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The Promised Color Temperature Test ...

HBB

Phoenix, US
8783 posts

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HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Charter Member
Fri 10-Jun-11 11:01 PM | edited Sat 11-Jun-11 03:23 AM by HBB

As promised, here is a hastily contrived test of the custom WB color temperature settings in my D3, under a constant color temperature (2650K) light source.

Nikon D3, 1/15 sec, ISO=200, 24-70mm @ 79mm and F/8.0

Target was an 8.5 x 11.0 sheet of Epson Velvet Fine Art paper.

All images captured in RAW, As Shot, and with Tint set to zero for every image.

Images were assembled in Adobe RGB 1998, and converted to sRGB during JPG compression for use here.

Illumination = six overhead tungsten-halogen track lights burning at 2650 Kelvin.

Exposure was a constant 2 1/3 stops above metered value, due to meter's desire to render bright object as neutral tones, by underexposing.

Each of the small patches below has a sequential number on it and the WB setting that was dialed into the D3. The incremental Kelvin steps (K) are a constant 10 mired apart.

The R/G/B values for image 1, upper left corner, are: 247/248/250. These values are close to a uniform gray, 5 to 8 points short of pure white at 255/255/255.

The R/G/B values for image 20, lower right corner, are: 255/255/227. These values show an increase of 8 and 7 points in Red and Green, while Blue decreased significantly (23 points) to 227. Net result is the increasing yellow cast (white minus Blue = Yellow = Red plus Green) that the images display over the twenty 10-mired intervals.

I'm running my new 24 inch Eizo CG243WBK monitor which just passed the 100 hour smoke test and infant mortality period without incident. It has not been calibrated/profiled yet, but the out of the box performance is superb. It also includes a very nice hood that fits around the top and sides, and shields ambient illumination nicely. I doubt that calibrating/profiling will make much difference in this quick test.

If I overlay consecutive images, with 10 mired spacing, while assembling the matrix, I "think" I can see the difference between them on screen. This monitor claims to be able to present something over 98 percent of the Adobe RGB 1998 color space. The difference on a given row from the far left image to the far right image is clearly visible. This would be a span of 50 mired. The difference bettween the middle image and the two ends is also visible, but not nearly as apparent.

The 10 mired interval between images 1 and 2 is 60 degrees Kelvin, or an increase of 2.4 percent. The 10 mired interval between images 19 and 20 is 200 degrees Kelvin, or an increase of 4.6 percent. Going beyond this chart, the difference between 6250 Kelvin and 5880 Kelvin (a ten mired interval) is 370, or 6.2 percent.

This confirms my suspicion that the degrees Kelvin difference for a given mired interval, ten mired in this example, increase as color temperature increases, thereby producing what appears to be a constant perceived change between consecutive steps to an average human eye/brain system. It may also be the average minimal change in degrees Kelvin that a series of observers under carefully controlled laboratory conditions were able to perceive, when the mired interval was being determined.

While I don't have access to the Nikon color management algorithms running in their cameras, what I am seeing more or less fits what I was expecting to see.

I captured a total of 25 images, from WB setting 2500K to 6250K. I wanted to keep the individual samples large enough for all to see. Twenty images seems enough for this experiment.

Remember: your monitor and the ambient illumination in your work area will affect your perception of these images. We are talking very subtle shifts in color here.

These images may appear quite differently when printed on my Epson P7900 printer, with its eleven different ink colors. I'll try and print this chart over the weekend and report back.

I've blithered on long enough ... thanks for listening.

Comments and suggestions for an improved test greatly appreciated.

Regards,

HBB in Phoenix, Arizona
Nikonian Team Member

Photography is a journey with no conceivable destination.

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