RGB Values

O.K., another newbie question.
If I were to shoot an 18% grey card and open it in CS2, would the RGB values of that photo be 128 for all three channels? Or would it be a different number?
Thanks for helping!!

Interesting question, and reasonable assumption. I can't think why it matters, shoot the grey card then transfer the settings to manual. I tried to find the last grey card I shot but it seems to have gone, it was helpful tho.

focuspocus wrote in post #3341466
If I were to shoot an 18% grey card and open it in CS2, would the RGB values of that photo be 128 for all three channels?

If not 128 for each channel they will be v-e-r-y close to each other. The important thing is that no one channel dominates, hence the color balance.

Here's a 18% card I shot. I opened it in Photoshop and used the Eyedropper tool to check the R-G-B values at 4 different spots on the card. I was frankly surprised that there was as much difference as you see here but I think the bottomline is that the colors are balanced.

Cheers!

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Thanks Tim & Steve!
I am trying to learn exposure by the Zone System and needed a point of reference.

ayotnoms wrote in post #3345306
I was frankly surprised that there was as much difference as you see here but I think the bottomline is that the colors are balanced.

Was the eyedropper point sample or 5x5 average?

Since Canon cameras tend to slightly underexpose camera-converted jpgs (although there can be variations even between cameras of the same model), the values will usually be around 118 in sRGB and slightly less in AdobeRGB.

The exact value may vary but they should all be the same value for a true gray color. If WB is off even a bit, you will get a color cast and the values for the different channels will then be different.

René Damkot wrote in post #3346983
Was the eyedropper point sample or 5x5 average?

Can you explain why it would matter for a solid grey card?

ayotnoms wrote in post #3345306
I was frankly surprised that there was as much difference as you see here but I think the bottomline is that the colors are balanced.

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Looks like the card is still wrapped in plastic and it is picking up some reflections. That would probably explain the different values you are seeing.

The use of the grey card is to get an accurate exposure for the whole scene in given lighting (possibly difficult) conditions. But the fact is that the metering system should resolve *any* scene to the equivalent of 18% grey whether it is white snow or dark woods. Surely the test could be carried out as easily with a sheet of white paper as for a grey card. The camera metering will still try to render the scene as 18% grey and the exposure time will be shortened. But shouldn't you still end up with the same RGB values for a grey card, white card, dark grey card, etc., as long as you have set colour balance accurately and the card has no colour cast?

If your colour balance is off then there is no reason at all to assume you will get equal values of RGB, whether 128, 128, 128 or anything else. Surely before you can perform this test meaningfully you have to set manual white balance off the card you are shooting, be it white or grey.

René Damkot wrote in post #3346983
Was the eyedropper point sample or 5x5 average?

3 X 3 average: CS3

Bodog wrote in post #3347946
Looks like the card is still wrapped in plastic and it is picking up some reflections. That would probably explain the different values you are seeing.

I took the card out of the plastic. The card was leaning against a vase with the lower portion closer to the camera and the shot was taken in uneven, available, indoor light.

Here's a question: how much value variation between channels would it take to consider the colors out of balance? On the example I posted I thought the values were "close enough for government work".

focuspocus wrote in post #3341466
O.K., another newbie question.
If I were to shoot an 18% grey card and open it in CS2, would the RGB values of that photo be 128 for all three channels? Or would it be a different number?
Thanks for helping!!

You have not mentioned whether or not you are shooting with a Custom White Balance or just using one of the WB selections, such as daylight, incandescent, etc.

If you are shooting with a carefully done Custom White Balance, then the 3 channels should come through at approximately the same number, but that number is likely to be much lower than 128 (due to the fact that most Canon cameras underexpose).

If you are not shooting with a Custom White Balance, then the three channels will be almost any value, because whatever light is illuminating the gray card might be far from white light.

tim wrote in post #3347752
Can you explain why it would matter for a solid grey card?

Color noise.

René Damkot wrote in post #3351883
Color noise.

Good point.