Here's another wild idea. How about people that are interested in dynamic range and bit-depth go out and pick up a book to learn something about the subject before discussing it on a message board. Once they discover that analog dynamic range and digital bit-depth are two completely separate things (one being a measurement of data and the other a resolution scale applied to collected data) they will understand why you want 16-bits of resolution in a 12-stop dynamic range image.
Simply put–you get more tonal levels in each f-stop of image data. This becomes increasingly important as you move down the brightness scale and the number of levels become increasingly smaller in the shadow areas. The "unused" bits of data are the ones that you don't want to use because they have the fewest number of levels and is where banding occurs.
An excellent post. Analog DR and bit depth are two separate things, but they are related in that to record a given DR, a minimum bit depth is needed. The number of levels required in the darkest f/stop of a given DR is also determined by bit depth. [a href=\"http://www.normankoren.com/digital_tonality.html]Norman Koren[/url] discusses these concepts on his web site. Since the eye is less sensitive to levels in the darkest f/stop, he suggests that 8 levels are need there to prevent banding.
This chart illustrates these concepts for 12, 14, and 16 bit images with a linear tone curve. For a 12 bit image the maximum DR is 12 stops, but there is only one level in the darkest f/stop and the data here would not be usable. If 8 levels are required in the darkest f/stop for a usable image, then the DR is 9 stops max. The f/stops where there are insufficient levels are shown in red in my chart. Usable DR is related to bit depth, but the bit depth will be of no avail if noise obscured any visible detail in the shadows.
However, the engineering definition of DR is full well capacity/read noise
, usually expressed in electrons, but since sensors are linear, one could also use ADU (analog to digital units, i.e. raw data numbers).
An example may help to clarify things. I will use the Nikon D200 for which I have data, but the same considerations would apply to medium format digital cameras for which the chip manufacturers supply data.
The full well capacity and read noise for the D200 are supplied by Roger Clark
and are shown in the table below. By this definition, the D200 has 11.7 stops of DR.
However, this DR is overoptimistic for photographic purposes. First of all, the DR of the image convolves the DR of the camera and that of the raw converter, and some DR is lost during the conversion. Secondly, the photographic DR is determined by how much noise you can tolerate in the shadows. One can photograph a Stouffer step wedge and determine DR with Imatest Stepchart
. The results are shown below.
The camera can detect 11.6 stops of DR, which is consistent with the engineering definition. However, for a high quality photographic image the effective DR is only 6.51 stops.
Data for the KAF 39000 chip used in some high end medium format backs are published by Kodak
Full well is 60K electrons and read noise is 16 electrons, giving a DR of 71.4 db or 11.9 f/stops, not that much different from the D200. The DR for excellent quality photographic images would be less. I have not seen actual test results, but from a consideration of theory, I think the DR advantages of medium format may be overblown. This does not take the noise spectrum into account. With a 39K pixel image, the noise will be at a higher frequency and less objectionable to the eye than with a 10K pixel image, where the noise pattern will be coarser. In the end, one must confirm the image quality with actual photographic tests.