stephen Benskin
(post #43) The tone reproduction chapter in The Theory of the Photographic Process 3rd edition covers both objective and subjective tone reproduction in great detail. Along with the mid-tone gradient, one other critical condition is required. The print tones need to be lighter than the original subject.

Originally Posted by Mr Bill
(post #51) Steven, I don't think I can go along with this part, "print tones need to be lighter..." I don't have that edition, so can't confirm what it says, but I really think you may be misreading something.
Apologies, I should have said "Stephen."

I don't think you addressed this point, which is important (IMO). I suspect that you concluded this from an inappropriate use of the reference lines used in various graphs. If you want them to represent actual density equivalents the line would have to pass through a point of 100% diffuse reflectance (pure white) on both axes. For the printing paper, this means a density value of zero, not paper-white. Most of the graphs seem to pass the reference line through paper-white, with a density value ~ 0.10.

If you were to shift these reference lines (use a straight edge, hold it parallel to the existing reference, then shift it over to cross zero density) you'll probably find that the print density values are essentially NOT lighter than original subject density equivalents.

Here's an example of how those reference lines can mislead: if you were to use the existing reference lines, which pass through paper-white, you might photograph a gray card (measured density ~ 0.74) and print it to produce the same density (~0.74), but someone else concludes that the print is lighter than the original because the reference line is 0.10 units higher (~ 0.84 density).

As I mentioned before, a print should be able to accomodate some specular reflections, so a scene white should be printed a slight amount darker than paper-white. When this is done, everything on the print gets a bit darker. Here's an example of such a graph, showing clearly that nearly all print densities are equal to or higher than the original scene density equivalents. It's from Digital Color Managment - Encoding Solutions (Giorgianni, 1998).

tone repro curve - Giorgianni.jpg

In this graph, the reference line should be a legitimate comparison for density. The section where the two lines run together is roughly where flesh tones of a light-skinned person would be. To the right of that, the curve is getting flat; this gives specular highlights some "separation", albeit compressed. (Without this compression, the rest of the print would get much too dark.) My most experience is in portrait work, where I would say this is a very good general-purpose tonal response (my work experience confirms this.) But I don't know how well it would hold up for landscapes, and the like.

(Gosh, it sure takes a long time for such a small amount of writing.)

I pretty much agree with everything else being said.