This subject has come up for me because of a recent project. Most color casts are never noticed as long as your subjects are colorful. However, when you are dealing a neutral subject, or background, a color shift of a percent, or two can make a difference. One of the things that made this shoot challenging, was that it was spread out over several days, with other shoots interrupting. This meant that the lights were taking down and re setup several times. From experience I made a careful diagram of the light settings and placement. When I first was aware of the color shift, I assumed it was caused by camera angles moving up and down, picking up different levels of color from the background. After testing that theory and coming up without a color shift, I focused on my lights figuring one of them had a color shift. This seemed like the next logical answer, since the lights were possibly switched around when doing a re setup. The good news is, all of my lights are color correct. The next possibility were my lenses. During the shoot I used four different lens combinations, because of the extreme differences in product sizes. Staring with a Canon 24mm to 105mm zoom lens for efficiency, but had to move to tilt shift lenses to maintain a greater depth of field. The main lens became a Canon 24mm tilt shift lens adding either a Kenko 1.4x teleconverter, or a Sigma 2x teleconverter which have always worked great for me in the past. As I said in the beginning of this blog, it only takes a percent, or two to throw the color off and that was what happened when the Sigma converter was added to the 24mm tilt shift lens. In the days of film I would only use one lens brand for that reason. What also complicated this project, was a mixture of flash, tungsten and gels. Below is an example of the color shift. Notice the warm color shift on image to the right.