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Author Topic: Zone System, light metering and neutral gray  (Read 12474 times)
bjanes
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« Reply #20 on: September 30, 2007, 03:15:36 PM »
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However I didn't understand too well why you corrected your camera's metering by 1/2 stop. Aren't we finding out in which level the camera allocates any metering on a uniform surface? independently of the reflectance of the card used. You are using the built-in camera's meter, don't you? If so the camera doesn't know how much reflectance the card has, but actually doesn't need to know, it simply measures the amount of light entering its metering system to allocate that luminance as a middle gray in the final image.
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Here is an explanation that is better than I can give:

[a href=\"http://www.bythom.com/graycards.htm]http://www.bythom.com/graycards.htm[/url]

Bill
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bjanes
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« Reply #21 on: September 30, 2007, 03:25:18 PM »
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Perhaps it is not relevant for the reasons you mention, but it is my understanding that BPC is always used when converting to one editing space to another (ProPhoto to sRGB, etc.) and that it was used when converting linear RAW to the destination space.
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BPC is an Adobe extention, that has not been formally adopted by the ICC. An [a href=\"http://www.color.org/AdobeBPC.pdf]Adobe Paper[/url] on the ICC site gives some useful information. In Adobe applications, BPC is done by default with the relative colorimetric rendering intent and is not applicable to the perceptual rendering intent. If the black points of the two profiles are the same, BPC has no effect. Since ProPhotoRGB and sRGB can both represent a black point of zero, it would have no effect on the conversion.

Bill
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Guillermo Luijk
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« Reply #22 on: September 30, 2007, 03:51:39 PM »
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Here is an explanation that is better than I can give:

http://www.bythom.com/graycards.htm


Jesus! so please correct me if I am not right (and I won't bother you again   ): the camera meter actually meters gray 12%, i.e. it will hypothetically allocate the measured area in a gray colour corresponding to a darker gray than the expected 18% gray (standard middle gray in the print industry). So by exposing up by 1/2 stop, which is the physical ratio between those two gray levels, you expect to reach in your camera something close to the standard 114 value.

Is that OK?
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bjanes
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« Reply #23 on: September 30, 2007, 05:47:52 PM »
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Jesus! so please correct me if I am not right (and I won't bother you again   ): the camera meter actually meters gray 12%, i.e. it will hypothetically allocate the measured area in a gray colour corresponding to a darker gray than the expected 18% gray (standard middle gray in the print industry). So by exposing up by 1/2 stop, which is the physical ratio between those two gray levels, you expect to reach in your camera something close to the standard 114 value.

Is that OK?
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Yes, I think that is correct. You have to be aware that many meters are calibrated for 18%. It always pays to do a bit of testing yourself.
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Guillermo Luijk
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« Reply #24 on: September 30, 2007, 06:20:07 PM »
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My Nikon D70 and D200 comply closely to the ISO standard. Here is an experiment with the D70. I took a Kodak 18% gray card and Q14 target and placed them on a background. Since the ISO standard for light meters pegged to 12% reflectance, rather then 18%, I took an exposure reading from the gray card and gave 1/2 stop more exposure than indicated. The gray was rendered at about 118 in aRGB and the steps in the Q14 target were more or less as they should be. I used the default ACR tone curve

FYI

I have been given a gray card shot RAW from a Nikon D200 and after gamma 2.2 conversion yields a gray level of 86. With +0.5EV exposure correction the gray level raises up to 103.

The D200 seems to have ~3 stops of overexposure safety margin before starting to blow the metered area:

[span style=\'font-size:8pt;line-height:100%\']D200 gray card linear histogram[/span]
« Last Edit: September 30, 2007, 06:21:02 PM by GLuijk » Logged

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