I haven't read the guide but in the review Hogan doesn't really nail the dynamic area issue.
The algorithm is different, it's not just a question of the camera's speed. The "old" dynamic area AF algorithm would focus on the primary subject and hold focus on the subject using the auxiliary points if the primary point was placed on the background as long as even one of the auxiliary points was still on the subject. The "new" dynamic area AF will quickly focus on the background in the same situation (after a brief delay). If you have a new, closer subject move in front of the primary point, both systems will focus on the new, closer subject, but the "old" will hold onto the new closer subject (as long as a part of it remains within the dynamic area) whereas the "new" algorithm will refocus on the subject under the primary point after a short delay and will not prioritise holding on a closer subject indefinitely like the "old" algorithm did. This is not a question of speed or processing power but simply executing a different algorithm. I like the new algorithm and did not like or use the old, as the new is symmetrical: it helps you keep focus on a moving subject when the subject's position wiggles within the dynamic area as long as you make a concious effort to return the primary point on the subject as quickly as possible, and it doesn't "drift" to closer parts of a three-dimensional subject whereas the "old" algorithm had the tendency to slip focus towards the subject's closest part (instead of the eye, for example) if you even momentarily slip the primary point to such a part, and stays there (even when the primary point was held on a more distant part of the subject). However, the "old" algorithm had the advantage that it would not easily lose focus of the subject in favour of the background as long as the subject occupied at least a part of the dynamic area. In fact I did not like the old dynamic area algorithm and preferred single point and group area on the D810; at least they were perfectly logical in behavior and easy to understand. With the D5 and D850 my favorite AF area mode is 9-point dynamic area, for fast moving subjects with somewhat difficult to predict trajectory I may use D25, group-area, or auto-area AF. In very low light dynamic area seems to slow down to think at times and in such situations, group area seems to keep working for a while longer as it gets darker, and is very fast.
- Casual movement, walking, etc. - D9
- Extremely low light and/or fast movement - group-area
- Erratic movement with complex trajectory - D25, group-area or auto-area
Anyway the good thing is that there are enough options to cover most any situation.
A further point about terminology: when discussing dynamic area in the D5/D850/D500, it's best not to think of the selected point as "starting point" rather I call it the "primary point" since the (new) algorithm doesn't track the subject's movement across the frame - it focuses on the primary point if it can, and only briefly will use the auxiliary points to assist (and does not linger there). By contrast 3D tracking tracks the subject's movement across the frame, starting from the selected "initial point" which is what the photographer uses to select the subject for the camera.
