Setting a custom white balance on the E1 is really easy, but its
automatic white balance has been so good that I've hardly ever bothered.
While photographing birds the other day with the E1 and Sigma 55-200mm
lens, however, I noticed screwed up colours on a few photos. (The bird
is a Noisy Miner, Manorina melanocephala.)
I think I've worked out what caused this.
The filter I have on the Sigma telephoto lens is a Skylight 1A
filter, which attempts to reduce the blue cast from sky illumination.
The external white balance sensor on the E1 has set a reference "white"
off the bright background light, and against that the light through the
lens doesn't seem blue enough — so the camera has "corrected" that by
making the image bluer!
Without the bright background light, the through-the-lens white balance
sensor must dominate.
The moral of the story is that UV filters are probably better for most
purposes than skylight filters.