On 7/3/04 6:18 AM, "Bob Smith" <[EMAIL PROTECTED]> wrote: > But you're assuming that a bit has some meaning as a unit of measure; > or at least a relationship with a unit of measure. It doesn't.
Surely there's a relationship between bit depth and dynamic range when available bits are required to describe tonal variations in our subject? For example: Imaging we only had an 8-bit capture device and I set up a shot where the highlights were underexposed by say two f-stops. So instead of having all 256 8-bit levels available, I now have only 64. My subject happens to have a tonal scale of 8 f-stops (measured with a spot meter) and the brightest part of the subject, thanks to my shoddy exposure technique, fell on 64, then 32 represents half that brightness or one f-stop less, 16 represents two f-stops less, 8 is 3-stops, 4 is 4-stops, 2 is 5-stops 1-is 6-stops until we run out of available bits with which to describe tonal variations in the subject. Had I "exposed for the highlights" by placing them as far up the scale as possible, I could have captured all 8 f-stops of information - without turning up the fill light. Many recent medium format based digital backs have 16-bit A/D converters, so there's a huge f-stop range available. Just make sure you don't overexpose, as it's very hard to put detail back once this happens. So isn't there a relationship between bit depth and subject contrast in f-stops? David Kay =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
