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White Balance Question/Help
Old 01-25-2003, 12:45 PM   #1 (permalink)
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I was finally getting around to trying our my custome WB function with my Canon EOS D60. I had been using Auto WB or the Tungsten if I was shooting inside with Ambient Lighting.

So I went to shoot an image encompassing a white card in the given lighting that I would be using and a question arose.

If you try to use your camer's built in reflective meter and shoot a 100% white surface, it's going to make it 18% gray, correct? So my question is, when shooting the image that one would use as your reference for a custom WB setting, do you use the "correct exposure" image of the white card, which is really not white, but rather 18% gray, or would you use a "corrected exposure" image of the white card, in which you would increase exposure a couple of stops to get a "true" white image.

If I haven't completely confused you with this and you have a definative answer for me, it would be much appreciated.

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Tim H
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Re: White Balance Question/Help
Old 01-25-2003, 07:46 PM   #2 (permalink)
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Hey, Tim -

If I understand correctly, you're asking whether or not you should use an 18% reflective (possibly non-white) surface to set the (possibly non-white) white balance on a D60.

I can't answer that question, because I don't know anything about the Canon D60. I CAN say that you appear to be confusing two things - exposure and white balance. I know that they are separate concepts and I suspect that they are dealt with separately on the D60. (Excuse me if the following is a bit didactic. I try to teach for a living and as I get older, I find that I'm trying harder - and being less effective. So I tend to over-compensate.)

Exposure deals with the total number of photons allowed to strike the sensor, i.e., the intensity of the light and the time the shutter is open. White balance deals with the relative numbers of red, orange, yellow, etc., photons AND the fact that the sensor (film, eye, or silicon) does not respond equally well to each color of photon. A truly white object should reflect 100% of each color of photon. A NEUTRAL gray object will reflect the same percentage of each color of photon, but it won't be 100% - instead, 18%, etc.

Setting the white balance is equivalent to telling your camera to adjust the amplication of the red, yellow and blue sensors such that the recorded values of those sensors are the same. In other words, you force the light to be recorded as if it had equal numbers of each color of photon.

You can do this with a neutral gray and get the same result. In principle, you could do it with a true black and still get the same balance - but I suspect that the dark noise of the sensor would cause some problems, as well as the algorithms the designers used, which, in all likelihood, do anticipate someone setting the white balance on a nonreflective object.

In short, no - don't increase your exposure unless you're setting the white balance on a color that absorbs some significant fraction of the total light.

Sorry to ramble on so -

John


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Re: White Balance Question/Help
Old 01-26-2003, 09:59 AM   #3 (permalink)
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You confused me at first...But then I read the other reply about photons and warp drives and plasma cannons (sorry, I couldn't help myself). I then went back to the book (D60 owners Manual) and it says to use "white" and it makes no metion of 19% gray. I have used custom WB several times when just taking snapshots around home because of my mix of tungsten and flouro lighting. I have been pleased with the results using pure white to set the balance.

this is just my $0.02 (they took the cent symbol off of keyboards when the typewrither went out of style).
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Warm Cards Should Pay Me........
Old 01-26-2003, 01:58 PM   #4 (permalink)
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for all the exposure I have given them on this forum.

WB has nothing to do with f/stops. It simply tells the CCD what white is, no matter if it really white or not, so the camera's Firm Ware then interpolates the colors for the rest of the scene.

Go here White Balancing 101

This will tell you everything you NEED to know about White balancing.

Professional videographers never use the Auto White Balance function unless things are happening too quickly for them to make the changes. The same should go for Still photogs too!

I highly recommend the Warm Cards system. I have been using them for over a year now and love them. However I am not shooting still digital, but I do use them everyday in TV production. Warm Cards had an article about them in ShutterBug late last year too!

Clint Smith
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