sk66 wrote:
kimballistic wrote
it seems as if there's a dramatic reduction in usable focus points as the aperture gets smaller than f/5.6. If you are truly limited to just the central 25 cross-type points and a single row of standard focus points, I assume tracking or other advanced AF options are out of the question. Anyone care to shed some light on this? Thanks! It doesn't quite work like the books/charts indicate. There's a transition between apertures where the AF points are becoming masked/vignetted, but they are not completely blocked. And f/6.3 is only 1/3 stop beyond f/5.6... I expect there is pretty much full f/5.6 point availability but the 5.6 points are probably not quite as effective as they should be.
The problem is that there is almost no way to know for a fact. And there are so many variables that subjective opinions/impressions are also highly variable/questionable.
I did some tests with it on my D850 yesterday while out looking for Harriers/SEO's... I put the AF point selection to auto and placed high contrast objects at various locations w/in the scene. Below ~200-300mm zoom the camera would happily/quickly/accurately use any/all of the 153 focus points. Beyond ~300mm the accuracy/speed began to drop off. And by 600mm it would not use/select points beyond the central ~72 AF point area at all, other than the odd momentary wander. Trying to force it to focus using AF points outside the central area was abysmal, resulting in nothing but hunting and failed focus.
I'm not saying it absolutely can't use the outer AF points at 600mm... in optimal (studio type) test conditions it might have some success.
But in the field I would focus on central compositions. IMO that's not much of an issue because most wildlife/sports/action is central AF/composition. But it's going to struggle more with small/fast/hard to track subjects. It's also worth keeping this in mind because the Nikon AF system will allow you to select any of the AF points; whether they are usable/functional or not.