On: Tue, 09 Mar 2004 David Kay wrote:- > Dear Bob and list: > Forgive me for going on about this again, I just have to make my point...
Bob M and Thomas H are two of the brightest minds I have ever met. However, they are busy people, so I don't think they will mind if I speak on their behalf in mutual agreeance. >> but adding more bits doesn't change the threshold of the >> device. > Correct, but adding more bits does influence the number of subject > brightness levels in f-stops we can record. Wrong! > Here's what I mean: > Take a fully saturated pixel of any bit depth. > Let's say it has a full-well capacity of 90,000 electrons. > Now, divide this number by a factor of 2 and keep halving the remainder. > Each division represents one bit or one half the light, until you reach the > noise floor. (It's here where good data is polluted by random "noise" from > the electronic circuits and/or sources of heat within the device.) So if your imaginary chip pixel has a headroom of 180,000 electrons, it will give you just one extra stop. Correct? > Surely it's obvious a 16-bit device can record more f-stops of subject > information than an 8-bit device? No, it will give 1024 levels of grey instead of 256! There are limitations as to how much light a sensor can absorb as a meaningful signal, very much in the same way a piece of film can absorb light - before reaching maximum base density. What I am describing is dependent on the signal to noise ratio of a chip - any chip, and is what governs its dynamic range. This is why the Hubble telescope is parked in a geo-stationary orbit in the shadow of the Earth, where the ambient temperature is a few degrees above absolute zero. As Hubble records extremely low levels of light, it needs a low operating temperature to give its sensor the extremely high signal to noise ratio it needs to do its work. BTW, most earthlings don't know this, but Hubble's sensor is only 2meg in size......the same size as my terrestial Canon PowerShot A40:) > This is the point I have been attempting to make! I am sorry, but your logic is flawed. > Shooting outdoor scenes in high-contrast Aussie light with a 16-bit device > records everything with detail as long as I expose for the highlights. > Actually, I need to add image contrast to make the result look more > photographic. Having travelled in many parts of the world (artic circle/equator/NZ) I can assure you the level of sunlight remains the same. The contrast of the light is determined by humidity, elevation of the sun, dust and/or atmospheric pollution. Aussie light is no more inherently contrasty than anywhere else. I think what is confusing you is the linear response to light with digital chips.....that is a straight ramp at an angle of 45degrees. This is why digicams can see into shadows with relative ease using a dynamic range of about 12 useable stops of light. When you say that you add contrast to an image in post-production to make it look more photographic, it means that you are tailoring the file with a curve (toe/slope/shoulder) to make it appear like it did to your minds eye at the time of capture. Hence Chris G's brief but worthy mention of in-camera profiling - something you may wish to consider. I swallowed my commonsense pill this morning, so I hope this helps. <G> William Curwen http://www.william.ws PS: as an aside, I am currently working with Ilford FP4+ processed in Rodinal diluted 1:100 as a compensatory developer which puts a very round shouldered curve to highlight detail - without any blowout. As I routinely expose for the shadows while shooting into the light, I am getting around about 16stops of dynamic range! My Nikon Coolscan has a D-Max of 3.0, and is more than able to strip out anything and everything thrown at it. The result is quite incredible luminosity. Of course, this is dependent upon the agitation technique used during processing to reduce fogging and therefore give a high signal to noise ratio. This is the same as for digital capture, as both scanned film and digital chips are dependent on the quality of the analog to digital converter being used. If I want 16stops from the PowerShot, I can quite easily shoot for shadows/midtones/highlights and composite together afterwards. Basically, there is no difference. My own point in this debate is that there will be a day soon when processor clock and bus speeds will be fast enough for digital sensors to record seperate shadow/midtone/highlights exposures sequentially and composite on the fly in real-time to give whatever dynamic range as required. =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
