confused with auto focus points

Discussion in 'Nikon' started by raihan_malik|1, Sep 25, 2017.

  1. Hello Everyone.

    My question has two parts. I have been a D7000 and D7100 user for last few years yet I'm not sure if I understand the function of Auto Focus Points correctly. I have been googling and youtubing and there are numerous articles and clips that explain things pretty well BUT I couldn't find one clip or article that explains what's the benefit of using single point or 9, 21 or 51 one over the other!

    I have some decent idea about afa, afc and afs. Since i do mostly
    wedding and shoot lots of portrait and group shots, i prefer to keep it in AF S with Single focus points but can someone please explain what would be the benefit of shooting at 21 or 51 af points over the Single? I mean what's the differences and what focus points gives better sharpness for single portrait and group shots?

    Thanks in advance. Raihan.
     
  2. My simple (undoubtedly too simple) take:

    If you're shooting stationary subjects, you'll only get single point in AFS (ignoring auto area, which I would avoid). In AFA, it will go to multiple points if the camera switches out of AFS automatically, but the setting does not apply if it stays in AFS.

    In AFC, If your subject is hard to track or moving irregularly, single point focus can lose it. The further away it is, the easier it is to lose, and the less critical exact focus is, so multiple points make more sense.

    If your subject is fairly large in the frame, multiple points can allow the focus to move to the wrong part of the same subject - e.g. a bird's wing instead of its eye. So the usual advice is to keep points as few as possible.

    I've never had great luck with 3D focus.

    On lower end cameras such as the D3200, which has no setting for release priority in AFC, the camera may settle for less accurate focus in AFC than in AFS (according to the manual). No such mention is made in manuals for higher models that I know of, and I've not noticed a difference in the D7100. When using the D3200, I've not noticed a significant issue either. I usually use back button focus, and stay on AFC, using either single or 9 point dynamic area.
     
  3. For your type of photography, there is generally no benefit in leaving AS-S single focus point. You may very well end up reframing the image or refocus more often than you would be helped by using more focus points. That is why I never really had any problems with the "limited" focus sensor area coverage on the D7000 or f D610 compared to my D800. I am simply used to it.

    There is no theoretic advantage in using a specific mode in terms of what gives more sharpness. The algorithm the camera uses do decide when a subject is in focus is the same, regardless of the number of sensors involved. Granted, using more sensors that all give the same information increases the chances of an in focus photo. In practice it may well be the opposite as the camera can decide to use the focus distance information from another focus point than the one you would want. Generally speaking cameras are programed to think that your subject is the one closest to the camera.

    Single point, vs 9, 21, 39, 51 or whatever in AF-C is more a question of how large a subject you are trying to track and how easy it is to keep the selected AF sensors centered on it. For moving subjects, such as animals (or dancing wedding couples), you might consider the risk of getting the bird's wing in focus rather than its eye a better compromise than an entirely out of focus photo. For horses, I usually select 9 or 21. I like the theory behind 3D focus and for some sports, such as track and field, it has been useful.

    As Matthew Currie wrote, keep the active focus points as few as possible.
     
  4. Dynamic area AF is meant for dynamic subjects in situations where you may have some difficulty keeping the selected focus point on the subject; it allows the auxiliary points to cover for the primary point for brief moments when the primary point can't see the/a subject. I would continue to use single point for the kind of subjects that you mention.
     
  5. ShunCheung

    ShunCheung Administrator

    I agree you would like to use the fewest number of AF points to get the job done. When you are photographing a moving subject, it get easily move away from the coverage on the one, single AF point so that you want to get some AF points involved so that your subject is always covered. I typically use 21 for moving subjects, more with the D500 since that camera has more, denser AF points.

    AF tends to slow down when too many AF points are involved as there is more calculation for the camera, but if you have a subject that requires all AF points to cover it, so be it. Slower AF is still better than totally out of focus.
     
  6. Using multiple points, the camera will focus on the nearest object in the field, not necessarily the subject you have in mind. It is, in effect, a snapshot mode. If another person or object intervenes, it may determine the focus point. Under ideal circumstances, must-point focusing eliminates the need to focus and recompose. A good example of use is a flying bird or air show. I've used it at social events with the risk that an extended hand may steal focus from the face. My camera has face and eye recognition, which works even with single point focusing.
     
    Last edited: Sep 26, 2017
  7. Dynamic area AF doesn't just focus on the closest subject; group area does that (within its limits), and auto area (however if face priority is on it can focus on the face instead). Dynamic area focuses on the selected point if it can, and only uses the auxiliary points if the selected point doesn't have a sufficiently focusable subject under it. That's the theory at least. However, there are different versions of dynamic area which differ in some undocumented ways. In some cameras the dynamic area is more reluctant to focus on a subject behind the current subject even if you move the primary point to the new subject. If you point and hold the primary point on a subject that is to the front of the current subject it will eventually focus on the new subject (after the lock-on period is passed). However, in theory the camera should focus eventually on the subject under the primary point if you hold it on the new subject long enough. With at least some Multi-CAM 3500 cameras there is a greater tendency to switch to a more frontal subject than a subject to the back of the current subject if you move the primary point around (while maintaining at least some auxiliary on the current subject).

    Multi-CAM 20k dynamic area flips rather quickly to a subject either behind or in front of if you move the primary point away from the current subject. Only for a brief time does it hold focus with the auxiliary points before switching to a new subject. This is convenient when focusing on an athlete in the middle of a pack; it will not prioritise the leading athlete like group area does. In the older implementation of dynamic area if you momentarily slip the primary point on the leading althete it can hold onto it and not easily return to the middle subject. In this way the older dynamic area doesn't actually implement what is described in Nikon's sports AF tips pages and the corresponding manual. However, some bird-in-flight photographers prefer the reluctance of the older system not to focus on the background so easily. The new (Multi-CAM 20k) system requires greater care in holding the primary point on the subject but it has the advantage that it will be true to what you aim instead of slipping to the front.

    Group area on the other hand seems to always focus on the closest point within the diamond at least in AF-C operation (there is some face recognition and priority available in AF-S) and does so quickly and works very well in low light. However, it's a bit less precise than e.g. 9-point dynamic area in the D5 which has become my favorite autofocus area mode.
     
  8. Thanks once again, much appreciated.
     
  9. Last edited: Sep 26, 2017

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