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Subject: Re: raw 3.1 professional custom profiles for highend digital cameras

1.) The two main sources of unit-to-unit variation in cameras are the chip sensitivity, and the filter densities. You can compensate for the chip sensitivity variations by determining the real ISO speed of the camera, on which more later. The script, and the Calibrate tab it drives, are designed to account for variations in filter density. They move the R, G, and B primaries in XYZ space, so they're just moving the corners of the built-in profiles ot ore accurately match your specific camera.

2.) When you run the script, all you need to do is to plug the numbers into the Calibrate tab (and, ideally, save them as a settings subset). You don't need the reference image, and you don't need to adjust anything. You seem quite intelligent, and since you were smart enough to buy my book, I certainly won't call you an idiot, but you may want to re-read the relevant section....

3.) I find I can get by happily with 1 calibrate setting for daylight and another for tungsten. Tungsten needs to be special-cased because of the double whammy of a blue-starved light source and relatively inefficient blue filters. Strobes generally have a pretty full spectrum and behave much as daylight does.

4.) Gossen-type meters are essentially useless for digital. They assume a film-like tonal response curve, and measure average scene luminance. DSLR's don't have that film-like TRC, so you won't get good results from that type of meter. I use the built-in meter, spot-metering for the highlights (In overrange situations, I spot-meter for where I want the highlight to be.) First, though, you have to determine the real chip ISO sensitivity—I've seen nominal ISO 100 be anything from ISO 75 to ISO 150, and there are plenty of cameras I haven't seen.

I used a Minolta Spotmeter F I borrowed from Jeff Schewe to determine that my Kodak DCS 460's ISO 100 was really ISO 80, my Canon 300D's ISO 100 was actually ISO 125, and my Canon 20D's ISO 100 really was pretty damn close to ISO 100, as follows.

Meter a bright highlight with the meter set for ISO 100, and take a shot with those settings. Repeat at different ISO settings on the meter. Then look at the images and see which one captures the highlights as hot but not blown with no Exposure adjustment in the raw converter. That's the real ISO of the camera, so if it's different from the nominal ISO you can dial in the appropriate exposure compensation on the camera. (My 300D is set permanently at -1/3 stop, though I'd prefer -1/4 but it's not an option on that camera.)


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