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Need advice on how to best shim adapter to fix past-infinity focus

Discussion in 'Adapted Lenses' started by Martytoof, Aug 12, 2013.

  1. Martytoof

    Martytoof TalkEmount Rookie

    12
    Feb 23, 2013
    Hi all,

    I just bought a Konica AR 40 1.8 which is a fantastic little lens, but the adapter I picked up seems to have a problem where it lets the lens focus past infinity. True infinity is reached one or two degrees before the end of the focus rake, so it's a little annoying to have to remember to rake to the end and then back up a little if you're scale focusing.

    I read that you can shim an adapter to increase its distance from the sensor which would effectively fix this problem, but I have no idea how to measure how much shim I need. The adapter itself looks like it can easily be shimmed: It has three screws which separate the Konica end from the E-mount end, and can easily be undone and propped up slightly.

    Any advice would be appreciated. Thanks!
     
  2. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Anything from printer paper to brass shim stock can be used. Shims made from soda cans seem popular. It's a hit-or-miss process, you add shims until you get where you need to be.
     
  3. Bugleone

    Bugleone TalkEmount Veteran

    210
    Aug 21, 2011
    The best way to measure this is to use a 'dial indicator'...otherwise known as 'engineers clock gauge'.....like this;

    http://www.mini-lathe.com/Measurement/Dial_indicators/Dial_indicators.htm

    When the lens is focussed from infinity to nearer points of focus the front element moves outwards...thus, a dial indicator set up on the rim of the lens (on the filter mount for example) can be used to measure the movement of the lens between two focus points. In this way the mechanical error of the adaptor can be accurately measured. This is the method and engineer would use.

    Alternatively, if a 'clock' is not available, you can measure the pitch of the focusing thread,..thn measure th emovement of the lens barrel as you focus between the two points and then, by math, calculate the movemnt of the barrel...the resulting figure is the dimension of the shim needed.

    Thirdly,...I should tell you that the focus ring of most good lenses are adjustable for error of focus point by means of the locking screws......easing of these screws allows the focus scale to be moved on the barrel so that infinity point can be brought to accuracy. Only do this if yo uare sure you will always use the same adaptor with that particular lens as you will effectivley have 'accurised' the lens for the adaptor.

    EDIT:.......Perhaps the best and simplest way is to use a 'vernier caliper' or it's modern equivalent the digital caliper like this;...

    http://www.tooled-up.com/product/si...oductlisting&gclid=CMSP293p-LgCFQTHtAod_T0Arw

    Simply measure the camera and lens from front to back with caliper tines on both camera back and front lens rim...take two measurements, one at the first focus setting and the second at the 'error' position...now subtract one from the other and this will give a reasonably accurate dimension for the shim.....
     
  4. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Great post bugleone, but shims is easier. More time consuming maybe, but easier. ;)
     
  5. xXx1

    xXx1 TalkEmount All-Pro

    Jan 15, 2013
    I used depth of field calculator that had a extension tube measurement too. By hit and miss (actually halving the entered extension tube) I found out that with my Samyang 14 mm there must be error of 0.3 mm in the adapter. I don't know if entering values reverse (I put focusing distance to 200 m and adjusted extension tube length until real focusing depth was 0.7 m [that lens was in infinity focus when it was focused to 0.7 m]) does work but error of 0.3mm seems reasonable.

    Paper is a poor choice for shims. I will use soda cans if I wont find suitable plastic foil.

    I used this calculator:
    Lens Magnification and Depth of Field Calculator