I was at the San Francisco Museum of Modern Art getting ready to photograph an exhibit of prints by the late Henri Cartier-Bresson. As soon as I entered the exhibit, I made a custom white-balance reading which was, for all intents and purposes, the same as the camera's Incandescent pre-set. After scouting the exhibit for possible photo opportunities, I was directed to Martine Franck, widow of the late Cartier-Bresson, who was discussing the photographs from the exhibit and her experiences as a photographer for Magnum Photos. I decided that I wanted a photo of Franck with one of Cartier-Bresson's better known photos in the background. Because the main lights in the museum were coming from tracks in the ceiling I knew there would definitely be shadows in her eye sockets. I decided I needed a tiny bit of fill light to add some life to her eyes. I thought the SB-800 speedlight in my bag would be overkill, so I decided to use the camera's built-in flash. I knew that the light from the flash would give the shadows a bluish tint, so I taped a small piece of Rosco's Full CTO gel in front of the flashtube. This brought the color temperature down to level more compatible with the current (incandescent) white balance.
I set the flash to fire in the manual mode, at 1/32 power. Since I was shooting at ISO 6400, I did not trust TTL when flying this close to the edge of the envelope. This was just a guess, since I knew that if it was too powerful for the given shooting distance, I could partially cover the flash tube with my finger to reduce the output. As it turned out, the setting was just about right.
In a later post I'll talk about light and lighting, but there thing you should keep in mind: Fill light should never over-power the main light. You use the main light to accentuate form and texture in a photograph. You use fill light to provide detail in the shadows. Don't push your fill light too hard or you'll blow out (over-expose) your highlights.
As you can see from the illustration, it's a small piece held in place with some Gaffer Tape. Gaffer tape is tough and can be attached and removed without too much difficulty. I just position it to fully cover the flash tube.
One last suggestion. Be sure to remove your lens hood when using the pop-up flash. This goes against my belief that the hood should be left on at all times, but when a lens is used at wide settings and short distances, the hood can cast a shadow on the lower edge of your photo, something you'll probably want to avoid.
If you're interested in learning more about color correction gels, I suggest you go to the the best source on the planet: Strobist.com. Click here for David Hobby's explanation on how and when to use gels. You can buy an inexpensive sample pack here.
I always have this gel taped to the back of my press pass, just in case.