Misfocus with A7 and native lens, what gives?

Discussion in 'Sony Alpha E-Mount Cameras' started by addieleman, May 31, 2015.

  1. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    After repeated unexpected failures to get proper focus with the A7 and the FE 16-35mm on it, I did a number of experiments to find out its possible cause.

    I normally use the A7 in AF-S mode with the small flexible spot and I ended up with way too many pictures that just were misfocussed, usually the image was sharp before the intended focus point.

    Experiment 1
    Compare between MF and AF-S, wide-open at f/4, subject with near and far. Using AF-S actual focus ends up before the intended focus which was at the far end. MF was dead-on.

    Experiment 2
    Flat subject (bookshelf), lens wide-open at f/4, compare between AF-S and MF with all sizes of flexible spot => No discernible difference, all OK.

    Experiment 3
    Near and far subject matter, i.e. bookshelf under 45° in horizontal direction.
    Lens at f/8, Live view setttings effect OFF => Front focus.
    Lens at f/8, Live view setttings effect ON => Perfect.

    Why this last experiment?
    If I remember correctly PDAF is supposedly disabled as soon as the aperture is closed beyond f/6.3 and that is effectively what happens when you set Live view settings to ON and set the shooting aperture at f/8.
    I suspect that when the camera does not respect the focus area of the flexible spot when it uses PDAF.
    If this is true, I again curse Sony for not providing the option to disable PDAF!

    I'd like to hear other ideas and opinions on this.
     
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  2. metalmania

    metalmania TalkEmount Regular

    123
    Dec 3, 2014
    USA
    Interesting, does it only happen with the 16-35 lens ?
     
  3. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Not sure yet; a quick test with the FE 28-70mm didn't seem to show this problem but I'd have to test more to be sure.
     
  4. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Further experimenting reveals that it may also be focus shift: at f/8 focus is spot-on with live view effects ON and front-focused when OFF. That is when using the lens on a tripod with OSS off. Couldn't see any significant differences in hand-held shots between OSS on and off; both series of 8 shots each contained some shots with sub-optimal sharpness. Motion blur isn't very likely given the shutter speed of 1/250 s and the subject was a building, not really moving I think :).

    The more I do different tests, the less I know what's going on here :(. Repeatibility isn't just there yet. The only things I'm not totally uncertain about:
    • It's best to shoot with Live view effects ON when shooting at f/8.
    • Manual focus at maximum magnification is necessary to get optimal results at wider apertures.
    To be continued...
     
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  5. michelb

    michelb TalkEmount Regular

    197
    Oct 27, 2013
    Greater Montreal area in Quebec, Canada
    Michel Brien
    From my readings in both Sony Guide and Gary Freedman's eBook on A7/A7M2:

    PDAF can only kick in if you are in AF-C mode AND your aperture if greater than F9 with compatible lenses.
    Quote: " Sony has tried to narrow the gap between these two AF mechanisms by “baking in” 117 pixels capable of phase detection right into the sensor. And while it is an improvement, I caution you against expecting too much from this feature.

    The A7 still uses CDAF for its initial autofocusing needs and for all video shooting. The PDAF pixels only kick in when you’re in Continuous AF mode (“AF-C”) AND after the camera locks onto to the subject via the conventional CDAF mechanism (AND if your f/stop is set to less than f/9). So it’s only an advantage when shooting stills of things that move. And it only works with native E-mount lenses."


    So the source of your front focus must be elsewhere if you are using AF-S mode. I find intriguing the fact that it occurs only with Live View OFF.
    Maybe in this mode, the camera changes the lens aperture to improve viewing and once it closes down to the specified aperture, this fools the AF system.
     
  6. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Thanks for pointing that out about PDAF, didn't know this. Concerning the influence of Live view effects, if it's set to ON the aperture is permanently closed to the value set for shooting; it's easy to see when looking into the lens. When it's OFF the lens is wide-open until focus has been achieved and only then closes to the shooting aperture value. My observation is confirmed by Lloyd Chambers who says exactly this about it. Also, Lloyd Chambers mentions that the FE 16-35mm has pronounced focus shift in the close-up range, but my experience with my sample suggests it's also there at longer distances of several meters.
     
  7. Nexnut

    Nexnut TalkEmount Top Veteran

    I had issues with focus shift with quite a few lenses in the past. Sounds like a reasonable explanation to me but doesn't exactly help to build a good relationship with a lens. Thanks for posting this, Ad - keep us updated!
    (The 1635 has been on my radar for quite a while but for some reason it just disappeared and the 10-24mm Fujinon showed up in the same place.)
     
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  8. Lisandra

    Lisandra TalkEmount Veteran

    216
    Jan 28, 2015
    Cdaf aren't supposed to front/back focus, its either focused or nit focused at all. So its interesting that is happening...
     
  9. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Strange indeed: with the FE 16-35mm set to f/4 I consistently get front-focussed pictures. Forum member robbie36 mentions a backfocus issue with the FE 24-70mm, which is equally strange.
     
  10. Lisandra

    Lisandra TalkEmount Veteran

    216
    Jan 28, 2015
    So maybe the on sensor pdaf is chiming in after all?
     
  11. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    That's what I still keep suspecting despite Michel Brien's remarks in this thread. It's a question only a Sony technician can answer. It would be a good thing if PDAF could just be disabled in the menu (not the PDAF area indication but PDAF itself). I've had the same struggle with the NEX-6 and all this focusing hassle is the main reason I left DSLR-land :mad:.
     
  12. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Here's a test. Lens FE 16-35mm wide-open at f/4, OSS off, Live view effects off, medium flexible spot size. Spot size and Live view effects setting makes no difference BTW. I'd say optimal sharpness sits around the 40cm mark whereas focussing was for the 50cm mark where the pen is. Click on the picture for a full-size image (11 MB).

    i-2JpCzwL-X3.
     
  13. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Another test, now at f/8. Medium flexible spot, OSS off, Live view settings OFF and ON resp. First shot is way off, second shot is almost spot-on.

    i-VmMDMZs-X3.

    i-3cNwCgt-X3.
     
  14. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    I'm sorry, Ad. I can't see what you're seeing. And I've scoured your full-size images with my reading glasses on. The ruler looks most in focus to me right where the pen is pointing.

    But I also wonder whether what you are seeing is going to make any difference at all in "real world" shooting. This is, after all, a wide-angle zoom, and my guess is that it will spend most of its time on your camera shooting landscapes with massive amounts of DOF. Are there really any circumstances at all under which you're going to be able to detect this flaw?
     
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  15. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Problem is, I started testing because I came home with misfocussed shots, so I have a real-life problem. Those were landscape shots made at f/8 as well as a few casual snapshots at f/4 that failed miserably due to focus errors while I'm 100 % sure I placed the focus spot in the right place. And it wasn't just the occasional mishap; never had this problem with MF legacy glass and I'm starting to regret this colossal outlay for a lens that's causing me headaches, literally by now. My criterion is that any good shot should be technically good enough to be enlarged to 40x60cm and that means focus must be spot-on.
     
  16. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    Well then. I'm sorry. Can you return it to the seller?
     
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  17. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Not really, I bought it in a store and I know they're not the kind of people to just take it back or even swap it for another one. If they have a second one at all. Might be a case for sending it in to have it checked, meaning I'll have to wait some 6 weeks before I get it back.
     
  18. michelb

    michelb TalkEmount Regular

    197
    Oct 27, 2013
    Greater Montreal area in Quebec, Canada
    Michel Brien
    Now you got me thinking so i pulled out my new A7MII with 24-240 to try some stuff.

    Non scientific test shots i just took are showing me some level of front focus whenever Live View is OFF. Probably not to the point of considering it a major issue but i can see on my test pics that with Live View ON, the same subject is actually a little sharper and with Live View OFF, the front zone of the item i focused on looks sharper than the front zone with Live view ON. And this both at max aperture (depends on zooming on the 24-240) AND at F11.

    It could be more a live view issue rather than a lens issue.

    Will need to try same in MF to determine if it actually is front focus by the lens or by Live View
     
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  19. Nexnut

    Nexnut TalkEmount Top Veteran

    I'd say the max. sharpness in the first image is way in front of the pen, about half way down. Pretty unusual, but it's there and can't be neglected if you do large prints and look close enough.
    Seconded - I do A0 and A1 prints on a regular basis and want my max. sharpness where I have put the focus, ideally wide open. Sorry I can't replicate the issue with any of my current gear other than my SEL20f28 where the focus is all over the place in some shots but for other reasons - different story altogether.
    As I've mentioned above I've had issues with focus shift (different focal plane when stopped down due to spherical aberration, not to be confused with PDAF related focus shift) with some of my primes, most noteably with my 50mm/f1.5 Zeiss Sonnar and the 28mm/f2 Ultron. I sold both lenses. What I see here looks a bit different though and might take some more experimenting to nail it down.
     
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  20. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Did some further testing today by shooting various scenes, taking care that the whole focus area was filled with subject at about the same distance. No problems with focal lengths from 16 to 24mm. At 35mm focussing is just erratic, plain and simple. It happened more than once that one picture was sharp and the next wasn't of the same scene and same camera settings. Also sometimes the picture was unsharp at the right edge; I also have one shot where the lens is ravishingly sharp at 35mm.