It’s a cool idea—using your ordinary shoe-mount electronic flash (or two, or three, or 8 of them) in conjunction with one still mounted on the camera (or the built in flash on a D200 or some other bodies) to make a wireless flash network that delivers precise exposure metered by the camera TTL.
It even mostly works (I think iTTL in general underexposes, and CLS works best for me with fairly strange ratios, and they vary a lot with the surroundings, but that’s life).
CLS works by sending information between the master and the slaves using modulated flash bursts. Each “group” (you can assign each flash to one of 4 groups) is triggered separately to pre-flash for metering, and then later informed of what power level it should deliver for the real flash, and then finally told it’s time for the real flash. I find this sequence highly visible, including through the viewfinder, and it takes a long time.
And those pre-flashes are the problem. They delay when the exposure is made by enough to matter a lot. It’s fairly risky for unposed pictures, people are likely to change their expression while the flash is blazing away. It can be a problem if you have blinkers who can’t control themselves even for posed pictures.
And, based on my experience this afternoon, it’s disastrous for cats. I didn’t get one single CLS picture of either cat with her eyes open. When I switched to manual flash control and MU-4 mode, with no pre-flashes, I started getting every single photo with the eyes open.
So my conclusion is pretty clear—do not try to use CLS for cat pictures, and be cautious with other animals until I have more experience with specific species and individuals.
(Yes, the ones with the eyes open have been adjusted and cropped far more carefully than the ones with the eyes closed.)
It’s a pity; this sort of situation is exactly when the automation might be worth it. Quick setup is of the essence, animals don’t generally have the patience of humans (and humans are pretty impatient when you get right down to it). But at least for these cats, it’s entirely hopeless.
Added 2007-09-01: It has been pointed out to me (list membership required to read the archive linked to, I believe) that the FV function will perform the pre-flashes and then wait for me to hit the shutter release for the actual flash, which means I can wait until the cats open their eyes again. I haven’t tried this yet (I’m fairly tied to that button being non-chipped lens selection right now), but it sounds very likely to be useful, and I need to try it soon.