White balance - higher color temperature makes subject cooler or warmer?

[Colin Southern]

If I am setting custom white balance, why should I use an 18% grey card?

No need. What you're after is something that's spectrally neutral ... it just happens to be that most 18% grey cards fit the bill nicely.

The 18% bit is only used for spot-metering / exposure calculation.

[garypoole]

I am new to posting to this site , but have enjoyed the discussions. I felt I had to get in on this one. Let me introduce myself. I am Gary Poole, a long time professional photographer. I am passionate about color accuracy as that is how I have made my living for 34 years. I work from home color correcting professional digital files from clients in several U.S. states and one province of Canada. www.yourbestcolor.com

People are making digital; and especially color way too hard. Some people like to make everything photographic or computer related into techno geek speak to impress I suppose. I will try to make the warmer cooler color temp argument easy to comprehend.

Kelvin temperature is a scientific scale based on the theoretical color of a black piece of metal heated to incredibly high temperatures. As any metal object gets hotter it begins to glow a dull red/yellow then a bright red, then white hot eventually going bluer as temperature rises. Notice the colors on a gas flame.These theoretical temps. apply in photography as Kelvin degrees. Yellowish light has a "warmer" look but a lower kelvin temp. Bluer light has a "cooler look" but a higher Kelvin temp. In the film days Kelvin degrees were listed on the film pack as "daylight" of approx. 5500 degrees Kelvin.

Examples:

2200 K candle flame
2500 K incandescent light bulb
5500 K daylight or flash
7500 K clear blue sky
10000 K clear blue sky on a mountain top (clear atmosphere)

Most all strobes, on camera flashes, off camera flashes, mono lights etc. are rated at 5300 to 5700 degrees Kelvin. This is the "standard" if you will for color. Your digital camera is able to compensate for the light source which is a great thing if you understand color , but can BITE you if you don't. If you set your camera say to 7000 degrees Kelvin, you are in effect telling your camera you are under 7000 K light
(BLUER than normal) therefore your camera should bias toward the warmer side to compensate. A HIGHER KELVIN SETTING YIELDS A WARMER PRINT). A lower Kelvin setting yields a cooler print. try this on your camera as proof, I just did.

I do not recommend using Kelvin for color balance. I always use custom white balance and a lastolite grey target or balance smarter target. If you capture an off color file esp in JPEG, you can't really fix it properly later. This grey target works 100 percent of the time for me. The only time I ever use Kelvin is when I an under fixed lighting indoors and I want consistent color in available light situations. I use about 4000 Kelvin under incandescent and candlelight which gives me a pleasant natural looking warm but not orange image. I always use a flash indoors when photographing people I also drag a mono light outdoors when shooting outdoor portraits. I know this is a lot of work but after all I am after the best image I can produce, not the easiest.

Don't make this too hard folks, keep it simple and enjoy the art of photography. It is also a science but it should be a pleasure and not a brain cramper. Gary Poole