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Favorite Classics Camera & Repair Articles Servicing the Auto-Miranda 50mm f1.8 lens
Servicing the Auto-Miranda 50mm f1.8 lens
by Mark Stuart

This was the procedure required for servicing the above lens, necessary when the aperture blades became covered in oil. This, in turn, had occurred because an inappropriate grease was used previously by me on the internals, from which oils migrated to the aperture mechanism. Here we go...

  • Set lens to infinity focus and measure length between mount and filter ring with a vernier or similar. Write this down - it assists in reassembling the lens so that you can correctly achieve infinity focus.
  • Remove the three screws holding the black trim ring around the outside of the lens mount, and remove. Do the same with the black shield around the rear element - it also has three screws.
  • Remove the three screws around the "depth of field index" ring - one will be hidden by the aperture ring finger tabs, so just rotate this to access the screw. These screws are soft brass, so be gentle. Carefully lift off the metal lens mount and aperture ring together, gripping them together with your finger pads. Warning - do not lose aperture detent ball bearing which is sandwiched between these parts ! Unless you wish to grease the aperture detent ball, these parts can stay together. To grease the ball, you can get away with placing a small amount of grease on the tip of a toothpick and inserting it through the gap where the ball resides. If it's gritty, you'll need to separate the parts and wipe the track and ball with lighter fluid before greasing.
  • Remove the focussing ring by turning it (as if you are focussing) until it disengages with the corresponding focussing helix (thread) on lens barrel. Important Note - carefully observe where the focussing ring helix actually releases from the barrel by sighting across some markings on the barrel, and remember any relevant markings that are aligned as it disengages. This will assist when re-assembling.
  • Now to the rear element group. Unscrew by gripping the rear element surround by hand - it will unscrew anticlockwise. There are no left hand threads on this lens. When this is removed, you will reveal the rear of the aperture blades. Note that this rear element group could be removed without any of the dismantling undertaken thus far, if required.
  • Now the front element group. Unscrew the whole outer barrel by hand, noting the position of the large spring tab washer as it releases. Remove the tapered ring by unscrewing - his will release the front element. Carefully remove and store in tissue paper, noting curvature (convex to front).
  • You've now made it to the intermediate element group. This simply unscrews - the black metal ring has two obvious slots. Careful storage is required again.
  • The aperture blade retaining spacer just lifts out, revealing the blades. The blades may now be removed. Warning - carefully note the way they fit together for reassembly! This is the trickiest part of the lens. The aperture blade pivot/seating ring may be removed by removing two screws accessible from the rear of the lens barrel.

Note 1 - Element groups may be further dismantled by unscrewing appropriate rings if necessary, eg. for fungus contamination.
Note 2 - brass focussing helix may be removed from focussing ring by releasing three brass screws around inside but NOTE exact location (mark with scratch) as the ring is infinitely adjustable and the screws operate by clamping down on the edge of the brass ring. Depth of field ring (focus limit ring) is removed by turning until lug matches cut out in brass helix ring. This can be quite fiddly. If you look carefully around the inside of the focus collar where the screws go, you may see the rectangular cut-out where the lug is released. When reassembling, use fine lube for depth of field ring (eg. teflon lube) as this contributes to focussing 'feel'.

  • Reassembly is the exact reverse procedure. Note to carefully check infinity focus as it's easy to be one 'thread' out on the focussing ring. What this means is that the focussing ring picked up on the wrong thread when you reassembled it. This is where the marks you observed when removing the focussing ring come into play - align the barrel and focussing ring and try to pick up the right thread. At infinity according to the lens barrel, the measurement you took before dismantling should be correct. Note that if infinity focus is out, note how much of a turn, and in which direction, you need to turn the focussing to obtain infinity focus. You'll need to partly disassemble the mount end, realign the focussing helix by the amount you observed, and reassemble the mount end. Note that if you are 'out' by one thread, it is very obvious when focussing the lens far away.

As a final point, I used an unconventional method of cleaning the heavier 'hardware' parts of this lens - I simply washed them in dishwashing detergent and very hot water, rinsing them in copious amounts of warm water, and allowing them to dry in front of a fan heater. The aperture blades were cleaned with lighter fluid and cotton buds. I have also subsequently used the dishwashing liquid method (very carefully) for fungus riddled lens elements on another lens (not allowing anything but my hands to touch the glass, and followed by lens cleaner) and the elements have been returned to pristine condition.

Good luck.