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My own experience suggests the benefit of not relying on wireless TTL flash exposure.
Reason: There is a perceptible flash delay during wireless communications at the moment just prior to actual exposure. Use a dedicated synch cord to reduce this source of lag between shutter depression and actual exposure. Yes, we are talking about a small fraction of a second, but it can make a big difference in your ratio of keepers. For my Nikon-based outfit, I use an Sc-17, which is an older cord, but it offers perfect compatibility with the D-series of SLRs.
Here's the rig I typically shoot with:
Body's a D70 or D200, and the lens alternates between a Sigma 180 (as seen here) and a Sigma 105. I usually mate whichever lens is mounted to a Kenko extension tube, either the 36mm or the 36 and 20. The bracket is a hightly recommended "CB Jr". CB is short for Custom Brackets, and Jr as in Junior. Well-built and great for macro stuff; it provides adjustable height for the flash, can rotate the flash bracket 180 degreees for vertical composition, and is designed so that the operator can access lens and body while shooting. it was purchased from B&H. The synch cord is the older SC-17, which provides full dedication to the D70 and is cheaper than the SC-28. You can find the SC-17 on eBay. The silver thing directly under the flash unit is an old lightweight ballhead, perfect for setting the necessary angle for flash illumination. It's a "Bilora" brand. Actually, a very nice product, locking easily with little effort. The flash is the nearly legendary SB-800 attached to a Lumiquest Lightbox diffuser. This diffuser softens the light nicely and provides a natural soft shadow from above.
Bonus tip: If you just love the whole wireless TTL thing, but find that people shut their eyes, get a dedicated cord and abandon the whole wireless luxury. Your camera won't provide a long enough period for people to shut eyes, or at least, not as frequently.
Alternately, many DSLRs provide a means to trigger a pre-flash manually, allowing you to set your flash exposure at any time prior to the actual images. Check your camera’s custom function options.
A great way to avoid the black background is to put your flash on a lightstand (or tripod) and position the flash so that its lighting the primary subject area AND background. You'll find that the light looks much more natural when you've lit the whole area:
For this shot I positioned a single dedicated flash on a lightstand and positioned it about 45 degrees to my left and about a foot higher than my position.
The idea of a slave (or more specifically, an "optical slave") is that there's a little electronic gizmo that's hooked up to some ol' ratty flash you have laying about, and when your gee-whiz super flash fires, the secondary flash with the slave will also fire (at virtually the same time) and the two lights will illuminate your subject and background. Modern flash photography has made this a bit of flash relay a bit more problematic for a few reasons:
The main flash, likely dedicated, doesn't "know" about the secondary flash.
The main flash, if in its "TTL" auto mode, must fire a few test shots in a fraction of a second prior to the actual exposure. Poor ol' slave flash is clueless to this, and dumps its load prematurely (premature eflashulation).
These issues can be solved by shooting each flash in manual or fractional power mode, and this works very well once you've worked out the proper settings. The only downside is that you're locked into keeping each flash at a fixed distance to the subject, or you'll need to adjust the flashes' distance or output.
There are some slaves that are programmed to ignore the pre-flashes, but your TTL system will still be basing calculations on what was seen at pre-flash time, so if the slave is adding to the primary exposure (and not filling background), then you'll likely blow out some foreground elements.
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Copyright 2006 Eric Delmar