object tracking vs. continuous auto focus

Discussion in 'Mirrorless Digital Cameras' started by mark_amos, Aug 19, 2012.

  1. I have a Sony NEX 5n. I like it a lot. The quality of the digital imagery I can achieve with it finally motivated me to adapt to these automated type cameras, and furthermore, I am willing to try to take advantage of some of the auto focus benefits I previously shunned, especially to take advantage of the amazing video capabilities. Mainly I just use single focus mode and just lock the exposure and focus on static subjects, but I need better facility for moving objects or video. My question is what is the difference between continuous auto focus and object tracking. On my camera with a touch screen, I find that for video, it is convenient to touch the screen in object tracking so I can shift the focus point. With respect then to still shots of moving objects and also video, I don't have a reliable workable technique. I know that the contrast detection of my 5n isn't as fast as phase detection, but can somebody outline a good procedure for setting the camera for stills of moving subjects and perhaps a different approach to video of a moving subject(s)? Don't worry about conveying the most artistic aspects. I just mean with respect to settings, what should I be starting with?
  2. I do not have a 5n but on my cameras the difference is that tracking allows the focus point to move across the frame while AFC does not. AFC needs to be enabled for tracking to work: once you lock focus on the selected point the camera will track the object (far too slowly on my GX1).

    You can test this very easily by panning the camera to simulate movement of a focus-locked object. The LCD will show that the focus point moves across the frame. This should also prove that AFC needs to be enabled for tracking to work. Conversely, moving the camera towards and away from the object will also show that the focus point stays fixed but that focus adjusts. This needs AFC but not object tracking.
  3. I neglected to answer your full question: I dont do video, so cannot comment on those settings, but I rarely use object tracking. I prefer to use AFC and move the camera to keep the object under the focus point.

    It's worth understanding what happens for object tracking:
    1. camera uses AF system - essentially an edge finding process that moves the lens to get maximum contrast - to get a set of sharp edges in the focus area.
    2. While storing these, camera does the same with the next frame from the frame refresh
    3. Camera then compares these two sets of edges to find edges in the neighbourhood of the first set that look like it. If it finds these, it deduces that the object has moved and updates the location of the focus point.
    This is computationally intensive and is likely to be slow. it is also likely to be more successful if there are few edges to track: the camera will find it easy to track a spider on a wall, and it will find it hard to track a jumping squirrel in foliage (too many edges). It will also find it easy if the image of the tracked object moves slowly across the frame (the neighbourhood search area is small).

    Therefore, whether object tracking will work for you will depend on what you're try to photograph. If the image of the object moves slowly across the frame and has relatively few edges it should work for you.

    I do not do video but I can see how object tracking would work for this and reduce the focusing workload.
  4. Thank you Abib for your response. Most of what I am asking is thoroughly addressed in what you wrote, but it may be that for me to apply it, I might need more Sony NEX specific input. Also, I don't have my camera with me at the moment, and I need to try a couple things so I can accurately represent what the camera seems to do. One aspect I can clearly describe now is that when I start to shoot video by pressing the video button, I don't know what the camera is focused on, and the only way I know to set focus is to have the camera in tracking mode and touch the touch screen, and it shows a little square, which moves around to track the object like a person's face, but often it get's confused and bounces to someone else, and then I have to click the speakers face again. Ironically, the easiest thing so far is to use a manual focus lens. I focus on the person's face then hit video record button, and everything is good....unless they start moving too much.
  5. Perhaps you are panning the camera too quickly... It's worth doing the test I described above as that should give you some idea of the limits of your AF system. I found that, for my camera, the pan had to to be very slow for the object tracking system to cope, too slow for the things I try to photograph.

    Good luck with it.
  6. Abib, thanks for helping. I think I am slowing figuring this out. I did the test of what you suggested about continuous af. I have done this before, but I can better fit this into the scheme of things. I see now that with the af/mf select set to dmf (instead of just af or mf), which allows you to refine focus using the focus ring to get an instant magnification and manual fine tuning, which I like, you don't have an option to change the "af mode". The "af mode" is a different menu option where you choose between single or continuous af. So apparently in dmf, the camera automatically focuses on what is in the center of the field even without pressing the shutter half way down. Therefore, in taking video, the lens focuses initially on what is in the center of the field, and then after starting, I can use the touch screen to touch a new focus location which uses object tracking to shift focus without me having to recompose.
    I have to admit, I don't have much of any experience with af cameras beyond point and shoots, and maybe the nex menu options are just confusing by any standard like some people say because frankly, this sucks. I love the camera. It's worth using for the amazing image quality, but some of the set up is a little bit rediculous IMO. I've been using the Sigma 30 EX DN, which I bought to have a high quality af option, and I like the lens a lot, but frankly, the way some modes won't allow other modes, meaning I'm never sure how af is going to work, makes manually focusing my legacy lenses on the 5n all the more attractive again. Again, I need and enjoy this camera and highly recommend it, but I really miss the ole M6.
  7. I really do not know about your camera but I would think that the AF system has at least three modes:
    1. It chooses what it thinks is the best focus area given the scene;
    2. A user-selectable focus point;
    3. Object tracking.
    Modes 1 and 2 should work with both single and continuous AF, and the focus point for mode 2 should be selectable prior to starting stills or video.

    DMF is very useful because (I find) the AF system is more accurate with it as one can see exactly where it is choosing to focus and because it seems to have a tighter tolerance for focus. On my camera I get much better results using it but I find that it does not work with continuous AF.

    If your subject is not moving much you may well be better off using mode 2 and Continuous AF and pre-setting the focus point.

    I like manual lenses very much - I have several - but find that AF is much faster than I am and indispensable for things that move. If you have lenses for your M6 you should think about using them on your camera (with an adapter).
  8. Thanks again Abib for your diligence to be helpful. I do have adapters for both my Leica and Nikon lenses, but I also appreciate the speed of AF for certain situations. Typically I find that mode 2 as you describe it is what works fine for most of what I shoot. I mean I usually just use single focus by using focus lock with half pressed shutter. What I discovered with your encouragement was that in this mode (at least with DMF on) the camera focuses on what is in the center even if you don't press the shutter half way down. That was helpful to discover because now I realize that is what is happening in video mode. You don't half press the video button; it just starts and automatically will always try to focus on what is in the center of the frame. This is not ideal for video because it means you have to start shooting with your subject in the middle; then press the touch screen to get object tracking to lock on the face (for example) and then recompose with the subject off-center if you want. Anyway, that is all I can figure out to do. I think with this contrast detection af, there is little value for me to try to object track or continuously focus because it's just too slow and inaccurate for still shots, and typically unnecessary anyway. This is why I suggest that my camera isn't a replacement for a really good dslr, but that isn't what I was looking for anyway. With a manual focus lens, none of this is an issue. Instead of trying to remember where in the menu the focus settings are or what button I've customized the rare ones to, I can just focus and then shoot with confidence that the camara hasn't changed the focus on me. If the subject moves, I know it. Still, I'm not giving up on af for good. Thanks again Abib.

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