> 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. > David Kay wrote:
> Surely there's a relationship between bit depth and dynamic range when > available bits are required to describe tonal variations in our subject? No. > > 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. Ok Let's play this theory game. You have all 256 levels available as you always do in 8 bit, but everything from 64 to 0 will be pitch black, or filled with noise if we were to follow you (flawed) logic. If you underexposed the image two f-stops, your image would be underexposed by two f-stops, which, depending on the dynamic range of the camera can mean a great deal of things in regard to which bit will hold the last discernible detail. Increasing the bit depth of the camera would allow you to draw out the bits that you couldn't discern in 8 bit, but you couldn't pull out details that are totally black (0 in any bit depth). Where 10 bit is an advantage is when you in 8 bit, after 6-7 bits (256,128,64,32,18,8,4,2) have to decide whether a dark colour will be rendered with a value of 1,2, 3 or 4. If you have 10 bit, the choice of that same pixel colour is anything between 32 and 1. This will lead to more accuracy in correctly rendering shadow details. It doesn't lead to the camera being able to see darker colours though. > > 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. No! Without knowing the Dynamic range of the capture device (2 f.stops, 6, 7, 8 9, 10, 11, 12 or umpteen) there is no way of saying which bit's will be black! If we exclude any gamma tweak in the software (deviation from linear) any curve adjustment (shoulder and toe contraction as on film) and colour adjustment matrices (other than adjustments to the quantum efficiency after colour filtration/bayer mosaic to adjust the spectral sensitivity of the camera to something akin to what humans see - and removing IR and x-Ray sensitivity) and a few other issues, AND we know that the camera has a dynamic range of exactly 8 f-stops without introducing noise, then your assumption is correct. (Sorry for the long sentence). But it isn't. > > 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. If your camera had a 10 f-stop dynamic range you would have anyway (assuming linearity, no gamma/curve etc.). But if your sensor had had a 10 f-stop dynamic range and you exposed for the highlights, then your absolute shadow area would have been lighter than black so you would have needed post shooting adjustment to fill the 8 bits fully. Or you would have been able to see detail that your spotmeter couldn't measure out of the shadows. Had the camera had 12 f-stop dynamic range you image would probably look pretty flat and uninteresting without post adjustment, as your wouldn't have any good black anywhere in the image. If your camera had an 8 bit f-stop tonal range, but a 16 bit bitdepth, and you exposed for the highlights your image would have been perfectly captured, with accurate shadow detail, and no post capture adjustments would be nessesary. > > Many recent medium format based digital backs have 16-bit A/D converters, so > there's a huge f-stop range available. These are separate issues. It's like saying my car with 4 wheels can go 100 mph. Now if I get 12 wheels I'd be able to go 300 mph... Not exactly true, but more wheels would without doubt increase the road grip and make sure the car will be more stable when you press it to the limits. If you have a 12 bit chip and a 12 bit A/D converter there is no way you will fit all 12 bit unharmed through the A/D converter, and you'd be left with less than 12 bits. Therefore most vendors fit an A/D convert of a higher bit depth than their CCD/CMOS sensor. If they fitted a 139 bit A/D converter it wouldn't increase the dynamic range one bit though. Dynamic range ARE different from Bit depth. > Just make sure you don't overexpose, > as it's very hard to put detail back once this happens. True. > > So isn't there a relationship between bit depth and subject contrast in > f-stops? Nope, not one bit <pun intended>. Best Regards, Thomas Holm / Pixl Aps - Photographer, Educator, Colour Management Consultant & Seminar speaker - Remote Profiling Service (Output ICC profiles) - www.pixl.dk � Email: th[AT]pixl.dk -- =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
