|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
- 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
- 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
- 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.