I ran a few 'quick-and-dirty' test shots of an ISO 12233-type chart (I have the same type of charts as Bryan at TDP uses for his tests) over the weekend. I didn't spend a lot of time on the setup, just natural light from the windows - so, while the light was essentially constant within each set (6-9 shots taken over less than 2 mintues), between each set of comparisons there were time lags (kid-related) so the light was changing - the afternoon got progressively darker, so ISO values had to be increased - like I said, quick and dirty.
Shot RAW, processed in DPP with Neutral picture style, sharpness set to zero and NR off. Each set of shots was taken without refocusing, and three shots per setup were captured, with the sharpest shown for the crops. For each comparison, a set of crops from the center of the ISO 12233 chart is shown, followed by a set from the upper right corner.
This set of shots was with the 100mm f/2.8L
Macro IS, shot handheld at 1/25 s, f/5.6 and ISO 800, and addresses the need to let the IS system start working.
- Top panels: IS on, half-pressed the shutter and allowed ~2 seconds for the IS system to operate before fully pressing the shutter.
- Middle panels: IS on, fully pressed the shutter in one smooth motion, no delay for IS 'warm-up'
- Bottom panels: IS off
It's very clear that it's important to let the IS system operate for a brief period before the shot is taken.