Fix infinity focus: tamron adaptall SP 28-80mm 27A

marcusBMG · Joined: 07 Dec 2012 Posts: 1265 Location: Conwy N Wales

A newly acquired 27A proved to be out on its infinity focus, in spite of being a particularly minty one, still in its original box. However fixing this proved to be easy.
Tools: just a modellers screwdriver with a flat head bit (may need filing to fit the narrow slot).

Take off the rubber grip on the focus by easing it out of the groove and sliding it over the mount. You may need to run the shaft of a small screwdriver under the rubber if it's a bit stuck to the tape underneath.
Peel off the tape joining the distance scale to the focus ring and park it on eg the table leg before it curls itself up into a ball. It is a good idea to scratch a reference mark across the join before its all off.
The focus ring can now be rotated up the helicoid and the focus stop - see pic - exposed (turn the zoom ring if its tucked under the distance scale). The whole front end will come off the helicoid but it is not necessary to do that, if you do you might lose the correct join point to screw back together and have a faff figuring it out (but this is what you would do to clean fungus etc).
Loosen two screws to adjust the stop, it is unlikely that anything more than a few mm of adjustment will be required. I needed all the movement range to make the adjustment: ~5mm. If your lens needs a lot more, you should suspect either something else wrong with the lens eg rear optical group reassembled/replaced incorrectly; or that someone might have had the front end off and not reassembled correctly in the correct thread. In this case you may notice that the focus doesn't come against the focus stop correctly in one or both directions of focus. Fix by unscrewing the front end off the helicoid, backing/forwarding one notch (~30°) and reassembling.
Make sure the distance scale infinity mark lines up correctly when you replace the tape. Check the focus, if aok replace the rubber grip.

55 · Expire: 2022-06-15

Thanks for taking the time to document this, marcusBMG.

bychance · Joined: 24 Apr 2013 Posts: 344 Location: Kent, England

Brilliant! Thanks for posting this Marcus.