Subject: Re: ACR Calibration
A reading from a mid gray would give a result similar to an incident light
You mentioned increasing exposure by 1/2 stop thereafter. I have not found
that a mere half-stop of additional exposure is enough (not consistently)
to avoid underexposure. I find the whole business mysterious. As I said
before, for a long time (with film) straightforward incident readings
served me well enough. Incident readings' having recently produced sometimes
serious underexposure has been downright perplexing.
You can check the calibration of your meter. If it is calibrated correctly for the equivalent of 12% reflectance, a reading from a Kodak 18% gray card exposed with +0.5 EV compensation should give a pixel value of 118 in sRGB. This is calculated from the sRGB equation, where pixel value = (1.055*reflectance^(1/2.4)-0.055)*255. For my own camera (Nikon D70) I get a value very close to this. As you go further from the midpoint, the tone curve applied by the camera or raw converter will increasingly depart from the calculated result.
If you want to place the highlight, it is best to perform a series of test shots with bracketing and use the ISO that gives the desired results.
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