I am happy to report here that after doing more testing of FRSP + flash (for macro focus stacking purposes) I got much better timings. Specifically, I managed to reduce the camera
exposure from 0.8s down to
0.25s, with the same level of reliability (only one black frame out of 284 shots - similar to my previous best case scenario with 0.8s exposure). I actually did a few changes compared to my previous setup, not sure which of them was the main culprit for much better timings. In particular, I switched to MLV from DNG; I put to "Auto" long exposure noise reduction in my camera (it was "Always on"); and I significantly extended the shutter half-press time (from ~1s to ~2s). I am still using the same 1 shot every 6s frame rate. (Will try to reduce it in future tests.) By the way, I didn't realize that the timing of when you depress half-press also matters (before I assumed only the timing of engaging the half-press matters).
The new setup has the following sequence of events (camera: 50D; exposure: 1/4s; MLV format; Lexar 1000x card):
1) 0.0s: the half-press is enabled
2) 1.1s: external flash is triggered
3) 2.0s: the half-press is disengaged
4) 6.0s: going back to step (1)
A small caveat: the first shot is always black with the above settings, if you start from the Live View screen. There is a simple workaround for that: before starting focus stacking, briefly press the camera's half-shutter: the camera will take one shot (dark; one can use it later to fix hot pixels in your stack) and will exit LV (black screen). Now if you initiate focus stacking with the above timings, all the frames - including the first one - will be flash exposed.
Second important finding: this morning I made a number of dark frame shots at different exposures (from 1/160s to 0.8s), using both regular shutter and FRSP. When I processed them in the same fashion (using command line dcraw utility: dcraw -T -6 -W -b 500 file_name ; this command increases the brightness of dark shots by a factor of 500x, to make noise much more obvious), I didn't see a significant difference in the noise levels of all these shots, made both with shutter and FRSP.
Specifically, I wanted to compare the situation when I am doing focus stacking using shutter with 1/160s + external flash versus FRSP at 1/4s + flash. Be the judge: here are the two corresponding dark frames, brightened by the same factor (500x), and the noise seems to be pretty much at the same level in both cases:
Noise test: FRSP at 1/4s by
First Last, on Flickr
Noise test: shutter at 1/160s by
First Last, on Flickr
This alleviates the biggest concern I had with using FRSP + flash for focus stacking - the noise level (because of the longer exposure).
So now, with significantly shorter exposures (1/4s - meaning your studio doesn't have to be very dark) and the quality of the shots being the same or even better (same noise levels; mirror/shutter caused vibrations are eliminated), FRSP seems to be a great tool for macro focus stacking with an external flash - if you have the proper gadget (like my
Fast Stacker) to trigger both camera and flash with specified time intervals, ans also to move the camera between shots. I will be releasing very soon the next public version of my rail's software (v1.14) which adds the FRSP support for Canon cameras.