Hi Yauman
Thanks for your reply. You got that right. It works now. But I still don't quite understand why it needs a reference image. Most videocameras just need to be told that what is white. What is the reference for? Should the reference image be something white that has the same color temperature that I want the white balance set to? Or is it just any picture from any place?
The simple answer is that if you are telling the camera you want a custom white balance, it needs to know what is "white." In all the other settings, it got "what is white" preprogrammed in - but with custom setting, you have to tell it. How do you tell it is where the reference image comes in.
"What is white" depends on how that "white" is lighted! Our brains remember color and interpret our vision based on that memory. If a person stands under a leafy tree surrounded by a green lawn, you looking at the person sees flesh tone because your brain tells you that his flesh is flesh tone. But in reality, he's really green from the reflected green light around him - and if you take a picture of him, he will be greenish! Thus the importance of have a proper reference for a proper White Balance.
If you take a picture of a white piece of paper (filling the frame) and by using that as the reference, you are tell the camera that "hey this is image should be white" (or grey) even though it will not be if the picture was illuminated by incandescent (it will be reddish) or fluorescent light (it will be greenish). So unless you have an absolute reference, your camera is "guessing" by invoking some averaging algorithm. For the D7100, the auto white balance actually do a reasonable good job.
Now, there's is of course absolute kelvin setting you can use and most professional videgraphers prefer this method of setting and remember all the Kelvin number for daylight, cloudy, incandescent etc. D7100 allows you to set by absolute reference too - just go to the K settings and dial in a number to whatever you think the kelvin setting for the scene is. (I have a videographer friend who makes fun of me for using a grey card reference - he looks at the sky and rattles off the Kelvin numbers off the top of his head!)
The proper way to use that custom preset is to buy yourself a 18% grey card (8x10) from any photo supply place. When you need to calibrate for a scene, take a picture of that card under the shooting light condition - make sure it fills the frame completely - turn off auto focus; blurred preferred - and take a picture of it using Auto WB setting. Now, when you use that image as the reference image for custom white balance, you are telling the camera that that image should be 18% grey and the camera computer will then auto correct the color for all your images based on that reference. Then when your light changes, you'll need to do another one.
If you don't want to be bothered with doing the reference image thing, you can do it post production if you use something like Lightroom, Aperture, etc. If the picture have some grey or white, or have someone holding a grey card in the image, at post production just tell the program that that little white or grey area should be the balanced white and the rest of the picture will be corrected. I alway have my models hold a grey card under her chin as the first picture of a series and then correct at post production. If you have an image without a grey card, the whites of the eye and the teeth are good white reference points. If the image have a part of brush aluminum (like the MacBook Pros) that's a very good white balance area to use for reference. (btw white, grey and black are the same thing in color physics - just level of intensity.)
Here's a trick when you use speed lights - at least the Nikon ones - set WB to "Cloudy" Works perfectly every time - much better than AWB or Flash setting!
Hope that helps.
