Recently in this Lighting Forum there was a post about exposing for color gels. There were some suggestions about using an incident or reflected light meter to expose for correct color and density. The idea being that if you use the indicated meter exposure when measuring just the light from the gel, what you see will be what you get. That's not often the case. Depending on the density and color of the gel, the exposure often has to be adjusted and sometimes the light from the gel actually changes color as the exposure is increased or decreased. For Example: If you want to create a dark blue glow or background color, even if you start with a dark blue gel, and you use the indicated meter exposure the blue will be too light. The light meter will place the blue as a mid-tone when you want a darker, richer blue that's really more of a shadow tone. Or perhaps you want a bright warm glow or background color so you use a light yellow or orange gel. Again, the indicated meter exposure will place that gold color as a mid-tone which is not light enough to create the bright warm effect you or the client wants. In fact the metered mid-tone exposure setting for both the blue and yellow gels will be different camera settings but they will produce the exact same density and tone. One will be blue and the other will be yellow but they will both be the same mid-tone density. Take a look at this chart of gel exposures (see image below). In the studio I lit a black seamless paper with no gel, deep red, medium purple, deep green and a deep blue gel and bracketed expposures in one stop increments. Along the bottom of the chart columns you'll see an exposure value for the exposure brackets. N is the indicated exposure from an incident meter held at the black paper and pointed back towards the lens. N-1 is one stop darker than indicated. N+1 is one stop lighter than indicated and so on. All brackets were done by f-stops and the brackets were exposed by strobe and captured digitally. The top row of brackets was shot without any color gels to have a neutral reference and show exposure changes only. The Red gel is best represented by the N-1 exposure. The Purple gel looks like the N exposure. The Green gel really looks like the N-1 exposure. The Blue gel is quite dark and looks most like the N-2 exposure. There's another effect that's visible here. These shots were done with a digital camera but I've noticed this very same effect when shooting transparency film. As the exposure increases, the color of some gels also changes. The red gel turns to orange at the N+2 exposure and becomes even more straw-like at N+3. The purple gel shifts to magenta at N+1 as it's exposure becomes brighter. Surprisingly the green gel stays consistently green though it does become noticable yellow at N+3 exposure. And the blue gel has a mind of it's own. At N+1 there's a shift to purple and then at N+2 another shift, this time to cyan. This change in color as exposure changes is a good reason to run a few tests if you find yourself using color gels in your work. If you have a means of proofing exposure and color while you are shooting with gels these changes will become apparent as you set up your lighting. If you're shooting film Polaroid helps to some extent though Polaroid color and especially contrast are not very accurate. Some gels are a completely different color on color Polaroid film, yet they are accurate on transparency film. Shooting digitally with captures appearing on a computer screen rather than the lcd screen on the camera is a good way of testing if the final photography is also digital with the same camera. If you don't have access to Polaroid or digital capture then an exposure test such as the one I've shown here is very helpful. If you're shooting transparency film, bracket from N-3 to N+3 in 1/2 or 1/3 stop brackets. You'll see some very subtle and useful changes by using 1/3 stop brackets. Sleeve the processed film, label the exposures and you can refer to the color and exposures changes for future reference. You can do the same for print film if you first shoot a neutral shot on each roll without a gel for color reference when printing the film. If you're shooting digitally then do the same but carry it through to your preferred method of print output whether you make your own prints or send them to the lab. Check out the chart of exposures and colors below and you can see the color shift and exposure changes that I've mentioned. Then we'll take a look at some examples of color gels in use.