UPDATE: 9/6/12 - See Aokatec IR panel under alternatives
Wouldn't it be great to dramatically reduce the number of shots where your subject is blinking?
I rely heavily on wireless TTL flash via Nikon's CLS Advanced Wireless Lighting and really like the ability of the D80 (and higher-end cameras in Nikon's line) to use the pop-up flash as a commander. One drawback to Nikon's wireless system is that there are longer preflashes that occur (for the commander to communicate with remote units) and many times, our son ends up blinking in the shot. The preflashes also create visible specular reflections. Great if you want an extra catchlight in the eyes but otherwise potentially problematic.
Nikon has a dedicated infrared commander (SU-800), which I'm assuming emits invisible preflashes, but it's not cheap. Moreover I sometimes use the pop-up to add a little bit of fill (generally -2 stops flash exposure comp), which the SU-800 can't do.
The SG-3IR (that's "ir" as in infrared, not "1r") flash panel looks like a simple and cheap piece of plastic, but it's an effective and affordable solution to the preflash problem. It's not easy to find but I got one from B&H and it works as expected.
From pictures, I thought the panel was just a piece of solid black plastic with small slits to let some light through. But the panel actually has no holes and is made of a material that blocks visible light but is translucent to infrared (which is what the slave units need to see). The panel is also larger than I expected - about 2/3 the size of a credit card.
The panel attaches to the camera's hot shoe. Its plastic base has a spring-loaded pin that secures the panel into the hot shoe. Removing the panel is kind of tricky because of the pin. The way I remove it is by pulling the unit up to lift it very slightly then sliding it off.
DOES IT WORK?
The panel is very effective in reducing blinking shots. All that the subject will see from the popup flash is a dim red light from the panel (much dimmer than the AF assist light). There is still a preflash from the slave (when in TTL or AA mode) but: 1. If the slave is off on one side, it's less likely to cause the subject to blink, 2. using fv lock fires the preflash, so that in the actual shot there is no preflash. If even that preflash bothers you, you can switch to Manual mode - no preflashes.
In real life, I've used the panel to take about 200 shots in a variety of ambient light conditions - daylight to dim indoor light. I've had literally a handful of pics with blinking subjects. I speculate that those few blinking shots are due to the subjects anticipating the flash even though they can't really see it.
What if you want to use the popup as fill? The panel has a hinge that lets you flip it up and it will stay up. Nice!
EFFECT ON RANGE
In tests I conducted, maximum range was reduced by about 1/3 to almost half, but probably won't make a big difference for family photographers. The farthest distance I was able to trigger an sb800 in near-ideal conditions (dim parking lot lights at night, in direct line of sight, sensor window facing the camera) with the popup covered by the panel is about 75 feet. With the bare popup flash I was able to trigger the sb800 under those conditions at about 110 feet. In daylight, with the panel covering the popup flash and the camera facing toward the sun, I was able to trigger the SB800 at about 45 feet. With the bare popup flash in those conditions, I was able to trigger the SB800 at about 80 feet.
In real life, in some shots in difficult conditions for optical triggers (bright daylight) the paneled popup didn't trigger the sb800 when it was off to the side. However I was able to trigger the sb800 if I moved the sb800 closer to the camera axis.
I wish to emphasize that IMO the reduced range is a small disadvantage compared to the benefit of virtually eliminating blinks.
WHAT ABOUT NON-NIKON CAMERAS?
I haven't tried it but in theory the panel should also work for other cameras. Issues:
1. Is the shoe mount the standard size shoe mount? (For example, Sony cameras use a different shoe mount.) Note: The panel has a pin but it is spring-loaded so I think it will just not drop down if there's no corresponding hole. Please note the panel might not sit securely.
2. Do the slave flashes sense infrared light? If they only sense visible light, they won't be triggered with the panel.
3. Will the popup flash physically fit behind the panel? The panel sits about 2 inches forward from and 2 inches above the shoe mount.
9/6/12 Update: Canon and Sony shooters can try the Aokatec IR panel (see below under alternatives).
1. Exposed negative film
Get undeveloped 35mm film, overexpose it, have it developed, cut it to size and cover the flash. I haven't tried this myself. I've read that the film may heat up and emit a burnt odor.
Nikon also produced a similar flash panel called the SG-2. The difference is that the SG-2 panel is made of white plastic. But I don't know if the white plastic completely blocks visible light.
3. Aokatec IR panel.
There is now a third party product that is similar to the SG-3IR, produced by a company called Aokatec. The most significant difference for Canon users is that the Aokatec foot has a recessed area so that it won't push down the TTL pins on a Canon hotshoe. Aokatec also produces a version of the IR panel for Sony/Minolta cameras with the Sony/Minolta hotshoe. For a more detailed review of the Aokatec panel go here.
Pros: blocks the visible light effectively, greatly reducing the chance of subject blinking. Can be flipped up if you need to let the popup contribute light. Reasonable cost for the quality.
Cons: a little hard to remove. When attached, camera may be hard to store in the bag. Triggering range reduced. Not easy to find.
Rating: 4/5. Strongly recommended for anyone who uses CLS Advanced Wireless Lighting, or uses the popup flash as an optical trigger for infrared-sensitive slaves. Would be 5/5 if the panel were easier to remove (e.g. with the same kind of lock as the sb800) and the camera could be stored in a bag with the panel attached and was completely transparent to IR.