Comments about ExpoImaging ExpoImaging ExpoDisc 52mm Digital White Balance Solution Filter for Digital Still & Video Cameras:

I use a Nikon D90 which gets auto white balance right about 90% of the time. I kept getting frustrated in unusual indoor lighting situations (e.g. church, rooms with mixed incadescent and fluorescent) becuase the D90's auto WB doesn't always interpret correctly.
And, of course, this camera - and I guess just about all DSLR's - have an internal light meter that measure REFLECTED light.
This neat little gizmo meters incident light (if you use it according to instructions) and has thus far resulted in virtually perfect white balance for me!
The use of this disc is not intuitive. You have to read the instructions carefully and follow them. The process is somewhat cumbersome at first, but is no big deal with practice.
My only quibble - it's outer diameter is a squiggy too small. It doesn't snap nor press fit as well as it should on my 52mm lenses. Thus I have to tip it upward a little to assure it doesn't fall out.
However, as dealing with all creative geniuses, with all it does so well for me, I can deal with a little temper tantrum.
Highly recommended!