Focusing - How does it work?

Discussion in 'Canon EOS' started by dave_c__manchester_, Dec 10, 2010.

  1. Hi, I took my DSLR and lens in to a local camera store to talk about a problem I was having with focusing.
    The guy who served me seemed to be knowledgeable and tried to explain the relationship between the lens and the camera when it comes to focusing.
    I've got a Canon 350D with a Sigma 18-50mm f/2.8 EX DC lens. Can anyone tell me what communication happens between the camera body and the lens as I press the shutter button?
    For instance the camera must "ask" the lens to focus, does the lens make the decision on what to focus on? If so what does it report back? etc etc.
    I'd be grateful if anyone could give me an insight on this.
    Many thanks
     
  2. In a nutshell: The camera tells the lens what to do: move. The lens moves and tells the camera when it's finished.
    If you search the archives for "autofocus feedback loop" (without quotes) you'll find some good threads.
    http://www.photo.net/canon-eos-digital-camera-forum/00ERXl
    Steve Dunn's comments toward the middle of the thread are good. Other treads abound with additional bits of information.
     
  3. B.t.w. what sort of focus issues are you having?
     
  4. I've noticed there is a problem with my Sigma lens. When I focus on infinity and press the shutter release half way down it focuses on what it thinks infinity is, this may be in focus or not. The problem I have is if I press the shutter release half way again the lens/camera shifts the focus ever so slightly. I'd say 50-75% of the time the focus is pin sharp, the rest of the time it's slightly off.

    I'd expect, if I keep the camera still and focused on the same object, for it to focus once and stay focused regardless of how many times I press the shutter release.
     
  5. [[I'd expect, if I keep the camera still and focused on the same object, for it to focus once and stay focused regardless of how many times I press the shutter release.]]
    When you do this, you have selected only a single focus point?
     
  6. Absolutely - single focus point (doesn't matter which one, usually the centre one though).
     
  7. Make sure you make AF mode set to "single shot". The default is "AI Focus", where if your subject moves, the mode is changed to "AI servo" (tracking) mode. If you press the shutter button half-way down and re-frame, the camera may think that your subject has moved.
     
  8. The focus is definitely "single shot", I hardly ever use "AI Focus" as I tend to take shots of static objects. Besides I wouldn't expect "AI Focus" to shift around if the object is static.
     
  9. I'd be curious how often this happens with a static subject and with the camera on a tripod.
    Because you're asking the camera to re-evaluate focus every time you press the shutter button (assuming you've not moved focus to the * button). So it's going to take a look again and, if you're just standing, you're not actually that steady (by comparison) and the framing will probably have changed slightly, thus the camera issues a command to the lens. It seems likely that there will be a chance it gets it wrong. For a bit of discussion about the phase-detection system in a (D)SLR, see Wikipedia:
    http://en.wikipedia.org/wiki/Autofocus#Phase_detection
     
  10. It happens the same frequency even if I've got the camera on a solid tripod (or say resting on a rock) and use the wireless shutter remote.
     
  11. Dave, you are really lucky that Rob got here first.
    Even though I am not an engineer, when I saw the title of the thread, I was mentally preparing to write a detailed discussion of USM, the miracle of the little lens motor that does the focus, complete with diagrams and pictures.
    But
    In a nutshell: The camera tells the lens what to do: move. The lens moves and tells the camera when it's finished.​
    is simply a masterpiece in giving you all that you really need to know. Short, sweet, and to the point.
    Bravo Rob! It's a little masterpiece. It deserves to go down in history with "git there firstest, with the mostest" and similar aphorisms.
     
  12. [[It happens the same frequency even if I've got the camera on a solid tripod (or say resting on a rock) and use the wireless shutter remote.]]
    Is the subject matter also static?
    Certainly more problems can arise when talking about 3rd party equipment. Sigma doesn't have access to Canon's lens specifications. They reverse-engineer it. Perhaps there is something about that process that can lead to a higher percentage of out-of-focus shots.
    I know that with my Sigma 17-70 I've had the occasional hiccup where it claims the subject is in focus when it's clearly not. I usually focus on something else and then back to the subject and it's fine. I don't worry about it as it's never caused any problems for my photography.
    Also note that the AF indicator in the viewfinder is smaller than the actual area of the AF sensor. You've got a larger area around the square than you think for the camera to use to look for contrast.
    Just for fun here's another good page on AF. http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm
     
  13. [[Is the subject matter also static?]]
    Yes, everything static, think landscape on a sunny cloudless day.
    I guess it's a problem with the lens. I have a Canon EF 70-200mm f/4.0 L IS USM which works fine.
    Like you say, the camera asks the lens to move, it does and then reports back. Seems the process of how far to move is broken. :(
    Don't suppose there's an economic way of resolving my problem?
     
  14. I don't think there's enough known to determine where things are falling down, but regardless, if it's impacting your photography, why not contact Sigma about it?
     
  15. I was told by my local dealer it would be cheaper to buy a new lens (of the same type) off eBay rather than get it fixed. Can anyone comment on this?
     
  16. It would depend on the nature of the fault, depend on the warranty, and depend on your willingness to play Ebay Roulette.
    I think it would cost you postage to ship the lens to Sigma for an estimate and if you choose not to pursue the repair some nominal fee is charged. Their website should have more information.
     
  17. I believe my Sigma manual states that their lenses will have trouble focusing on blank blue sky and I notice mine continues to try and focus on sky but never really gets it right. I have to find an area closer but still out there at what would be considered as infinity in order to get good focus. If you are doing landscapes you may experience this problem. You may want to switch to manual focus when you have the camera mounted on a tripod.
     
  18. I believe my Sigma manual states that their lenses will have trouble focusing on blank blue sky and I notice mine continues to try and focus on sky but never really gets it right.​
    This isn't necessarily a fault of your lens, it is merely a limitation in the which autofocus system. It doesn't have anything to do with the fact that it is focused at infinity, it is the lack of contrast in your subject (i.e.. blank blue sky). Try focusing on a black white wall, its the same thing. Luckily with something like a sky, manual focus to infinity is simple.
    AF needs contrast to work, so if your problem is similar to the one described above, it may just me a misunderstanding of how the autofocus system works. This would explain why your lens always racks in and out, never quite getting in focus. Its searching but cant quite find it.
    If you really want to test this try shooting something like a newspaper on a wall or something like that, something with a lot of detail for the AF to catch on to. Shoot in good light, as it has a huge effect on this as well. Try something like this and see if the lens behaves differently.
    oh and in my experience and many other, Sigma lenses have their little problems, and AF hunting is one of the big ones.
    Good luck,
    - Richard
     
  19. A crucial point has not been mentioned in this thread:
    The deterimination of "in focus" is done by the camera. The AF sensors are in the viewfinder area, which is why you lose AF capability with some incarnations of the "live view" function. The AF works by detecting changes in contrast in the area of the image covered by the sensor. This is why the camera can't focus on solid colored objects and in low light. Literally, the camera can't see changes in the subject as the lens moves.
    Now, the lens plays a huge role in the focus in that the camera needs to move the lens to achieve focus. How well the lens responds to the camera is a huge issue, and some lenses are very much better than others as noted in the responses above.
     

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