"Strange AF focus point choices" Tue 29-May-12 11:23 PM by TBF
I recently shot several score photos of a friend's child in my back yard in good afternoon backlighting with my 5100 and a 28-300. Most shots were at base ISO, 50-200mm with the aperture wide open for the focal length. I was reasonably happy with the results, but thought that the focus could have been sharper. When I looked at NX2 Focal Point choices (All at taken at AF-A and Auto) I was rather shocked to see that the face was almost never chosen as a focal point no matter where in the frame it was located. From reading K R and T H I had come to believe the "truism" that Nikon AF, with its color sensitive algorithm would seek out and focus on the face. Not so in this case. Can anyone out there explain this? Thanks in advance: the community on these forums is a marvelous resource for photographers. Tom Forker TBF UPDATE: Screen Caps don't seem to work so I photographed the monitor and posted it to my gallery UPDATE; Just now it only seems to come up under Focusing Issues in the Gallery Search
#1. "RE: Strange AF focus point choices" In response to Reply # 0
AF-A is not dependable if you want a specific object in focus. I don't know what the algorithm does or is supposed to do. It seems to me that it picks the most "prominent" thing in the scene, whatever that is.
But anytime I want a face in focus, I use single point focus & exposure. Then move the focus point as needed to focus exactly on the eyes. Yes, this can be difficult with a child. But it will get you a better % of properly focused and exposed photos.
working on it in Middle TN Nikon D3100
35 mm 1.8 Nikkor 18-55 mm Nikkor VR 55-200 mm Nikkor VR 55-300 mm Nikkor VR 150-500 mm Sigma OS Feisol CT3471 & Markins M20 ballhead
Leonard62 Hatboro, Pa, US Nikonian since 15th Mar 2009
Wed 30-May-12 08:37 PM
#2. "RE: Strange AF focus point choices" In response to Reply # 1
Good point, Diane. That's the way I would do it.
But as an experiment, because of this thread, I tried it. When using the viewfinder with AF-A and Auto, facial recognition worked maybe once in 5 or 6 shots. Otherwise the focus point was all over the place and in multiple places.
It was better in LiveView as when you acquired facial recognition focus the focus square would show on the screen and on the face. It's pretty slow though and not designed to chase little kids around. This is the first time I used face recognition and while I find it's easy to set up it's not very reliable. Diane has the right idea.
#3. "RE: Strange AF focus point choices" In response to Reply # 2
Thanks to both of you for your responses. The photos were, in general, pretty sharp given the relatively wide open aperture and the odd choice of focus points. Because these were not strictly "portraits" but rather "child playing on lawn with dog", I wrongly assumed that the focus system would at least choose the face as one of the focus points. And given what I have read about the Nikon AF system I find it quite remarkable That the eye is never chosen as a focus point and that in only 2 out of 40 "keepers" is a part of the face within one of the focus points. Another observation is that in about 15% of the shots NX2 shows NO focus points: not sure whether this is absence of camera data or NX2 failure. I have used the live view system with good success on stationary subjects but with moving targets I much prefer the viewfinder. I think I have read that some of the more expensive FX cameras have rather more visible focus points that light as the camera follows a chosen subject. Next similar opportunity I will be using single point AF as I usually do for wildlife. Tom