In my effort to calibrate the autofocus after updating my camera's firmware, I noticed that my exposures were fluctuating. It seems the light source (house hold lamps with energy saving bulbs) is the culprit. Fluctuating exposure seem to influence the result of the test using Michael Tapes Design's
FocusTune as seen in this graph. It shows a fast sequence (10fps) of 54 images taken with 1/250 sec and the
exact same focus setting (I set the focus to manual focus after initial focus was acquired). I believe the clear fluctuation is caused by interference with the modulation of the light source. I redid the test with a shutter speed of 1/60 sec and saw much less variation and no apparent regularity. Slow shutter speeds are not very suitable for critically testing auto focus of long lenses or long lens and teleconverter combinations - even with sturdy support - so I will now reconsider using flash while testing autofocus indoors.
