For benefit of those who dont know what is custom white balance, in a nut shell, here it goes (lau chaio here pls correct me if I am worng) :
Different types of light give off their own color cast, this is due to their construction and the mat'l and gas used.
eg, flourescent light (not the day light balance type) typically gives slight green cast. ie, if you shot a white paper, chances are you will notice it comes out abit green.
Most books will tell you to put a magenta color correction filter onto the lens. This is correct if you dont use flash. Reason is simple, normal flashtube are daylight balance. So double -ve becomes +ve (not very good illustration).
The magenta filter will correct the green cast, so white will appear white. But when flash is fired, "white light" goes through magenta filter gives magenta cast on the white paper.
To correct this, you need to mount a green color correction fliter on the flash. So now the color cast of the flash is the same as ambient light.
Too much trouble right? That where CWB comes in for digital cameras. Again in a nut shell :
With or without flash, shoot a white paper, use this image to set you white balance through CWB (pls read you camera manual to know how to set). Now your camera is color corrected.
But if the ambient light changes, ie, turn off flourescent and turn on the tungsten light bulb or from indoor shoot to outdoor shoot. You will need to re-set the CWB again. Thats why most people dont like to use this function.
Another use of CWB is to enhance Black & White photography in digital camera. Once again, in a nut shell (me going nuts, ha ha ha) :
Traditionally, we use green, blue, yellow, orange, red to increase the contrast in B&W photo, depending on the color of the subject and its surrounding.
Now, IMO, adding any filter infront of the lens will degrade the image quality, though human eye may not be able to tell. But if a low grade filter is used, the expert can tell, only if you compare both shots (with & without) side by side (of cause the contrast will also be different).
Adding color filters also reduce the light reaching the film / sensor. Modern camera wil auto everything will take care of this with you noticing. For cameras of yester-years, one has to compensat this diff. manually.
Digital camera has the power to over come the a/m. You guess it, CWB!!!! So grandfather story continue :
Place the filter infront if the lens (dont need to screw on). Set CWB. Remove the filter. Shoot. Will see color cast, but never mind. Load to PS, use grey scale. Done.
Actually those camera that has color temp adjust. function can just offset the color temp, shot, load to PS, use grey scale. Done.
Do you think 20D (with B&W function) actually has collor filters inside the camera??? Ha ha ha.
I stress again, a/m is my opinion. You can disagree. No problem. :thumbsup:
Well one thing's for sure. Nowadays, for HID gas discharge stuff, i will definitely shoot in RAW. Then tune the colour temp as well as hue. D&Ds esp notorious for a single or double spotlights lighting the stage in near blackout conditions to maintain the mood.
This is especially important if you are shooting at high shutter speeds (be it to freeze motion or you need it coz you are handholding or stuff like that). There is very little chance jpg is gonna turn out nicely. The colour temp will just go haywire in tune with the AC sine wave no matter how anal you are with AWB, CWB or expodisc, it's physics you are going against.