04-05-2017, 02:42 PM
Quote: So there you have it, Nikon went from a very useful dynamic mode on the D750/D810 which I use successfully to a crippled version which doesn't have real use.
Anything written in the user manual is vague at best and could well be just erroneous at worst.
I've been studying this a bit in the past month or two. I am normally a single point AF shooter and never got accustomed to dynamic area AF in the past. When I compared the D7100's 9-pt dynamic with the D810's dynamic and group area AF, using 200/2 in shooting figure skating, I got the best focus keeper rate by far with the D810's group area AF. The 9-point dynamic would often focus on the background when the skater was far away and I wasn't able to keep the main point on the face steadily enough. At the time I had no clear idea of what the intended distinction between the modes was, but I became a fan of group area AF mode in situations where the subject's face was too small and moving in a fast and nonlinear trajectory to keep focus using a single point. However, one drawback of group area AF mode is that the entire group has to be within the AF sensor array whereas dynamic areas can be "clipped" by positioning the main point far off center to the edge of the array. This means in practice when shooting vertical subjects (such as people) with the camera in horizontal orientation, the main point can be placed on the face nearer the top of the frame without having to crop off as much of the image frame in post processing as with dynamic area AF. However, when I tried dynamic area AF in the D5, it let to further surprises in the form of shots being focused on background (I had the same experience with the D7100's 9-pt dynamic but to a lesser degree). I looked up the topic online and found several discussions on the topic.
Initially it seemed that there was little difference between the D5's dynamic area modes and using a single point, which can be easily verified using some static subjects. However, when I tried to simulate a dynamic situation by walking towards a static subject with continuous focus on, there seemed to be a difference with dynamic area mode the camera would hold on the focus to the subject while the main point jittered around it in an erratic fashion and single point did not. I do think there is a difference between the modes but it should not be tested with static subjects, and the time scale of the fluctuation between the relative position of the main point and the subject is quite short, i.e. it basically corrects for rather rapid fluctuations and if you point the main point towards a different subject and stay there it almost immediately refocuses on that new subject. I then tried to use the D5's dynamic area AF for synchronized skaters and the D810's dynamic area AF for some walking pedestrians. The new system allows dynamic area focusing on a subject which is in a crowd or group of people, but requires great precision from the user. It does provide some assistance to the photographer not being able to hold the selected point on the subject but this happens on a rather fast time scale. In the D810, I could not easily force the system to move from a closer to a more distant subject in the crowd; the system would take a while to think and basically be very reluctant to go for the more distant subject. This is a scenario where the new system is better. But it doesn't provide the same level of assistance, it can be only used successfully if the user is acutely aware that the main point must be held onto the subject for the duration of the tracking, and when reviewing shots one can see that it does in fact hold onto the main subject even for brief periods where the main point is not on the subject but on the background, provided that there is not a lot of contrasty detail in the background. If there is contrasty detail in the background, that background quickly becomes the subject, then, in the new system but less so with the old. Still, compared to group area AF, in the D810, the "old" dynamic was still more eager to focus on the background than group area AF which seems to solidly find the closest subject within the group and does a very good job at it.
So it would seem that the dynamic area AF in Multi-CAM 3500 family cameras has some tendency for closest subject focus (or that it doesn't quickly switch to focus on background if there is clear foreground detail) but not as much as the group area AF, whereas the Multi-CAM 20k family cameras give no priority to closest subjects in dynamic area AF but they do in group area AF, which seems to be similar across the newer and the older AF module (originally there was no group area AF in the first Multi-CAM 3500 module cameras, but this was added to some models in 2014).
I believe that what Nikon did was intentional and the NPS tips for D5 AF (there is similar documentation for the D500) explain some use scenarios for the different modes in sports photography:
For single figure skaters, there is a box which explains a recommended usage of reprogramming the lens button to activate group area AF: "You can then press a focus function button to select group-area AF when framing shots of distant subjects with the camera in wide orientation, and release the button to switch to 25- or 72-point dynamic-area AF when framing shots of nearby subjects in tall orientation." So there is the idea that a distant subject is easier to focus on using group area AF and nearby, one should switch to 25- or 72-point dynamic area. I find this to be sound advice although I have a tendency to just stick to group area AF most of the time anyway. The dynamic area AF would permit a more off-centered subject which can be useful when the subject is filling the frame, whereas the group area AF works great for faraway subjects which do not fill the frame and the whole subject is within the depth of field; its tendency for closest-subject-priority helps avoid a situation where the dynamic area AF would focus on the background. This kind of usage would also give greater control of the focus for a near subject.
For speed scating where there is a group of skaters close to each other, Nikon's AF tips page explains "Group-area AF is a good choice when your subject is in the lead, but 9-point dynamic-area AF may be the better option when your subject is back in the pack, as the camera may otherwise tend to focus on the leader." So the dynamic area AF allows focusing on a middle of the pack subject where group area AF would not. In the old dynamic area AF behavior I think there would be a greater tendency to stay on the closest subject than in the new implementation, based on my testing. So Nikon have tweaked the system to allow for greater control, but the cost is that the user may need to be more acutely aware of how the system is working and have greater skill in holding the main selected point squarely on the intended subject during the tracking.
I do not believe there is any error in the implementation of at least 25-point dynamic, it seems to behave as described. I have not used 9-, 72, or 153-point dynamic much yet, as the 25-point seems to be a good area for my needs, along with the group area AF which often is my preference.
I think what Nikon could do perhaps is offer greater degrees of customisation of the dynamic area AF modes and provide some control over closest-subject-priority within the dynamic area AF framework. Also, they could allow a setting which would be like 3D tracking or Auto Area AF but within a restricted area. With such additional options I think a configuration which would work for everyone could be found.
For me the key is that now I understand (sort of) the way dynamic area AF works and what kind of a situation it is intended for, and I believe I can better make use of it correctly. I just think of "skater in the middle of a pack" and it brings up "use dynamic area AF". "Faraway subject" => "Group Area AF". Slow or highly predictable / linear movement => use single point AF for greatest control. Unpredictable, fast, complex action with desired result of faces preferably in focus without having time to move focus point manually => Auto-area AF.
So far 3D tracking hasn't impressed me but I know some users really like it. Currently I've restricted my AF area mode selector to rotate between single point, group area, 25-point dynamic, and auto area AF as those have worked best for me so far, and having a limited selection gives me faster access to a particular mode, though I don't tend to switch between modes often.