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Expodisc vs. Pringles vs. gray card vs. whatever
I'd like to see a discussion of the various exposure and color balancing aids, such as gray card, expodisc, Pringles lids, etc. There are of course at least two aspects, one being exposure determination for incident light and another being white balancing (e.g. taking a photo of the light source with your expodisc on the lens, and using that info in the darkroom to get good light balance.)
To get the discussion rolling, last night I thought about using a mailing tube cap as a poor-man's expodisc. I measured the transmission of a translucent mailing tube cap with my densitometer. The overall transmission was surprisingly useful, i.e. 16% in the green channel, not too far off from the supposed ideal of 18%.
The white balance was a little off, reading about 0.06 absorbance units too transmissive in the red channel and about 0.09 units too dark in the blue channel.
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Gray card has always worked for me. Once you have your test print matching the gray, then I'd zero the color meter on the baseboard to the gray card for that pack of paper. I try to expose a gray card at the beginning of each roll. Then zero the color meter to the new gray card negative at the beginning of each roll with the color wheels on the head.
I'm not sure how one would use those other things. Perhaps you'd be out in the scene with the thing over the lens and shoot a frame pointing back toward where you'd stand to make the picture. Then try to match that negative to a gray card or something???
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If you are looking for a cheap white balance source, $5-6 will get you a heavyduty plastic portabrace white balance card. I'm using it for digital. Haven't tested it with a darkroom densitometer as I'm not doing color printing. I'm not sold on the concept of the expodiscs, though I agree they have some utility.
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Each Expodisc is (was?) individually filtered for correct percentage transmission, color balance, and even diffusion. Mine haven't shifted color balance in well over 20 years of use, and they also act as a lens cap. It's used in the same way as an incident meter, and can be used for pre-exposure (i.e. flashing film to a threshold value). It's basically turning your camera into a TTL incident meter with the addition of providing a reference for correct color balance. An Expodisc is much easier to carry along and lasts far longer than a gray card, even the plastic gray cards that I've tried over the years. An Expodisc is also much easier to illuminate easily at the correct angle, whereas gray cards often need to be more carefully angled and placed in the light.
White balance is the same concept as color balance, but appears to be used in a much more digital context, or by those who grew up on digital and don't know the more traditional film nomenclature. It's also typically aimed at balancing on a highlight (white reference) rather than a mid-tone (gray card), although I'm aware that it can be used on any neutral tone. Anyone who understands either system should be able to make the necessary mental adjustment easily.
I've not seen white balance referred to as a way to determine exposure, and that alone makes it very different from a gray card. White balance I've only seen used to set a color standard, and a gray card is often used both to set and exposure and color balance. Bleached white paper coffee filters are also often recommended over the lens for white balance in the same way as the Pringles lid, but I have not seen a reference to anyone using the filters for incident exposure metering through the lens. Has the Pringles lid (as I understand it production has changed from the one used for white balance) ever been recommended for metering as is, or with a specific compensation factor?
Lee
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a reflected reading of of the palm of one's hand if 2 stops off of 18% grey ( not matter one's race ).
seems fine to me to use a hand, instead of the other stuff
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 Originally Posted by jnanian
a reflected reading of of the palm of one's hand if 2 stops off of 18% grey ( not matter one's race ).
seems fine to me to use a hand, instead of the other stuff
for me it's a stop and a half. but maybe I just need to wash my hands more often 
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If your palm is a neutral gray in color (regardless of race) it might be time so see your doctor.
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lee i didn't say it was neutral gray, but 2 stops brighter than neutral grey.
eric
no need to wash'em if they are consistently 1/2 stop off
killer signature btw
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Originally Posted by jnanian
lee i didn't say it was neutral gray, but 2 stops brighter than neutral grey.
True, but the discussion is about relative merits/uses of the mentioned items, and at least two of them have a second function as color references, which the palm of a hand doesn't do well.
Sorry if I was too glib, just thought that aspect warranted mention, as you recommended the palm 'instead of that other stuff'.
Lee
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Originally Posted by Lee L
True, but the discussion is about relative merits/uses of the mentioned items, and at least two of them have a second function as color references, which the palm of a hand doesn't do well.
Sorry if I was too glib, just thought that aspect warranted mention, as you recommended the palm 'instead of that other stuff'.
Lee
good point ...
i was half there at least
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