Gray cards : the 12% - 18% controversy

Started Apr 13, 2013 | Questions thread
OP cquarksnow Contributing Member • Posts: 523
Re: The grey card is a reference for print making

xpatUSA wrote:

Roland Karlsson wrote:

xpatUSA wrote:

I think we all know that raw data from any camera will not be coded in RGB, but thank you for the reminder ;-). I was never talking about raw data with reference to gray card shots and the resulting histogram therefrom.

Hmmmm ... it fees like we are talking by each other. What histograms? On the camera LCD after taking the image? In SPP? In e.g. Photoshop after converting to TIFF/JPEG? Which one are you referring to?

OK, to keep things nice and simple and to avoid all these question marks in our discussion, let us take the example of a X3F file for a Sigma SD10 shot of a Kodak 18% gray card shot under D50 LED floodlighting with the metering showing 0 EV. Hopefully that is specific enough. If the spctral emission card is played, I also have halogens or an EF-500 flash. Take your pick. Next, we will extract the medium-size preview image from the X3F file. That way, no particular converter will be used and no post-processing will be done. The image will be exactly as God, er, sorry, Sigma created. We will open that JPEG image in ImageJ and we will immediately regard the ImageJ histogram.

Where will the peak value of the distribution lie? You tell me, and then I'll do the test.

Yes, when I said earlier "converted without further processing", I really did mean to imply a neutral image, converted from raw and properly profiled. Or even a neutral jpeg out of a camera, if there is truly such a thing . . .

So ... I assume you meant the histogram for the SPP converted image. Or?

I don't recall saying SPP - so, to simplify that question see my paragraph above.

Don't have LR. I know what ProPhoto is, but I have never heard of Melissa RGB before. Did a brief googling. Yet another color space! Yipie! Seems to be a wide gamut gamma 2.2 color space.

Neither do I, but elsewhere I've read that LR working space is named after a lady engineer at Adobe. The space is ProPhoto but the illuminant is D65 and the gamma is specifically sRGB, not 1/2.2.

courtesy of Wikipedia

Hi, xpatUSA -

As the gamma for this Melissa RGB (a.k.a. B@stard RGB) is not 2.2 , do you think the MelissaRGBD65.icc profile in Sandy McGuffog's site is nevertheless correct

based on his explanation :

The MelissaRGB profile deserves some explanation. Melissa RGB is not an "official" color space, but is the combination of the ProPhoto color space, with an sRGB gamma curve. It's important because Melissa RGB is what Lightroom uses for its readouts. Specifically, the RGB values you get as Lightroom readouts are in this space. Note that the Melissa RGB profile I'm providing here has the real sRGB gamma curve. The sRGB gamma is close to a 2.2 gamma, but is linear at low values. So, if you take a readout of the darkest patch of the GretagMacBeth chart, there is is about a 1.5% difference between the reading using ProPhoto at a 2.2 gamma and a true Melissa RGB profile.



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