Infrared wireless flash
A while ago, I wrote an article about attaching color gels to a camera flash. The idea was to color balance a fill flash, to make a simple bounce system or get downright wild effects. Adding filters to a popup flash can also have other uses: blocking all visible light to remove spill light from the popup flash during wireless flash.
Most SLR cameras (digital and film alike) have a way of controlling a compatible off-camera flash gun by communicating with it via light pulses from its popup flash or a didicated on-camera master controller flash. The camera first instructs the off-camera flash to emit a metering flash, then meters the light, and sends the calculated intensity of the final flash back to the off-camera flash. The last pulse of light from the camera sets off the off-camera flash for the actual exposure. This last triggering pulse often shows up in the final image. If this is undesirable, one can block the visible light from the controller flash, while the infrared part remains. As it happens, the off-camera flash is most sensitive to infrared anyway, so doing this won't even decrease the communication range much. A simple, but effective example:
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1. Under normal circumstances, the light from the on-camera controller flash spills into the frame.
2. With an infrared filtered controller flash, this is no longer the case, and all light in the exposure is coming from the off-camera flash.
So how does one block all visible light, while still passing virtually all infrared light? Some color gel manufacturers offer special infrared pass filter gel. It's expensive and hard to obtain. Some sources suggest the use of overexposed and developed photo negative film. I tried that, but for my tastes, even two layers still pass too much visible light. Maybe I didn't have fully overexposed film, your mileage may vary. In the end, I settled for two stacked dark color gels: Rosco #27 (deep red) and #90 (deep green).
3. I glued the filter stack and two magnets to a piece of transparent, preformed firm plastic and simply stuck it onto the tiny stack of magnets already attached to the bolt heads underneith the flash tube.
Both my cameras (a Sony Alpha 700 and a Konica Minolta Dynax 5D) have two tiny bolt heads just below the popup flash tube. A stack of tiny Neodymium magnets attaches directly to them. For your camera flash you may have to find the right spot where the magnets will attach or use a tiny bit of super glue to glue the magnets onto a position that will allow the flash head to move and close (if applicable). For more info on the magnets and where to get them, please read the page about attaching color gels to a camera flash. Another extreme example of how much visible light is actually filtered out:
4. Normal situation with unfiltered controller flash. The off-camera flash is lighting the wall behind the mirror.
As you can see, the first shot is completely washed out by the controlling flash, although it is correctly exposed. In the second shot, a very dim green glow is all that's left of the visible part of the controlling flash. Exposure values (ISO, shutter speed, aperture) are identical in both shots.