The disconnection between micro adjust AF settings and the nature of lens design...

This is a Sigma 50mm f1:1.4 lens with a Sony A mount. It's a great lens but my camera can't reliably focus it and neither can yours. Even if you use a focus align jig and take great pains to calibrate the hell out of it. Is there something wrong with the lens? Nope, I get the same behavior from the Zeiss 50mm f1.4 for the Canon and also the manual focus Carl Zeiss 85mm 1.4 in any of the major brand mounts. So what's the deal?

It's pretty simple really. All these fast lenses have a common attribute called focus shift and the simplest explanation is that the point of correct focus shifts as you stop the lens down. The micro-adjust AF controls in all of cameras are amazingly simple and stupid. They are all made to do one calculation per lens. But if you calibrate for the wide open setting (f1.4) which is the stop you paid all that hard earned money for the lens system's focus point will shift as you stop down. The setting at f2.8 when used on my Sony a99 is three or four points different than the f1.4 stop. In theory you could test and divine a calibration setting for each f-stop but there's no way to load more than one setting into the camera for each lens.

If you were amazingly compulsive you could calibrate all the critical stops and third stops and make a chart. Then when you grab your camera to shoot you can check your f-stop, consult your data and set the correct number of every f-stop. You'd probably only need to do the stops from wide open to about f4 because at that setting depth of field masks the errors. Mirrorless cameras set focus at the shooting aperture for their contrast detection AF so they tend to be much, much more accurate. I wonder if there is a downside to the inclusion of phase detection AF points on a sensor as relates to focusing point accuracy.

Auto Focus Micro Adjustment and the Sony a77

I couldn't really adjust for this lens but that's okay, 

I know from recent experience that it's "wicked sharp."

As I work more and more with the Sony a77 I find lots of things to like about the camera and very few disappointments. One of the reasons I chose to go with the a77's as my primary shooting cameras ( in addition to the brilliant EVF and really nicely implemented video) was the Auto Focus Micro Adjustment control.  I've been pre-occupied with other camera controls in my quest to really master the camera and I left lens adjustments to last.

But recently I've been working nearly wide open with the 50mm 1.4 lens and I noticed that it would routinely back focus. This led me to jump into the menu and get busy.  I also noticed that my 70-200mm G lens (a whopping $2000) wasn't as sharp as my previous Canon and Nikon zooms so I thought I'd take a crack at that one as well.  In the end I tried every Sony lens I owned on both bodies and now, after hours of being really compulsive and fastidious, I am even happier with my little Sony system than before.

When I first accessed the control the ability to adjust was greyed out on the menu. I finally decided to push the "clear" button and a message came up telling me that 30 lenses had already been registered and that I would lose all those settings if I continued.  Since I'd been using the camera with this control deactivated anyway I decided that it would be "no skin off my nose" to go forward.  I pushed clear.  Now I could make adjustments to any of the Sony branded lenses I put on the camera and it would save up to thirty lenses of my choice.  Do I think the camera was used before me? Decidedly not. I think the camera comes that way by default.

I actually kept notes as I worked.  The 50mm needed a "minus 8" correction.  The 70-200 needed a "minus 6" correction and the 50mm 1.8 DT lens needed a -3 correction.  Most of the lenses were right on.  The little 85mm 2.8 shocked me.  I've always used it to shoot portraits and nothing with sharp lines or edges. When I blew up my test file with the LensAlign it was so sharp wide open that I was temporarily giddy.

I tested all my lenses at 2.8.  I figured most gaussian lens designs will have a bit of focus shift from wide open to 2.8 and that trying to get them just right at 1.4 was perhaps a futile endeavor.  Happily, once I adjusted the 50mm 1.4 at 2.8 I went back and checked it wide open and was happy to find the same correction needed.  At least the lens is consistent.

I tested eight lenses and I did this on two bodies and there were mild differences between bodies.  I almost messed up the test entirely as I had inadvertantly set the focus mode to local which allows the camera to choose between a little cross of squares.  When I aimed at the target I would get different readings each time and when I tested at different distances I needed different numerical values as well.  Once I realized my mistake and set the  camera to center spot AF everything fell right into place.

The 70-200mm is now very sharp wide open and wickedly sharp one stop down.  I took the time to re-test at every marked focal length and found that, once you've set the right value, it tends to be the same for all.

In addition to the stellar performance of the 85mm 2.8 I was also amazed at just how sharp the 16-50mm 2.8 lens is.  It's the sharpest wide to short tele high speed zoom I've ever played with. I can go out shooting now with a sense of assurance that I'm getting the ultimate performance for myself and my clients.  And it was a bonding experience for me and my camera.