Gels

I’m often asked what that orange stuff on my flash is. In most cases, it will be a Full Colour Temperature Orange (CTO) gel. The CTO and other gels I use are made of the same coloured, heat resistant materials used for instance in theatre productions. You hold them in front of a light source to change the colour and/or density of the light.
The CTO is the gel that I find most useful for indoor work. The colour temperature of a camera’s flash is close to that of sunlight. The colour temperature of the bulbs you’ll find in most homes is much warmer. If you balance ambient and flash light, the picture is going to end up with either blue-faced people and a correctly coloured background, or regular colours on the flash-lit areas and a background that’s way too orange. The CTO matches the colour of the flash to that of a tungsten lamp.
The CTO isn’t the only filter that there is. Other filters will match a flash to halogen lights, or they’ll give it a strong colour tint that can be used for artistic effects.
I made my gels from sample packs of the Rosco and Lee filters. I got some from my flatmate who got them from the university’s drama department, others were sample packs that the companies used to send out when you simply asked them. They’re harder to get for free nowadays though. One of the easiest ways to get hold of a pack with a decent size and the colours that are most useful is the Rosco Strobist Collection.
There are dozens of ways to attach your gels to an external flash once you’ve got them. Some use paperclips, plastic holders, or velcro. While they have the advantage that you don’t need to trim your gels, most will negatively affect your options to use other light modifiers – an omnibounce or other diffuser, for instance, will likely not fit over your attachment mechanism.
I decided to stick with the option that takes more effort but in the end is much more flexible: I cut out the gels I need to make them fit the front of my flashes – one size for the Canon 580EX, one for the 430EX. The sizes for the I and II models are the same. The are templates somewhere on lightingmods.blogspot.com (just can’t find them anymore…)
I had to brush up my cutting skills a bit; the gels have to match the shape of the flash precisely so as to avoid bits sticking out on the one hand, and gaps throgh which non-coloured light leaks on the other.
If you’d like to learn how to use gels properly, I’d recommend over to David Hobby’s blog strobist.com. He’s got a section on gels in his Lighting 101 articles and general posts with info about gelling your flash.