Fixing infinite focus on a lens adapter

Discussion in 'Adapted Lenses' started by Ardent, Jan 11, 2015.

  1. Ardent

    Ardent New to Mu-43

    Dec 18, 2013
    White Plains, NY
    I got one of those really cheap Chinese lens adapters for $17 (the "Fotasy" brand) to use with the venerable Canon FD 50mm f1.4. It works great for my purposes and for most applications I have no complains. But I discovered I wasn't able to focus to infinity when I pointed the camera at some clouds. I read that the problem is caused by the adapter not providing the exact distance from the lens to the sensor. My adapter is apparently a bit longer because the focus does not reach infinity. Other people report that they can focus "beyond" infinity suggesting their adapter is shorter. I took mine apart and sanded it down a bit. I was pleasantly surprised to see that I was able to focus at further objects. I was also surprised to discover my whole adapter is made of aluminum rather than plastic as it appears.

    If your lens is able to focus beyond infinity maybe you can try to add a very thin spacer on your adapter to increase the distance.

    Does anyone know a test I can do to find out if my adapted lens focuses exactly at infinity.
  2. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    To see if the adapted lens itself achieves perfect infinity is hard, you really need an optical bench and the correct mount for said bench to suit the lens.

    If you make a matched pair with a lens and an adapter and a body however you can focus it on something very distant - I generally use the moon as the furthest object far enough away that's still bright enough and generally easily visible to use, the moon is lit by the sun which is roughly daylight balanced so it's a good target with the spectrum of light we most commonly use. It's also far enough away that it should be a good facsimile of infinity.
    Clouds work better than objects closer to the horizon due to mirage which can cause problems, however they may not be far enough away to achieve infinity on stars (especially with longer focal lengths).

    Some lenses will have an infinity stop adjustment screw which allows you to adjust the point the focus ring stops at(samyang/rokinon generally have them under the rubber grip, I'm unsure on the metal finish versions - my 7.5mm achieves infinity on stars and the moon although not on closer objects in the landscape at it's infinity stop... which makes me wonder if when people complain about going past infinity they're talking about objects they think should be at infinity that are not actually far enough away to be at infinity), if the lens doesn't have one (or you need to pull it apart to access it) you're basically down to shaving or shimming the adapter.

    The best way to remove small amounts of metal evenly is to use a lathe, however failing that a piece of flat glass with very fine grit wet and dry sandpaper on it. Wet the glass with water and put the paper down so that it sticks to the glass and is kept flat. Keep it wet to wash away metal filings and move the adapter over the surface making sure to rotate it so that if you're pushing unevenly it will hopefully even out. You should be able to achieve a flat (within tolerance) mirror finish by this method.

    Basically it's fiddly work, which is why most adapters are thinner than required so lenses just focus past infinity - while it's not ideal it means that people can't complain that their lenses won't focus at infinity and the maker of the adapter gets away clean.
  3. Ardent

    Ardent New to Mu-43

    Dec 18, 2013
    White Plains, NY
    Thank you for the detailed reply, this was very informative!

    Like you advised I had a sheet of fine grit sandpaper on a flat surface and slid the adapter back and forth while rotating it. I guess I will sand it down until I can focus on the moon and clouds. I don't think I will be shooting the stars at this focal length (50mmFF/100mmMFT).
  4. pellicle

    pellicle Mu-43 Hall of Famer

    Feb 10, 2010
    Southport, OzTrailEYa

    this is exactly what I did ... also if your adapter has a screw in plate on the front, take it off and work on that bit.


    if you have a 28mm lens too then that will be better to work with as a "test" because the small amount of extension will result in a bigger inaccuracy on the shorter focal length.

    buy something like 1200 grit emery paper and work with it taped onto a piece of glass (such as any cheap picture frame)

    Here are some of my explorations on this.
    • Like Like x 1
  5. listers_nz

    listers_nz Mu-43 Veteran

    Nov 22, 2013
    Christchurch, New Zealand
    The same trick also works with the L39/M39 mount Industar 69 lens. I followed the instructions (elsewhere on this forum) about how to adjust the focus of the lens by removing the focus ring, but that only got me part of the way there. Then I took the ring out of my adapter (its secured using 3 allen screws through the side) anbd proceeded to sand that down until I could get infinity focus. Obviosly the adapter is now only suitable for that lens, but at only a few $ for another adapter it is hardly a problem.
  6. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Fixing infinity focus is also one of the advantages of having the optical element in the cheap focal reducers, since you can screw it in and out slightly to compensate for focus problems. That obviously is a different situation (and more expensive), but just figured I would mention it since it's tangentially related.
  7. HarryS

    HarryS Mu-43 Top Veteran

    Jun 23, 2012
    Midwest, USA
    It' ws not obvious to me until I tried a second lens with the same mount. I shimmed my first FD adapter for my 85mm f1.8, and was happy til I found it no longer worked with my 50mm f1.8. Then I had a greater appreciation of the wide variation in lens tolerances that must occur after they have been in use for 40 years. I'm willing to accept focus past infinity, as long as it's somewhat reasonable. When a lens says 3 meters at infinity, that's way off.
  8. tjdean01

    tjdean01 Mu-43 Top Veteran

    Feb 20, 2013
    I like to be able to focus "beyond infinity" because I know that I have the capability to reach peak sharpness
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