And this is what we found ...
|Required AF finetune adjustment value for accurate focus depending on the focus point for one sample D800/E.|
|Required AF finetune adjustment value for accurate focus depending on the focus point for another sample.|
In the charts above, the "floor" represents the layout of the 51 AF sensors of Nikon's Advanced MultiCam 3500FX module. We evaluated the performance of 5 out of these 51 focus points, positioned according to the yellow bars. The larger the bar, the larger the focus error without an AF finetune adjustment. The bar denotes the required AF finetune adjustment to compensate for the focus error. Measurement done at 24mm, f/2.8, tungsten light and with 1.2m subject distance.
We conclude that the issue with the accuracy of the outer AF focus points of the D800 is real and probably affects all units out there to some (varying) extent. It is said to be pronounced at ultra wide angles. However, we found it to be (just) unacceptable at 24 mm as well. We guess that every camera with the "Nikon Advanced Multi-CAM 3500 FX" auto focus module is affected, i.e., D800, D800E and D4.
We evaluated the characteristics of the sample variation of the issue and found that the cause may not be consistent across units. There may be a mixture of a tilt problem (such as caused by a misaligned AF auxiliary mirror, possibly independent of the focal length) and a parallax problem (such as caused by a decentered AF relay lens, possibly dependent on the focal length) where both components contribute with variable strength from sample to sample (our statements about possble causes are speculative, our statements about tilt and parallax errors and sample variation are not).
Therefore, we express our concern if the issue can be cured by a simple firmware patch. As far as we can see, such a patch would require at least two additional calibration parameters. We rather see many units undergo a thorough re-calibration job at Nikon.
Breaking news: According to www.nikonians.org/forums/dcboard.php?az=show_topic&forum=430&topic_id=7054&mesg_id=7054&page=#7303 , a Nikon service center has been able to calibrate the outer AF points of one unit, much in line with our findings which suggest that each camera has to be cared for on an individual basis. It remains to be seen what procedure Nikon will propose to customers in general though.
Meanwhile, we recommend to be cautious when using all but the center 15 cross type AF points for any work relying on critical focus, at least on the wider end of focal lengths.
We provide full documentation of the lab testing which my friend Dieter Lukas from panobilder.de and myself did in order to classify the D800 outer AF sensor issue. Please refer to:
- http://www.falklumo.com/lumolabs/articles/D800Focus/OuterSensors.pdf (recommended)
- http://www.falklumo.com/lumolabs/articles/D800Focus/OuterSensors.html (HTML version)
- http://www.panobilder.de/artikel/nikon-d800-af-sensor-genauigkeit (Version in deutscher Sprache / German version)
- http://www.falklumo.com/lumolabs/articles/sharpness/index.html (some background)
Update (2012, June 20):As detailed in our full document (cf. above), we used FoCal software from Reikan to determine the AF adjustment values required for each focus point (we had to use a trick to work around the FoCal limitation to only test for center AF point accuracy). We actually recommend their software for normal AF lens calibration.
Interestingly, Reikan is now producing a version of FoCal which is able to test for the consistency of all AF point. We highly recommend all D800 users to aquire and use this software (when out) in order to check for their own camera. Read more about it here:
Meanwhile, we have officially asked Nikon Germany for a press statement about the issue but have received none to date. We'll keep you posted.
Update (2012, June 24):There is an interesting blog article from hifivoice:
Enjoy the read and thanks for stopping by,
Dieter & Falk