In the July/August 2009 issue of PT, we discussed the Ideal Tone- Reproduction Curve, a product of research conducted more than a half century ago that identif ies, for a scene element of any luminance value, the shade of gray (ref lection density) at which it is “best” reproduced in a black-and-white print. We also raised the question of how readily this ideal tone-curve is achieved in a purely digital workf low—the subject of the present article.
With the magic of Photoshop, any kind of tone reproduction can, of course, be realized with exacting precision. But what is inherent to digital in the absence of any manipulations? To answer this, we created a tone- reproduction curve for comparison with the ideal, using an inexpensive digital camera, inkjet printer, and photo-quality inkjet paper—no adjustments to the JPEG prior to printing, no printing profiles, just a straight print of an as-captured image. Creation of this digital tone- reproduction curve first requires construction of a camera’s characteristic curve as explained in the March/April 2009 edition’s Characteristic Curves for Digital Cameras. That article described how to characterize a digital camera in terms of the relation between camera exposure and brightness value (BV, on a scale of 0 to 255) of the resultant
image on screen. The curve derives from photographing a uniform card, such as an 18% gray card, over a wide range of exposures, spaced in one-stop increments, suff icient to create a series of frames ranging from BV= 0 to BV= 255 and plotting their BVs in one-stop increments on an arbitrary Log E axis. This graph serves as a reconnect between the world of digital and the characteristic curves we knew in the silver era. In the digital world it is also known as an opto-electronic conversion function (OECF) and is discussed extensively in ISO 14524:1999(E).