Qs re Using Hardware Store Paint Samples as Gray Card, Etc

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NOLA cam Junior Member • Posts: 25
Qs re Using Hardware Store Paint Samples as Gray Card, Etc

Qs re Using Hardware Store Paint Samples as Gray Card, Etc

I have an Olympus micro 4/3rds system. I took some gray color paint sample cards from Home Depot and Walmart. Various shades that I thought were of a medium gray color. I took some RAW + JPG shots of them under various lighting conditions, either indoors or indoors mixed with sun on a somewhat cloudy day. Used a sharp 45mm prime m43 lens (90mm FF equiv) at f5.6 and f7.1. Center spot focus and took a few shots of all 8 cards together overlapped along with a few other items at the periphery to check colors. Center spot focus. I found that the images varied in brightness, but some were clearly too dark, so I varied the EV to +0.3 or +0.7 as well.

I could only check the jpgs as I do not have a plug in for the RAW Olympus files in GIMP. Perhaps I was lucky in the paint sample cards chosen as several had tightly overlapped histogram R G and B channels when checked in GIMP (I do not have Photoshop). Small areas of interest were used. However, some were well above or below 128 (0-256 scale) in the channels (that is they appeared to be too bright or too dark to be in the middle of the R G B histogram at 128.)

The lighting also changed which sample was closest to 128 on the histograms. That seemed to be the biggest issue. Changing the EV also changed which sample was closest to 128 on the histograms. GIMP also provides a luminance histogram, and that appeared similar to RGB. I had some scans of 1997 Koday gray scale and color control patches cards to check in GIMP; but not a dedicated 18% Gray Card.

GIMP only seems to guess at the Kelvin temperature of the lighting and I have no idea what values to use for bright clear sunlight, overcast cloudy, nor run of the mill high efficiency fluorescent bulb for regular indoor use.

Qs for the group:

How do you check the Kelvin temperature of the lighting condition for indoors or outdoors? Do you simply apply numbers from a table, etc? What if you do not know what type of lighting was used indoors? What about use of flash?

How do you choose a preferred Kelvin temperature for the adjusted image?

Can I use most any “medium gray” paint sample card where the R G and B histogram values are sharply peaked at similar values? What is so special about a store bought card for us amateurs? Is the main use for Photoshop or other software white balance, lighting and color correction? Same issue regarding a color card vs homemade.

I understand why the lighting varied the histogram number for the gray colored cards. But how do I use the card in the real world and then correct in GIMP? I would likely be using a gray or color cards for taking pictures at museums or stained glass in old churches. I know I would have to take a picture of the scene with the gray card or color card in the scene to do the corrections later. For most other pictures, I would hope that my Olympus Pen7 and OM D EM5II can do appropriate white balances. Various family members have the old Canon T5, Nikon D3300, and a Sony alpha 6000. I assume that solutions will run a parallel path for any of these.

I hope I have posted to the correct Forum. Any assistance appreciated.

Canon EOS 1200D (EOS Rebel T5 / EOS Kiss X70) Nikon D3300 Olympus E-M5 II
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