In part 1 of this series, I promised to show the tone curves for the various raw developers that I'm looking at. Here they are:


The Lightroom curves, for various settings of brightness and contrast - brightness has by far the most pronounced impact the image. In Lightroom, to get to a linear curve, you need to do three things - set brightness to zero, set contrast to zero, and select "Tone Curve - Flat" from the presets.


Then the Aperture curve showing the effect of the Aperture boost setting; the effect is essentially the same as the Lightroom brightness setting. To get a linear curve from Aperture, all you have to do is to set boost to zero.


Finally, the Capture One V4 setting; Capture One is a bit different to Lightroom and Aperture. Where Lightroom and Aperture have slider settings that are non-zero by default (brightness and boost respectively), on Capture One all settings default to zero. However, also by default, Capture One loads a "Film Standard" tone curve, which has a very similar effect to the other two program's non-zero settings. To get rid of the curve, all you need to do is to select the "Linear Response" Curve setting.


The last set of curves show a comparison of the default curve for each program, all referred back to a sRGB/2.2 gamma curve to make them comparable. While all the curves are about the same shape, there's a distinct difference in the "aggressiveness" of each curve. Lightroom/ACR adds the most brightness in the mid tones, and Capture One the least, and Aperture's about in the middle. We'll come back to this issue later in this series, but the next step is to use these tone curves to allow us to calibrate colors from the GretagMacbeth test chart.

And actually using the tone curves to calibrate colors is quite easy. All that's involved is the following two steps:
  1. Convert the l*a*b* color values for the GretagMacBeth patches to RGB in the color space of the program in question
  2. Use the tone curve to adjust the RGB values in accordance with the curve
Once we've done this, we have RGB values that are what should be displayed for each program, if the color calibration is correct. I've done this for the three programs I'm testing - a spreadsheet with the values is posted here: http://chromasoft.googlepages.com/calibrationspreadsheets.

In the next post in this series, I'll take a look at how each program compares.
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