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If you buy the studio light it will be like standing on the dock with one foot holding the studio light and having the other foot in the boat with the 430ex. You'd have half of an ideal studio lighting solution and half of an ideal location / candid solution.
If instead you bought a Master capable Canon flash (550ex, 580ex, etc.) you'd have an ideal location / candid solution which with the addition of an umbrella, softbox, or just creative bouncing will create results similar to what you'll get with the AB400 key light and 430ex as fill.
Here's an unplanned comparison I did a few years ago I have on-line already. I have both a pair of 580ex speedlights and a set of four AB800. House guests arrived after a long day of sightseeing and while showing them the house I grabbed the camera with speedlights and grabbed the shot of the boy. A while later I set up the studio lights to shoot the girls and group shots. I didn't reshoot the boy because his speedlight shot looked OK:
I use this as an example because I was surprised by the results and learned from them. Overall the lighting on the girl seems a bit "harder" to me despite the use of much larger modifiers on the lights, more fill and better placement relative to the face (reaching under the chin better). What is creating that impression? It's due to her skin being oily which created more specular highlights than the smaller sources did on the boy's face. He had washed his face on arriving, the girl wearing make-up didn't.
Neither are great shots because they were 30 sec. "snap" shots without a great deal of preparation but they illustrate the results you'd see with the option you are thinking about or what I suggest won't be very different and in some cases, as here, variables you can't control such as glare off an oily face negates the benefit of a larger key light modifier.
The advantage of investing in the second speedlight is you will probably find more ways to use it than if you have static, foot on the dock, studio set up in the basement or living room. I shoot mostly with the 580ex flashes because it's more convenient. See: http://photo.nova.org/CanonPracticalUsage/ I break out the studio lights when a friend needs a formal headshot or family photo. The in-house model hides when she sees the studio lights come out.
If you do opt for the studio lights buy two and invest an extra $55 each and get the AB800 ($280 vs $225). The two lights make it logistically easier to shoot for both you and the subject because you simply put the fill centered at chin level and dial in the desired mood in the lighting via the ratio: normal, darker and "harder", lighter and "softer" as needed vs. futzing around positioning a reflector to catch the light, get the fill on the front of the face where it is needed and keep it out of the frame.
The AB800s will allow you to operate in the middle of their power band most of the time and allow a reserve for situations where you need more. By comparison with the AB400 you may find yourself shooting at max power and wishing you had more.
What will likely happen if you do opt for two lights and learn to use the is that it will open your eyes to the possibilities of what can be done in your "run and gun" candid shooting with a pair of speelights and buy that second speedlight.
Edited on Jan 15, 2013 at 12:37 AM · View previous versions