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Measuring Focus with Lumix 12-35mm and GH3

Discussion in 'Native Lenses' started by shanguli, Jan 11, 2015.

  1. shanguli

    shanguli Mu-43 Regular

    89
    Sep 16, 2014
    canada
    Hi,
    I am using:
    Panasonic GH3,
    Lumix 12-35mm f/2.8 Zoom lens
    Oly 17mm f/1.8 Prime Lens

    I was watching this video regarding measuring focus using a tape measure: https://www.youtube.com/watch?v=AjLfyxKeF40

    There is no specific ft or meter indication on the lumix 12-35mm zoom lens, but 17mm Prime has those indicated on.
    Questions:
    1-I wonder how can I measure focus using a tape measure with my GH3 & lense (more importantly lumix 12-35)?

    2-btw, how come other primes like Oly 45mm doesn't have those ft & meter indications on it (like the Oly 17mm )?

    Thanks very much!
     
  2. bassman

    bassman Mu-43 Top Veteran

    678
    Apr 22, 2013
    New Jersey
    Scott
    Many m43 lenses are "focus by wire" - there's no actual mechanical connection between the focus ring and the focusing mechanism - it's really an electronic switch that tells the focus motors to move one way or the other, and perhaps how fast. Sort of like the volume control on most modern car stereos. So there's no correlation between a specific turn of the ring and a particular focus setting. The 17/1.8 is one of several exceptions.
     
  3. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    The ring has no mechanical connection to the focusing elements, it's just an encoder of some kind (probably a potentiometer with an A/D converter, the number of bits is how fine the control on it is... so I suspect a pretty standard 10bit giving 1024 positions, it may even be a 12bit model with 4096... however I doubt it) - when you turn it the lens is told what position it should be in (distance it should be focusing at).
     
  4. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    as well (from what I've heard) the system is not actually precision aligned, as all native lenses re-calibrate on powerup
     
  5. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    I'm not sure what you mean by precision aligned, however the 12-40mm is consistent on mine.

    The distance on it is not entirely accurate (infinity is not at the hard stop, the distance measurements are never really accurate), however when you pull back it always focuses on the point you prefocused at. If I zoomed in, focused, then zoomed out, then zoomed in... focus would be maintained (not perfectly, however better than I would expect given the lens itself is not parfocal so it has to constantly adjust as you zoom, about the level I would expect given the limited resolution of the AD converter in the chain compared to the motor at being able to move the focusing elements infinitesimal amounts).

    Do you mean when you turn the camera off it may drift from the point you focused it at? I've never actually tested that... I guess it probably would, it would also vary somewhat if the reference voltage wasn't perfectly stable or drifted (assuming it uses a potentiometer) since the AD converter would get a different value (assuming it would be itself immune to said drift/stability issues). In all cases it's not a perfect solution, it's just a somewhat unique and well executed one when compared to other current offerings.
     
  6. budeny

    budeny Mu-43 All-Pro

    Mar 4, 2014
    Boulder, CO
    By default, Olympus cameras will reset lens on power off/on, hence loosing the focus. But this is configurable in camera settings (under AF/MF section), so if you set "Reset Lens" to off, then focus will be maintained even after powering camera off/on.
     
  7. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    While that's true I was talking about the 'physical' focus ring common to the 17mm and 12-40mm, it only moves when you have the ring pulled back (if the ring is pushed forward you can focus, then when you pull it back it snaps to where you had previously focused it).

    If you have it pulled back when the camera turns on it will automatically focus to whatever distance the ring is set at even if it had previously reset the lens.
     
  8. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    I was meaning that the distance from mount to sensor was not precision stuff as it once was. That the system was built to allow for small tolerances. Such must certainly be the case with ibis ...

    But I don't know this, as I said its an impression I was under..
     
  9. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    Yeah, it's very likely true otherwise we would still be using breech lock lens mounts... modern bayonets are often very sloppy at times.

    (This reminds me of something else that came to mind with the thread about infinity on adapters. While the redrock micro adapters for Nikon F to Canon EF do not allow aperture control for lenses without physical control so you can't use them with newer lenses, I love the fact that they have 3 tiny grub screws plus the lock screw so that you can adjust the adapter to parallel as needed, that sort of attention to detail is often missed in still photography and I only really see it with things designed for video).
     
    • Like Like x 1
  10. shanguli

    shanguli Mu-43 Regular

    89
    Sep 16, 2014
    canada
    Seems like you don't measure focus with tape measure (I thought tape measure is the way to go with my camera and lens comb).
    I just need to measure the focus accurately, using GH3 and Lumix 12-35 lens.