More D800 autofocus observations

After a couple more days of testing, I’ve got more observations on the D800’s autofocus system:

1. I think we’re reaching the limits of accuracy for CAM3500FX, and in fact, any phase detect based AF system. There are just too many parts that have to be precisely perpendicular and in exact alignment to achieve focus accuracy – the AF sub mirror assembly, the AF sensor itself, and the main imaging sensor. If any of these is out of plane by a few microns, then you’re going to see some softness. We’re now getting enough resolution that the planarity of the lens mount relative to the sensor becomes an issue – to say nothing of perfect alignment of optical elements. I believe there was an article posted a while back on the Luminous Landscape about shimming a sensor and how much resolution improved by both on-center and especially in the corners of the frame.

2. Future AF systems will have to be hybrid – i.e. use some form of contrast detect or phase detect embedded into the imaging sensor in order to work around these limitations. It doesn’t however solve the problem of mount planarity or lens element alignment.

3. There are some things you can do as a photographer to counter these limitations, chief of which is use live view for critical focusing, or stop down – or better yet, both. Live view eliminates problems of AF sensor/ sub mirror alignment. Stopping down covers slight sensor misalignment with depth of field.

4. AF fine tune is an absolute must to get the most out of the AF system.

5. Bad news for manual focus fans. I did my mirror alignment and calibration this morning – it was almost perfect from factory, which is a first; however, my joy died after removing the focusing screen. The focusing screen in the D800 is a different size to anything Nikon has yet produced. Worse still, it’s the largest one I’ve ever seen, so you can’t even cut something down to fit – it’ll just drop out. This is a real shame; I can only hope a third party produces replacement screens for MF aficionados.

6. Finally, lenses you thought were fantastic on the previous 12MP FX cameras may now only be mediocre or average on the D800 – you have been warned. MT