In the world of traditional silver photography, characteristic curves (also known as D-Log E or H&D curves) serve as the transform between actual scenes and the images we create of those scenes. They make it possible to meter an item in a scene and know the shade of gray it will lead to in a finished print. By using the elaborate calculations of four- quadrant tone-reproduction diagrams, that gray shade can be forecast in density units with a precision of two decimal points. Even a general knowledge of curve shapes and how they are influenced by development allows prediction of which of ten tonal ranges or Zones an object will appear in. Successful photography in no way requires an understanding of these curves, but they certainly make it easier to manage tonal relationships in moving from scene to negative to print.
Sadly these curves did not make the transition to digital. In fact the “D” of D-Log E (density) doesn’t even exist in the digital world of on-screen images. But happily there is a digital counterpart, a camera characteristic curve that provides photographers with similar information. Instead of density we have image brightness values (BV), readily measured on a scale of 0 to 255, with no need to own transmission or ref lection densitometers. And it is easy enough to relate these BVs to an exposure axis to appreciate how the camera translates between scene brightness and image brightness. Each of the many thousands of film and paper curves that have been published is unique to the product and processing it represents. Likewise, a digital camera characteristic curve is specific to a particular model of camera and the settings employed in its operation. In this article we will work through the procedures for creating and analyzing these curves for a pair of cameras. Both are basic fixed-lens units operating in JPEG mode. Our intent is not to share the characterization of these particular cameras but rather to illustrate the process that can be applied to anything from a camera phone to DSLR, from JPEG to Raw.