thanks for the info guys, including the very exhaustive and informative manual written by sammy888 on "painting with light" with flash strobes. lovely picture you have there streetshooter!
ok..let me clarify the purpose of asking such a question. let me simulate a shooting scenario, in order to put the question into context.
let's say i have a tall sculpture, over 1.8 metre tall, and one flash strobe. in order to capture the art piece in its entirety, i would probably need to light it from several different angles (side, back, top, bottom etc.) to bring out its form, shape and texture. obviously 1 flash exposure won't do the trick, and setting the camera on long exposure and 'paint' the sculpture with flashes from the strobe as sammy888 described presents a few problems for me.
1) during the entire exposure, any movement within the frame would be captured. hence, i can't move around the subject lighting it from different angles, unless i move really really fast.
2) painting it with light this way is unpredictable as i won't be able to see the effects beforehand...how the light falls, any stray shadows, unwanted flare, strength of flash etc.
it will be good if i want to do an experimental picture with unpredictable results. in this hypothetical scenario however, i would like WYSIWYG and a carefully choreographed lighting situation.
with a traditional 35mm film camera, careful calculations can be made before each exposure on the same frame. set up lighting -> open shutter, flash -> close shutter -> repeat step 1 as needed.
with a DSLR, the benefits are immediately obvious. the effects of each flash exposure can be reviewed and changes made accordingly. i can appear in the picture holding the flash, but i can be easily erased out later, as described by Streetshooter.
however, this is the part where i got stumped. how i do combine the several images i have into one coherent picture? certain light rays will overlap each other, in essence, naturally adding on exposure to the previous (aka 'brighter'). simply adjusting the opacity of layers on top of another won't achieve that effect, nor erasing away layers. i know some software that comes with digital backs can achieve that automatically, but obviously i don't have access to that.
i am sure photoshop can achieve that same effect of "building up" exposures, albeit a bit more manually. and i'm quite confident it's a matter of adjusting layer modes etc...but i haven't figure that out yet. :dunno: