I was looking into the new Nikon D600, which looks like a pretty compelling camera. I however noticed that its 39 AF points though quite a lot are all bunched up in the middle. I am used to having AF points more spaced out, usually because I tend to use the Rule of Thirds quite a bit in my compositions. I haven't had the chance to see the camera out in person yet.
So, focusing on that one aspect only can someone with experience advice me how much of a nuisance that would be?
Yes.
The phase-detect auto focus is based on comparing light rays reaching the focus sensor from two facing sides. The smaller the maximum aperture, the higher the chance that light reaches a peripheral focus sensor only from one side. Thus, dead center of image is the sweet spot for an auto-focusing sensor to be as it will receive enough light from two sides in the biggest range of apertures. Finally (somewhere around f/6.3...f/9), though, the directions will be so subtly different that focus cannot be determined even in the center.
To conclude, being bunched in the center is good because more AF sensors will work with a wider range of max apertures. For people that compare cameras by numbers (x AF sensors work at aperture f/y), and/or plan to use slow lenses (or fast lenses with tele-converters), this looks good. You'll also appreciate the higher sensitivity and smaller gaps between focus points when tracking fast-moving objects; in order to avoid accidentally cropping them, it's a good idea to keep them near the center of frame anyway.
But it also means that phase-detect AF by points further away is not possible, so you are forced to choose a workaround, all of which have some kind of downside:
With a slow lens or shooting stopped down, the second option (recomposing and ignoring) is usually pretty much okay. For moving subjects, you'll have some extra focusing inaccuracy caused by subject moving away from metered location while you're recomposing. Laying out cash for a full-frame camera, you might be intending to use some fast lenses to get that oh so thin depth of field, and that's where you would notice the focus being slightly off after recomposing; so in this case, having the AF sensors flocked up in the middle is bad.
Note that designing an AF system with widely spread focus sensors has been rather easy for APS-C DSLRs, since the all crop DSLRs so far use flange distance and rear element diameter dictated by lens mounts originally designed for a larger format (the full frame). Full-frame cameras don't have such extra room to spare, and getting light from the edge of lens is more complicated.
I think this could be one of those, it comes down to personal taste answers. Myself, I only use 1 central point of focus. This is quite an old school way of focusing and framing subjects but I like to have full control on what I'm focusing on. It's a different way of shooting. (but to iterate, I half press to focus on my subject then while half pressed I frame the shot, 90% of the time this is my style. Of course there are times where you need to shoot fast. In this situation i would switch to all, but rarely. )
Having this many focus points bunched into the middle can add more accuracy when shooting a moving subject, such as runners, Formala 1 cars. It can also provide absolute accuracy for when photographing portraits, the eyes, nose and mouth could be placed into focus with great accuracy.
So that said, with photography there's a right tool for every job. Having focus points so close together can be highly accurate, but if it doesn't suit your style or the subject matter, it's worth looking at alternatives. Even some Hasselblad's use central point focusing, but then it follows the subject when framing. Clever but if Hasselblad insist on using it, there's probably good reason for it.
