I decided to do some research into flash photography today. 'Learned quite a few amazing things. Flash technology took a leap when in 1988 Nikon introduced TTL (through the lens) capability in its SB23 and Sb24 Speedlights. Gone are the days when one has to compute the right settings based on distance, aperture, and film ASA. Every digicam now uses TTL in its flash, which makes flash photography a no-brainer, compared to the early days! The only thing grandma has to figure out is how to pop up the flash. The camera does the rest.
I was delighted to learn that my D90 incorporates Nikon's latest flash technology. It has a built-in commander mode, which means I can actually control many other flash units wirelessly. I can actually set my SB600 Speedlight to flash in the TTL mode at +3 or -3 f-stops. The settings are remotely controlled on my camera itself, while the camera's built-in flash can also be independantly set with the same options as the remote flash or flashes. This gives a vast array of options for creative lighting. I am sure high end Canon cameras will have the same options, either now or later. For me, I am simply delighted with my D90.
I also gained a renewed respect for slow sync and rear curtain sync. One writer on the internet suggests that all indoor flash should be done in the slow sync mode. I am not so certain of such a sweeping statement, but I did give slow sync a try today. You won't believe the difference until you've tried it yourself! I may be wrong, but from what I've gathered, slow sync basically slows down the shutter speed regardless of your shutter speed setting. Then, while keeping true to the exposure requirements of your aperture and shutter speed, it flashes in rear curtain mode. This allows you to capture as much ambient light as possible and still enjoy the aid of flash to finish the job of getting the right exposure for the shot. This is slightly different from pure rear curtain sync. My D90 has both slow sync and rear curtain sync (of course, in addition to other modes). For some samples of what it can do, see: http://digital-photography-school.com/13-great-slow-sync-flash-images .