Using CTO to Create a Blue Background
This is a trick I learned years ago when working as a corporate TV production manager/cameraman. The principle is quite simple - when your camera white balance is set to daylight, any tungsten or incandescent light source will render orange, conversely when the white balance is set to tungsten any daylight source will render blue. So, in order to render the foreground normal and the background blue you need to set the camera white balance to Tungsten (Canon users) or Incandescent (Nikon users) and balance your flash which is daylight to tungsten - remember that the daylight source will be blue and needs conversion to orange.
To achieve this you will need a CTO gel filter. CTO stands for Colour Temperature Orange and is available in densities varying from 100% or Full to 1/8th. I use either Rosco Cinegel sheets or more recently the pre-cut Strobist Collection gels which include a wide selection of colour correction gels which when working with hotshoe flash or Speedlite off-camera are more convenient to carry than large sheets or rolls of gel.
The image at the top of this item was shot for a local swim school. I started with a blue background from the pool water, but the window light was neutral, not blue. As I was working in a high-humity environment I had no choice but to work with battery operated flash - mains powered units are a no go when the atmosphere is so moist - 100% humidity!
The lighting for this shot is very simple - two off camera Speedlites, both corrected with full CTO. One light to camera left modified with a 24" (60 cm) softbox and the backlight snooted with Rosco Photofoil on a stand across the far side of the pool. The keylight was mounted on a VAL (Voice Activated Light stand) aka Laurie a very able intern. The camera shutter was set around one stop below that indicated for a "normal" look to render the background slightly darker. From the image meta data it looks as if I used a 24 - 105 mm f4 IS L Series on the 5D MkII at 84 mm with a stop (sorry aperture) of f5 and the shutter at 1/125 th second, ISO 400.