Digital Photo Printing
It's all in the details !
Consider the obvious viewing extremes:
 If you look at a digital print with a loupe magnifier you can distinguish
the printer dots.
 If you look at the same print at a distance, you may not be able to
distinguish individual pixels so you could have printed it larger.
At some viewing distance:
 Individual pixels will be just recognizable as pixels and ...
 The color of every pixel will be a precise as the eye can distinguish 
within the color gamut limits of the printer, of course.
Finding a reference viewing distance:
 The "Golden Ratio", 1.618, appears often in literature related to artwork
which makes it an ideal candidate for photography.
 1.618 represents an esthetically pleasing dimentional ratio
between two related distances found in nature. [(a+b)/a = a/b]. The 40D
sensor, having the same aspect ratio as 35mm film, would be close;
(3888+2592)/3888 = 1.666 and 3888/2592 = 1.500. However, the importance
atributed to this ratio may be of more usefulness than the value itself
since the exact ratios in nature are generally only close to this value.
 For photographic prints of 3:2 aspect ratio, like 35mm film, a 13x19 print
has an area of 247 sqin; equivalent to a square of 15.7 inches on a side
with a diagonal of about 22 inches. Using the Golden Ratio of 1.6 for this
13x19 print gives about 25 inches. How convenient!
 To make things even more convenient, let's just use the actual diagonal
of a print (here 13x19 gives about 23 inches) as the viewing distance. So,
it is not a surprise to find comments on the internet that the diagonal
measure of a print to be the recommended viewing distance.
