I have a E-M5 and GX7 together with the A7r, which I have been shooting with the FE35 extensively indoors, most recently in a dark event with people moving around. In daylight, I would classify the A7r's AF as excellent and a key feature. It's a tad slower than my M43 twins, but still excellent. It focuses quickly and accurately with street type scenes on people and I seldom miss a focus even at F2.8.
Indoors its a different matter. Sometimes the A7r misses focus and when it does, it tends to focus on an object in the immediate background. For example, I'm trying to shoot some flowers, but it focuses instead on the panel behind the flowers despite having the flowers being mainly in the foreground. With natural indoor light, its usually OK, but it seems to get confused with certain incandescent or LED lighting. Also if you look at 100%, the 36MP sensor tends to amplify any slight mis-focus, so you have really have to nail it. Print size and on the screen/HP slight mis-focus looks fine but not on a monitor at 100%. However, when you nail the focus on the A7r/FE35, the image really pops and looks 3D, something lesser lenses and sensors have difficulty achieving. If you look at many of the shots on the image thread,you can see the effect, but do notice too that many shots posted are mis-focused.
The M43 cameras focus much better in low light. It's either that both Panny and Olympus got it right or there is less computation required for a smaller sensor. Reviewers report that the A7 focus is faster than the Nikon D800 live view focus, so CDAF on a high end DSLR is slow too. It may be something that only time and faster processors, which hopefully don't eat batteries, will solve.
On a side note, the GX7 focuses much better than the E-M5 in very low light. Most people haven't noticed that the GX7 focuses down to -4EV whereas two OMDs focus only to 0EV. I'm amazed that the GX7 can focus fairly quickly without a focus assist light in almost complete darkness.
The solution therefore on the A7r is to manual focus in very low light. It's actually quite easy with FE lenses. All you do is hold down the AF/MF button, and turn the focus ring. You pop into magnified MF mode (with peaking) and it's a snap for me to get the picture in perfect focus. However, I've had a lot of practice with film SLRs AND I still have to hold very steady because of no IBIS. The A7r requires a lot more effort to get a nice sharp shot, but when you do it, the image quality far far exceeds the smaller sensors. So practice, practice, practice....
