Is there ever a need to take camera out of continuous focus AF mode if focus is off the shutter button

Discussion in 'Beginner Questions' started by jordan2240, Mar 27, 2015.

  1. I own Pentax DSLRs, but assume most work similarly with regard to AF modes, so this shouldn't be brand-specific.
    This is probably going to be more long-winded than it has to be.
    Let's say I want to take a picture of a statue, but I don't want it in the middle of the frame. Typical scenario, I believe, is to have the camera in Single AF mode (AF-S on Pentax), place the subject in the middle of the frame (assuming your focus is set for the center focus point), half-press the shutter, recompose keeping the shutter half-pressed, then shoot.
    My scenario is a little bit different because I've taken the focus function off of the shutter button and placed it on the AF button, so I will put the subject in the center of the frame, press the AF button to focus, release the AF button, recompose, and shoot. Only real difference is I don't have to keep the shutter-button half-pressed while recomposing.
    However, I also like to shoot moving subjects, so my cameras are often in AF-C mode (for continuous focus or focus tracking). This allows me to hold the AF button as I follow moving subjects, and the focus will keep changing (well, theoretically anyway - doesn't always work great with Pentax).
    But even with AF-C mode set, I can still perform the same process I described above for the statue because as soon as I release the AF button, focusing stops. So my question is this - assuming I want to use auto-focus, is there ever any reason for me to take the camera out of AF-C mode. Has anyone found that their camera actually focus better in Single focus mode vice continuous focus mode? No idea why it would, but perhaps the algorithm for each mode is different.
    Thanks,
    Bill
     
  2. I always leave it in AF-C if using back-button AF
     
  3. Well, in that specific scenario, indeed there isn't an added value to AF-S indeed. If you always work with the back-button to trigger focus, then leaving the camera in AF-C at all times seems just fine. I never really tried, but I can imagine that AF-C is more geared towards speeds and responsiveness, and hence could be slightly less accurate (but quicker) than AF-S - but this is just a thought, no fact at all.
    Personally, I do not much like 'focus-and-recompose' much, as I often shoot at wide apertures and close-ish distances, where this technique will get your perfectly out-of-focus images most of the time. But even if you just select one of the outer AF-points, the AF-C w/ back button would still fine as you described. However, the majority of entry-level DSLRs do not have a back button for AF, so in those cases, switching back to AF-S can make sense.
     
  4. I don`t know if there are different algorythms but I think that continuous mode with a dedicated AF button is the way to go for almost everything. The single mode has the advantage of focus confirmation (beep!), to me, nothing else. That`s all.
    As Wouter says, if there is not an AF button, the single mode could be more interesting.
    The continuous mode with a dedicated AF-ON button provide a separate independant control, so focus can be activated several times if needed, waiting for the shutter release moment. To keep the button half pressed seem to me an awkward procedure.
    I also use to select the closer AF point when focus and recompose, even in continuous mode. Focus speed is a must to me; I use to shoot non posed portraits, my subjects don`t stop, they use to be in continuous movement at a more or less closer distance. I get the higher percentage of keepers working this way.
    And there is another issue... should the stabilization system (say, VR on Nikon) be activated together with the AF, or with the shutter release button? For sure we`ll have different opinions here... :)
     
  5. Agreed that if you can't do back-button focus, it would seem you'd have to use AF-S unless you used selective AF points and picked the one that didn't cause you to have to recompose; otherwise, recomposing in AF-C would cause the focus to reset.
    I guess I'm mainly wondering if manufacturers optimize the focusing systems for AF-S.
    Interesting question raised by Jose. Pentax has in-body stabilization, but I've never checked to see if the results are the same with both back button focusing and shutter button focusing. I know what experiment I'll be doing this evening.
     
  6. And there is another issue... should the stabilization system (say, VR on Nikon) be activated together with the AF, or with the shutter release button? For sure we`ll have different opinions here... :)
    Believe the D800/D800E/D810 offer VR activation via AF-On button - my D300/D700/D7100 do not. And it is awkward having to remember to keep the shutter button half-pressed to activate VR. Have to check to make sure that doesn't lock exposure - can't recall if I did take care of that when setting the cameras up for back-button focus.
    is there ever any reason for me to take the camera out of AF-C mode​
    There isn't for me. As to possible different algorithms in AF-S vs AF-C mode - I never really checked. Moot point really when using focus-recompose - the error introduced by the subject not being in exactly the same plane as the focal point was during focus acquisition (or more precisely at exactly the same distance since the recompose movement puts area of sharpest focus on a curved (concave) plane with the camera at its focal point) is likely larger than any possibly higher focus accuracy achieved when in AF-S mode (vs AF-C). Before I went to use back-button focusing, I had a quite a few unsharp images because I forget to switch from AF-S back to AF-C when the subject started to move. Hardly ever use focus-recompose anyway - I rather move the focus area in the viewfinder.
     
  7. is there ever any reason .... to take the camera out of AF-C mode​
    Yes, to set up trap focus on Nikons, although this was crippled on the early D800/D800e models and belatedly fixed with a firmware revision.
     
  8. Since Pentax SR gives no benefit with closed shutter VR activation on AF button is a Nikon issue which most likely adds focusing speed for them, since it must be easier to lock on something not camera shaken.
    I'd set the camera to AF S on shutter release when handing it to others.
    As an early user of Pentax AF I haven't yet adapted to AF C on the rear button, but am trying to do so. - Don't you guys run out of thumbs to select your AF spots & AF spot selection? - I just checked: the K100D lacks an AF button at it's back entirely.
    I do a lot of focus & reframe with my RFs but with the SLRS in good light I give the outer AF spots a go too. - Who knows if a lens really has a flat focus plane? And the further I reframe the more my subject could move OOF.
    For me - not yet drilled to the rear button thing - it seems difficult to coordinate but I can see the benefits of it.
    I'd doubt that AF S ever focusses better than AF C. I'd assume if a camera has motion prediction at all (hard to tell with Pentax and I own nothing better) one can only benefit from it in AF C?
     

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