White Balance and Styrofoam

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Just made some measurements - due to noise made by some of participants of the taboo forum. Really, noise helps...

Styrofoam cups have non-linear response to different sources of light. The colour temperature measured for different kinds of light sources with and without Styrofoam cap can deviate as much as 600K from direct measurement; typical value for daylight is 270K.

Equipment used: Gretag Spectrolino spectrophotometer to see resulting spectral power distribution; Gretag EyeOne colormeter; Minolta Colormeter IIIF.

Miscellaneous equipment - Sun, clouds, different lamps, Nikon and studio flash, colour compensating filters from Lee Filters.
 
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Iliah,

So if I understand, some folks have proposed or are using styrofoam cups as a target for pre-setting WB, and you are saying that these are not valid due to the non-linear reflectivity?

Did I get that right?

Further, if this is the case, how does this compare with a 'calibrated' gray card, Expodisc, and Pringles lid?

Iliah said:
Styrofoam cups have non-linear response to different sources of light. The colour temperature measured for different kinds of light sources with and without Styrofoam cap can deviate as much as 600K from direct measurement; typical value for daylight is 270K.
Regards,

Frank
 
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Flew said:
Iliah,

So if I understand, some folks have proposed or are using styrofoam cups as a target for pre-setting WB, and you are saying that these are not valid due to the non-linear reflectivity?

Did I get that right?
Yes, and we are speaking not about reflectivity, but about transmission - they cover the lens with such a cup.
Flew said:
Further, if this is the case, how does this compare with a 'calibrated' gray card, Expodisc, and Pringles lid?
Kodak card is known to have minor problems. The best one can get is a sheet of Neutral Grey 8 from Gretag - response is linear from 430 to 700 nm, and no metamerism. I'll measure Pringles lid tomorrow - I do not have one. Expodisk I do have somewhere, let me find it :)
 
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Expodisk shows cooler readings, about 200K for incandescent, 400K for daylight, and up to 600K for electronic light. Resulting white balance will be warmer, as overcompensation occurs.
 
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Excellent info Iliah!!

Thanks for taking the time to post. Maybe we need to add a technical discussion forum for posts such as this one.

What do you think :?:

Regards,

Frank
 
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Iliah....

I'm not complaining at all. We are having some discussions about how we should organize the forums here, and I was just wondering what you thought about having a forum just for technical discussions.

Regards,

Frank
 
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I don't think there's a more appropriate forum just yet (at least at the cafe).

Very interesting discussion, though. Like many other people, I struggle quite a bit with WB and any help I can get is appreciated. Hopefully one day my eye will be trained better, but in the meantime I'll use whatever I can to get more accurate WB.

It's great that you have the appropriate measuring equipment and controlled lighting to measure this -- please continue to keep us informed of what you find Iliah!
 
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I Think a Technical Forum Would be The Cats Meow

Did I just say something corny like that?

Oh noooooo....I'm sounding like my mother!
 
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Iliah said:
Expodisk shows cooler readings, about 200K for incandescent, 400K for daylight, and up to 600K for electronic light. Resulting white balance will be warmer, as overcompensation occurs.

And it is exactly my finding as well, thx for confirming this for us Brother.
 
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And it is exactly my finding as well
I certainly remember your tests. I found them quite convincing. As I use gray card or colormeter, it never crossed my mind to make direct measurements of color characteristics of other "media". Your photos provided me with quite enough information that I should not bother.

Also, my feeling about Expodisk is that even if it has even colour transmission, it is hard to use it. With colormeter I take light, shadows, and midtones measurements and make my decision. If only Nikon would display the colour temperature coefficients, Expodisk can be used the same way. But lack of such indication makes Expodisk for me much less viable then colormeter.

Yet I bought Expodisk - was just buying something at local Penn, and it was there. I used it the same day, shooting cars at sunset. To speak the truth, I hate this type of shooting exactly because of white balance - customer wants cars to be exact colour, but light is very difficult. My idea was to get good balance with digital camera before using film. It takes long time with colormeter, and light is constantly changing. As you know, it is less then 20 minutes of beautiful sunset light you have here in DC. Results were not very good, whatever metering I used - pointed to the light, or taking standing near the car pointing to the direction of tripod, or to sky. So, Expodisk went to closet.
 
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If I were to use a pringles cap, would I put it over the lens and set it up as a custom wb and then use it the next time I take a picture or do I use it with each picture I take?

Sorry but I am new to using anything outside of filters.

Thanks,
 
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Melissa, as long as your lighting does not change you can use the same custom WB. No matter what target you used to set it. You just want to make sure the target is neutral in color.
 
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Thanks, Iliah, for communicating the results of your tests.

2 questions:

- what are the problems with the Kodak grey ? - just curious. as I don't use it for this purpose - but I've thought that they're reliable as long as the card is slightly out of focus during wb preset.

- I use Robin Myers' "Digital Grey Card". These cards according to Robin Myers have a very flat spectral reflectivity curve from ~ 415 nm to > 750 nm. Hence WB preset should be very accurate - no metamerism. Have you tested this card ?

Thierry
 
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Dear Thierry,

Kodak exhibits some metamerism. I have not tried Myers' cards. Does he have its spectral properties published?
 
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Iliah said:
Kodak exhibits some metamerism. I have not tried Myers' cards. Does he have its spectral properties published?
Iliah :

If I understand metamerism, it would then indicate that the Kodak cards' are not giving a consistent WB in different light ? Under which light does it change ?



John P.
 
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Dear John, I do not like the slight concavity in Greens between 550 and 590 nm. It shows under early morning light, overcast, and fluorescent lights.
 
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Catz said:
If I were to use a pringles cap
I thought Pringles lids were clearish. Am I mistaken? If not, is there anything else that would work? And I assume that I hold it tight against my lens for the custom WB.

Thanks for the info.
 
T

tamachan

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gbenic said:
Catz said:
If I were to use a pringles cap
I thought Pringles lids were clearish. Am I mistaken? If not, is there anything else that would work? And I assume that I hold it tight against my lens for the custom WB.

Thanks for the info.
The new lids are near transparent. The older lids, which are hard to find, are somewhat opaque.
 
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So, the Expodisc causes an overcompensation towards red?

Interesting, because that's what I've been finding in the studio too. It's not a huge amount, but a significant amount. Sometimes I try to correct it in NC, sometimes it's not bothersome, so I leave it as is. Always wanting to lesson my workflow, I'm very much interested in getting an accurate WB prior to doing any shooting. So, if I get this Gretag-Macbeth #8 Grey card, do I do a WB preset in the same way I did it with the Expodisc? In other words, do I set up the cam for a "Pre" WB reading, turn off my len's AF, then simply shoot a full frame of this grey card illuminating it with whatever light I'll be using for the shoot, be it strobes or natural light?

Sorry if this sounds all too elementary to some, but I haven't been at this all that long, and have no prior film experience to fall back on.

Edit: Where do I find one of these Gretag Neutral Grey #8 cards? I can't find one anywhere on the web.... so far.
 
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