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Subject: Re: Exposure to the right and tone placement



-->I recently made a post in this thread about the full well concept of sensors, which made some good points about ISO, noise, and exposing to the right

Bill,

I think I've been pretty clear that unless you've made the aesthetic decision to blow highlights in order to hold very deep shadow detail, you don't want to expose so far to the right that blooming becomes an issue. All along, I've been advocating correct exposure, not systematic overexposure.

The point is quite simply that with digital capture, the midtone, 18% (or 12.5%, or 13%) reflectance is enormously more fungible than the white point in post-capture tone mapping, hence the useful metric by which to judge whether or not an exposure is "correct" is the white point.

Forget for the moment the cushion offered by ACR's extended highlight recovery logic—it's enormously useful in practice, but it shouldn't be relied upon if you're aiming for correct exposure, and when you reach the point where the well is filled in all three channels (bearing in mind that the red filter transmits more light than the green, which in turn transmits much more light than the blue), there is no highlight to recover, and attempting to stretch the detail that is available typically produce rainbow artifacts that may be managed if you're willing to entertain only a narrow range of white balances, but is not something to strive for.

Rather, the point I've been trying to hammer home is that if you capture 18% reflectance so that it gets recorded at its "correct" value in linear gamma, down around level 47, you waste a huge chunk of the camera's response on highlight detail to which your eye is totally insensitive, while simultaneously starving the shadows. If you look at linear conversions of step wedges, I can't see how you'd come to any other conclusion unless you're misled by Nikon's lossy compression.

You have to learn the behavior of your personal camera body and its metering system, which involves two compensations, one for the difference between nominal and actual ISO, one for the difference between where the meter expects the highlight to be in relation to 18% or 12% reflectance and where the camera actually puts it. All the usual fudge factors needed to compensate for the fact that the scene with 12% (or 18%) average reflectance rarely occurs in anything worth shooting still apply....

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