ACR not applying linear segment w/ProPhoto

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i using imatest check dr of nikon d800e stouffer stepwedge. using acr 7.2 adobe standard profile , pv 2010 linear tone curve (sliders set zero, point curve = linear). when rendering prophotorgb, noticed there no linear segment @ low luminances , expected linear segment present srgb.

 

the imatest plot shown along calculated response using densities of step wedge , prophoto equations. linear segment starts @ target densities of -2.5. causes prophoto have greater total dynamic range srgb. no big deal, investigation investigate assertion prophotorgb has greater dynamic range srgb. true saturation clipping occurs earlier narrower space, luminance clipping should , dr should same.

 

http://bjanes.smugmug.com/photography/imatest/d800e-dr/i-trcscxv/0/m/densityresponses-m.png

 

http://bjanes.smugmug.com/photography/imatest/d800e-dr/i-5znzqst/0/xl/dr_compositeb_flat-xl.png

acr/lr use linear segment initial part (dark shadows) part of encoding curve prophoto.  source code shown in dng sdk, file dng_color_space.cpp, if you're interested.  briefly, transition point between linear , non-linear part (assuming [0,1] coordinate space):

 

x = 8.2118790552e-4

y = 0.019310851



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