AF finetuning - watch the light source!

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.

New firmware

I have updated the firmware for my Nikon D4 camera.

Nikon promises "Increased focus accuracy is achieved when a lens and teleconverter with a combined maximum aperture of f/8 are used."  and who wouldn't want that?

Before the update, I could not adjust the rear focusing with my 200-400mm lens in combination with the TC-20EIII tele converter. Now it seems I can! But I am getting inconsistent results using the FocusTune software and the LensAlign. I suspect it has to do with misalignment of the test shots. I am awaiting my Manfrotto Super Clamp (under way from Greenland). With that and a telescopic support I will stabilize the camera and lens using two attachments points. I will then redo the tests.

Someone reported on improved focus with D4 in combination with large aperture lenses after the firmware upgrade. I will still have to check on that.