Originally posted by pschlute I don't think you can use flash with Pixel shift.
I've done it once as an experiment and it worked (surprisingly).
As you say, there is the issue of recycle rate, but you can either use low flash power, multiple speedlights (each at lowered power, but overall adding up to useful levels), or a dark environment and long shutter speeds.
In my experiment, I took the latter approach and fired the speedlight manually whenever the camera proceeded to taking the next image. As a matter of fact, I'm pretty sure that there will always be a limit on the shutter speed as I don't think the camera issues a flash signal for each PS image taken, so the shutter speed will have to be slow enough for you to reliably get the timing right. EDIT: It may be possible to use four speedlights, each equipped with a manually set delay so that it fires at the right time in the sequence. The Cactus RF60X and/or the Cactus V6/V6II support such delays.
I expected issues with slight brightness variations due to the speedlight not being able to perfectly output the same amount of light each time, but most of the test images looked fine.