I use a Sony NEX-5R, and my single biggest problem with the camera is that it often mis-focuses under low light. I shoot a lot of low-light photos, and I work around it by using manual focus.
I have been told that cameras with larger sensors are more likely to mis-focus under low light, because of the physics / optics. Is that true?
I notice that my iPhone almost never mis-focuses no matter how dark, while my NEX does so often enough for it to be a problem.
And reviews say that even Sony's and Fujifilm's top-of-the-line APS-C cameras (A6000 and X-T1, respectively) don't focus as well as micro-four-thirds cameras. For example:
[...] As the light levels became lower the A6000 (like the XT1) slowed down and in dim conditions often struggled to lock-on, whereas the latest Olympus and Panasonic models continued to autofocus successfully even in very low light. For comparison, Sony quotes the A6000 AF works down to 0 EV whereas the latest Panasonics will still operate at -4 EV [...]
So, is it correct to say that larger sensors mis-focus more often under low light?
If so, the conclusion for me would be that full-frame would not be an upgrade over the NEX, since it does not address my biggest problem with the NEX, and is likely to make it worse. Yes, it would work better at higher ISO, but it would focus worse, which is one step forward and one step back -- I don't want that tradeoff.
Addendum: I'm not worried about the speed of focus -- I can certainly wait a second for it to focus. What I care about is what fraction of photos end up mis-focused.