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03/22/2005 11:24:47 PM · #1 |
I read that if you shoot a black piece of poster board and a white piece, the auto-metering systems in cameras will render them as neutral grey.
Did I miss something? I get black and white posterboard photos. Can anyone shed some light on this myth for me?
Thanks, |
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03/22/2005 11:25:53 PM · #2 |
| Your camera must be broke. If you read it on the internet, its gotta be true. |
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03/22/2005 11:52:48 PM · #3 |
i guess i depends on the white ballance setting and the available light, but I have never seen it turn my poster board (black or white) to netural grey. I have seen it turn my poster board to a not so white color, or the black poster board to a not so dark black.
Once you learn how to expose the shot with white balance settings and proper lighting and exposure your poster boards will come out real nice
black poster board
  
white poster board
white bed sheet (no edits, only resized for the chalenge)
James
Message edited by author 2005-03-22 23:55:10. |
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03/23/2005 12:02:44 AM · #4 |
| You have to spin them really fast. |
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03/23/2005 12:11:28 AM · #5 |
Actually, digitalknight is -- I suspect -- referring to a concept that I vaguely recall from my college days and B&W Technical Photograpy class. The basis for metering photographic reproductions is that if you blend all the colors together, that the resultant mix will be precicely 18% gray.
We used to test our in-camera metering system by shooting a light-colored wash cloth that was strongly lit from the side. The bright highlights and dark shadows of the nap on the terrycloth would create an "ideal" ratio of light and dark.
Once developed, we would use the school's densitometer to read the neg and determine if the image was over or under exposed. Hence, if the exposure was right, then the meter was working correctly. If the meter was wrong, we could deduce the correction factor in f-stops.
The very ambitious among us also checked the apeture, and by first checking the meter, we could set the camera to a variety of apetures, and they should all display the same exposure. I recall that my camera had pretty wildly varying yet exquisitely repeatable exposure differences around 5.6 -- one of the more commonly used f-stops.
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03/23/2005 12:13:23 AM · #6 |
In theory it's true, and it has nothing to do with white balance. The camera's meter has to have a point-of-calibration, it's not smart or anything, it's quite dumb actually.
Talking about simple, hand-held meters this is the gospel: the meter assumes that the average reflectivity of what it is reading is a zone 5 gray, and it gives you an exposure to render the scene as a zone 5 gray. Using film, with an outside meter, and setting the exposure manually to the meter's recommended setting, it is in FACT true that if you fill the viewfidner with a white wall, meter the wall, and use that exposure, the wall will be gray. Ditto a black wall. This is fact, it's actually part of zone system teaching process to prove this.
It gets a little more complicated with in-camera metering systems in the digital age, because they are relatively "smart". Still, in theory, if you use the spot metering function and meter a pure black and the exposure sets on that reading, the black will be rendered gray. And ditto a white. This is fundamental.
Now, back in the early days of automatic, professional SLRs (film) they started fussing witht he parameters, because it seemed that a more reasonable "average" of all tones in the typical shot was like zone 5.7 or something, I can't remember. So there was a lot of debate at the time as to whether this was a "good thing" or not.
Basically, a zone system photographer calibrates his meter first anyway. He runs tests on it, via exposure and nominal processing, to find out if it is accurately rendering zone V as zone V, and will assign a percentage adjustment to the ISO he inputs into the meter to adjust for any discrepancies. If the meter reads over-sensitive, by 10% say, then he'll set it at ISO 90 instead of ISO 100 when he's running a nominal ISO 100 film.
And the ZS photographer buys his film in bulk batches of a given emulsion run, and tests THAT too, to find out the actual ISO of the film as opposed to its nominal one. We used a lot Plus X 4x5 film, supposed to be ISO 100, usually tested at ISO 82-90.
So there's a lot of variables in this business of metering. And of course sensors aren't film, and digital files aren't negatives, and so forth. But the basic principle holds true. To some degree, your camera is assuming that what you are metering is gray. Think about it; in-camera metering is the measurement of reflected light; the camera has know way of knowing if what it is metering is a bright light bounced off a dark object or a dim light bounced off a bright object.
There are apparently alogorithms in matrix metering modes that factor this in to some degree and make semi-intelligent choices. But if you run the meter in spot moide, you can use it to "place" your exposures where you want them to be, once you understand the above.
Robt.
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03/23/2005 12:17:48 AM · #7 |
| If you want to render the white wall white, and the black wall black, just don't forget to use exposure compensation. That is, to get the white wall add a stop or two of exposure from the metered reading, and to get a black wall, decrease a stop or two from the metered reading. |
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03/23/2005 01:09:40 AM · #8 |
Originally posted by Olyuzi:
If you want to render the white wall white, and the black wall black, just don't forget to use exposure compensation. That is, to get the white wall add a stop or two of exposure from the metered reading, and to get a black wall, decrease a stop or two from the metered reading. |
... if you aren't using any sort of clever 'matrix' mode, which may or may not give you an exposure for 18% grey. Otherwise the exposure compensation approach is quite hit or miss. |
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