Initial Sigma 18-35/1.8 AF adjustments

Just thought I’d share what my AF calibration settings are, at least initially, for the Sigma 18-35/1.8 on a Nikon D5200.  And some comments on the process.

As you can see, on my particular D5200 it has an overall tendency to back-focus (negative adjustment values mean “the subject was closer than AF thought”).

It took a good two or three hours to arrive at these settings, using a variety of home-made focus targets.  Calibrating for infinity is particularly difficult, both because it’s difficult to find a suitable target that is that big, and because the depth of field is very high at great distances.  On the upside, the latter at least means that small calibration errors don’t really matter.

It’s also annoying how huge the jump is from 0.5 metres to infinity, which is basically your entire practical working range, and it does feel like the necessary adjustments for true infinity are substantially different to those for two metres.

I also discovered that there is substantial shot to shot variation in AF accuracy.  This could be the same problem that was seen in reviews of the Canon version of this lens, or for all I know it could just be normal variation for this kind of lens (i.e. large aperture).  I do have a Nikon 50/1.8G but I’ve never tested it in the same way (because I’ve never really had any issues in practice).

In fact, I didn’t actually intend to run this calibration to begin with it all.  It’s only because I discovered that there was a strong back-focus in general, while just playing with it, that I realised I’d have to do something about it.  That -8 makes the difference, in a typical two metre portrait distance, between someone’s eyes being in focus and the back of their head (and 35mm @ f1.8 means that’s a big difference).

Relative to that, the random shot-to-shot focus variation is about four units.  So even fully adjusted, a good third or more of the shots have the ears in focus instead of the eyes.

In all my past experiments with focusing I’ve found that phase detection (i.e. viewfinder use) is actually more accurate more consistently than contrast detection (live view).  My first impression with this new lens is that that cannot possibly be true, when using this lens.  I guess we’ll see.