Exposure control in PS

DS
Posted By
David_Skidmore
Jun 29, 2004
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With cameras, the camera tries to expose the image to match an 18% gray in the area of the viewfinder that is metered. Theoretically if you use, say, the spot meter, the camera will expose the image at that spot so that is represents an 18% gray. With centered and matrix metering, the same, except that the area averaged is larger.

Question…how can I take a portion of an image, lets say a 20px by 20px section of the image and adjust the levels so that the average gray for that area is 18%? First of all, how do you average the area (or better yet, how does the camera average the area) and secondly, how can you know that that area equals an 18% gray?

Dave Skidmore

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J
Jim
Jun 29, 2004
wrote in message
Question…how can I take a portion of an image, lets say a 20px by 20px
section of the image and adjust the levels so that the average gray for that area is 18%?
Color balance.
First of all, how do you average the area (or better yet, how does the
camera average the area) and secondly, how can you know that that area equals an 18% gray?
The exact method that cameras use is proprietary. Camera makers don’t discluse their method in other than the most general terms. For 18% gray, R = 128, B = 128, G = 128. By the way, photos of the 18% gray card don’t always yield these numbers even though they should. Jim
GH
Gary_Hummell
Jun 29, 2004
You could use your color samplers set to 5×5. Place them in the gray card part of the image. The Info Pallette will then show the color values based on the units you select in the Palette Options dialog. RGB units are easily used as gray is always equal values like 128,128,128. LAB, 50,0,0.

Gary

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