One of the key complaints many have about the D610 is the use of the 39-point focus sensor. Yes, between the reduced number of focus points and the FX frame, the area covered by the focus sensors is small, at best. Somewhat smaller than the 51-point sensor covers, and very noticeably smaller for anyone moving from DX to FX (the D7000 user moving to D610, for example, which Nikon has been promoting as an upgrade path). Note that the focus system doesn't perform any differently than before. If you've used the 39-point system in an earlier Nikon DSLR, you can expect pretty much the same performance in the D610. Actual focusing seems ever so slightly better in the D610 than it was in the D7000, all else equal (I suspect some additional smarts in the metering/focus sensor integration), plus the system will focus at the center point with f/8 lenses (and a subset between f/5.6 and f/8). So there have been some gains.
But that smaller "area" is what gets people: the focus sensors don't even hit the one-third points. The actual area seen by the D610 focus sensor is the same as is seen by the D7000 focus sensor, by the way. It's the change in angle of view (DX crop versus FX) that changes the apparent area covered. There's good news and bad news buried in the 39-point focus sensor for those of you contemplating the D610. This is the best implementation of the 39-point system so far, by a tiny bit. Many people also don't realize that the line sensors in the 39-point version are somewhat larger than those in the 51-point version, either. That has both positive and negative implications in and of itself. The positive and the negative is that it's more likely that sensor is seeing something that isn't perfectly centered on the viewfinder indicators. Sometimes that means the camera is finding the subject you didn't quite get the sensor on and that's what you wanted, sometimes it means that the camera is seeing something you don't want it to see when focusing. Remember, those viewfinder indicators are not indicative of the size and shape of the autofocus sensor.
But let me cut to the chase here: a lot of consumers just focus in the center. Good news: the nine central sensors are all cross hatched and large. Central focus is pretty darned sure and reliable. Even Dynamic Area 9-point with the center area selected is darned good. It's all the things that you can do outside those central nine sensors (3D Tracking, Auto Area, 21-point/39-point Dynamic Area, Single Point with one of the outer sensors, etc.) that would probably get you into focus situations where you might not like the performance as much. As it is, a lot of shooters are focus-and-reframers. Great, stick to the central area and learn how the geometry of reframing works and you'll be just fine.
