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Old Jun 19, 2004, 10:21 AM   #11
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Let my try to explain it this way because it would be wrong to let less knowledgeable people use your first statement as a basis of fact. As of right now, most modern cameras and hand-held meters and are set to read 18% gray (not taking into account the "k" factor as it will likely confuse the situation). This is done so that we have a defacto standard rather that each manufacturer setting their own standards. I'm not saying this is the best choice, but rather the standard. If you go to your local camera store, you can ask to see an "18% gray card". From there it should be obvious what it's used for. Just don't put absolute faith it the card being a "true" 18%. I've seen cards measured with a densitometer that can vary by well over 1% (which is actually close enough).There are probably articles on the Internet that could help you understand the subject better than I could explain it, try a Google search.
shene wrote:
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Guerito wrote:
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shene wrote: Keep in mind that many people who complain about exposre forget the fact that light meters are calibrated at the 50% grey level.

Not true...18%.

Guerito, Let me clarify this issue. The 50% grey level is the actual intensity and the 18% is the reflectance. In other word, a 50% grey level*NORMAL* object reflects 18% of the incoming light. Is this clear enough? Moreover, this 18% number is also questionable because there is no standard for light meter manufactures to follow. Some may use 12% while some may use 15%. So, the use of the so-called mid (i.e., 50%) -grey is the safest way to get the matter right.

CK
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Old Jun 19, 2004, 5:27 PM   #12
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Guerito wrote:
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This is done so that we have a defacto standard rather that each manufacturer setting their own standards. I'm not saying this is the best choice, but rather the standard. If you go to your local camera store, you can ask to see an "18% gray card". From there it should be obvious what it's used for. Just don't put absolute faith it the card being a "true" 18%. I've seen cards measured with a densitometer that can vary by well over 1% (which is actually close enough).There are probably articles on the Internet that could help you understand the subject better than I could explain it, try a Google search.
Since you are getting into technical issues, we can be more precise now. The only meter calibrating standard is ANSI PH3.49-1971, which was created by those manusfactures to provide an industray standard for handheld meters. Moreover, this standard is advisory rather than mandatory. It says that meters should be calibrated to *ABOUT* 12% gray, with an allowable error of +/- 2%. Note that "12% gray" is *NOT* the actual gray level/scale between black (0) and white (100). That is, this 12% does not mean the gray level is 12% from black. Instead, the 12% is reflectance of a subject. This has been a very focusing issue as we have seen here and elsewhere that newbies tried hard to create18% gray card by setting the (R,G,B) channel to (45.9,45.9,45.9), i.e., 18% of 255! However, this effort cannot be successful because the gray level is too dark. What went wrong here? It is because newbies frequently considered the reflectance value (18%) as a gray scale!

Nobody knows where the 18% value came from. There are several "stories" circulating on print and on the Internet; but, none of them seems credible. (I normally don't trust too much about information on the Internet because there are more incorrect ones than good ones.) Since the ANSI standard mentioned above is an advisory one, many manufactures could ignore what they wrote 30 years ago, and, as result, one may get a light meter calibrated from 12% to 18% reflectance. A concensus we have had in the past 20 years is that camera meters are usually calibrated at the mid-gray (50% gray). When one points a reflected (not incident) meter to a subject, the meter will consider the reflected level of light equal to this 50% gray, i.e., zone V in Ansel Adams' zone system. Don't you think this is more natural and accurate explanation than the "18% gray", which IMO is a misleading and confusing word. Most people don't know what 18% reflectance looks like; but, all of us can be trained quickly to recognize 50% gray level (i.e., zone V).

Lastly, making a gray card very accurate is only one part of the exposure equation. No matter how accurate the 18% gray card is build, if the meter that reads the card is not calibrated at 18%, exposure compensation is needed. Moreover, Kodak and other gray manufactures suggest that the gray card should be held at some degree (45 degree or 30 degree) to the light source when taking a measure. This will reduce the reflectance from the designed 18% to about 15% or 12% depending the chosen angle.

With this evidence, the word "18% gray" is very misleaning and confusing, because this "18%" is reflectance. On the other hand, "50% gray" is an absolute scale and can be easier to identify and learn.

A final note. The above discussion only applies tospot metering. It does not apply to the more popular averaging/evaluative and center weighted schemes.

CK
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Old Jun 20, 2004, 6:13 AM   #13
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Guerito and shene, this grey scale discussion is quite interesting, but I will have to research it more to see if it has any real impact on my experiences with my FZ10. I'm assuming this "grey card" you can buy from a photo shop would be ideal for manual white balance setting?

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I've had my FZ10 for about 2 months now and have never had a problem with over-exposure.
I've had mine about the same time. I spent today taking snaps at EV -0.66 and they all look much better than what I was getting before. I mixed in a few at EV +0 just to see how they went and all of them were overexposed. In fact, even some of the EV -0.66 ones were too, so I'll be using EV -1.0 as my default from now on.

I also took a few spot metered shots and when I went to the trouble to focus on the right place (MF assist) then meter on the right place, the shots worked perfectly. I will be practicing this technique until it comes naturally. Then I won't have to worry about the FZ10's wonky multisegment meter.

As an aside, I took my first slow shutter flash picture today (in quite dark shade near sunset), and surprised myself by taking quite a nice picture. The only other time I'd seen this mode used was by a friend who accidentally activated it on my camera when taking shots at a party. Every shot was smeared so he thought my camera was defective!
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Old Jun 20, 2004, 10:45 PM   #14
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You can just fold up a piece of white printer paper and put it in your camera bag or pocket. I've also seen people use a 4x6 index card, but those are rather small.
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Old Jun 23, 2004, 4:48 PM   #15
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click on original link for updated review.

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