Subject: Re: ACR Calibration
> In addition, setting exposure based on the white patch is not very reliable. The “neutral 5” patch is photographic middle gray (18%). This works out to 127 for Adobe RGB or sRGB or to 110 for ProPhoto or ColorMatch RGB. Middle gray is the basis of photographic exposure metering and ISO settings
I had to abandon "middle-grey" measurements for digital capture. I used that approach, and incident metering, successfully for a long time when I shot film. But my consistent experience with digital SLRs (Canon's) has been that incident readings and reflected "18% grey" readings often produce underexposure, sometimes to an unpleasant degree. I don't understand what the problem is in theoretical terms, have stopped caring about that, and got over worrying that my equipment was somehow defective. I've seen what I've seen and I've had enough of underexposure problems.
An incident reading might be effective with "below average" subject-matter, meaning an unusually flat scene. That kind of shot is probably salvageable during raw conversion even given underexposure. Different story with subject-matter containing a longer range of tonalities. Failure to hit the exposure right on the money is liable to produce blown diffuse highlights that would otherwise have been rendered correctly (at least on film). In those situations, "18% grey" just plain doesn't cut the mustard.
Yes, it's possible to do highlight recovery with a raw converter. But I keep encountering deliberate underexposure in the camera for that purpose, or making a second raw conversion with the EV slider cranked way down, producing color shifts that must then be compensated-for in Photoshop. Not doing so produces a "highlights-only" layer with a noticeable difference in color balance -- not acceptable. It's an unpleasant problem and I don't yet know how to deal with it.
The only metering technique that I find to work well with real-world images, as opposed to theoretical images referred to via numbers alone, is to meter the brightest diffuse highlight in which I want detail, then make whatever adjustment is needed to render as a diffuse highlight. IOW: an unscientific variation on the Zone System theme. I began getting consistently more accurate exposures out of the box when I stopped relying on incident metering and began spot-metering highlights. (If I go by an incident reading and it gets me "there," I consider myself merely lucky.)
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