What was that piece that just fell off the strobe?
A diffuser is an accessory piece of equipment for the strobe. It is a piece of plastic that usually snaps on or locks into place over the strobe face. See figures 1 and 2. It is designed to widen the coverage of the light as it comes from the strobe. A diffuser causes the light coming from the strobe to bounce around, be deflected, and spread out from its original path. It’s not unusual for many strobes not to be able to fully cover the field of view of a wide-angle or ultra-wide lens. Without the diffuser in place, you get a spot light effect. See figure 3 (15 mm lens/no diffuser). If that is what you want to create, that’s fine. But, if you want full coverage of the picture area, you’ll need a diffuser or a strobe that has a wider coverage area (more expensive and usually larger size strobe).
A diffuser will also soften and slightly warm the color of the strobe light. You can lose as much a one f-stop worth of light from the strobe while using a diffuser, because the light is being spread out and partially blocked. Keep in mind that you may need to compensate for this loss of light by either adjusting the strobe to subject distance or opening up the aperture as much as one stop.
A common problem with diffusers is that they are easy to lose. They become un-snapped or un-locked from their position on the face of the strobe, float away and are never seen again. There are a couple of ways to reduce the likelihood of losing the diffuser. If you’re going to use the diffuser for the whole dive, place two or three small pieces of plastic tape on the diffuser to help hold it in place. See figure 2. If you think you’ll want to take the diffuser off and on during the dive, use a piece of monofilament fishing line to tie a safety line from the diffuser to the strobe. So, if it falls off, it doesn’t get lost. Note the small holes in the diffuser (figure 2) that can be used to attach the line. They are originally designed to let air and/or pressure out of the diffuser when submerging.
Diffusers can be a useful accessory to change the coverage, softness, or even the color quality of the light coming from your strobe without having to spend the extra money for a larger, more expensive strobe.