What to do if your light source's white balance is not known - is my reasoning sound?

Let's say I'm using lights which are the same type. But I'm not sure exactly as

to what their colour cast/temperatures are.

 

Would it be wise to just set the camera's WB to daylight and photograph a white

card in front of the subject? That way I can figure out how much 'out' the white

card is from 5500K and apply that setting in software to all images from that

session.

 

Perhaps another way would be to make a couple of flash exposures of the setup

with the lights off. Then compare the two later.

 

I guess I'd have to be sure that my white card is in fact white!

You can use a white card, or a commercial grey card, or something like an Expodisc.

 

Or you could just shoot RAW and apply WB changes to your images in post-production.

Custom white balance. Read the instruction book for yuor specific camera. It involves photographing a white card and telling the camera to make it white and remember the the setting for future photos.

 

I resort to raw and auto WB frequently, but it depends on how well your camera does as to how much you have to fix later.

"I guess I'd have to be sure that my white card is in fact white!"

 

That can be a real issue. Some papers, fabrics and paints look "white" to the

eye, and indeed are in the "white" color family but may have a tint to them, or

a not visible to the naked eye slightly fluorescing agent in them to make them

appear whiter. But the camera will pick up on the bluish tinge even if you can't

perceive it. Using materials treated that way as a white reference value will

shift all of the other color values towards the yellow and reds.

 

Preferably you don't use even a truly neutral white target as a reference value;

you use a light gray (once again, a light gray that is a truly neutral gray)

instead. A white target, if exposed correctly, doesnﾒt leave the digital

processor much room to adjust the color balance. This is true for both in camera

JPEGS and raws shot for external processing.

 

For that reason, and a couple of others, preferably you use a light gray

reference target which is light source neutral - one that doesnﾒt shift color

under different types of light sources. Depending on what and where I am

shooting I use two different reference value targets: an X-Rite 24 patch Color

Checker and a WhiBal card. With the ColorChecker I use the either the third

lightest patch as a reference.

 

Like .[.Z and Ronald my recommendation is to shoot raw. With a good raw

processor -- Lightroom, Aperture 2, Adobe Camera Raw, Nikon Capture NX, Canon

DPP, Capture One, Bibble, Raw Developer --really just about all of them -- it is

very easy and very fast to batch correct an entire group of raw images, and

unlike in camera JPEG processing, it does not alter the photo permanently.

In post processing in Photoshop, you can grab a known white from the image, and use that to set your white balance. I've not done it, but

I've seen it done. Micharl Schwarz grabbed the white of an eye on one of the subjects, to be used as a known white. It worked very well.

A similar idea to Mr. Moravec's comment is to buy an ExpoDisc device and use it to create an "in camera" medium-grey tone image. Then you set the camera's White Balance to a Custom White Balance and use that medium-grey image as the point of reference. This technique simply captures the whole mix of light "temperatures"coming off your lights, as it falls upon the subjects, refracts and filters it through a traslucent white layer, and then feeds the resulting grey screen to the camera lens and sensor. You take a dummy picture to capture that medium-grey tone image. Then you dial in the custom white balnce on the camera to tell it to use that grey-tone image. This sets the custom white balance to closely match your immediate ambient lighting, regardless of your mixing of light sources. And this results in very good color tones in your color digital image. See www.expodisk.com for full details.