Pentax Super Takumar ƒ3.5 135mm
by Stuart Willis

Disassembly is from the front end.

The lens comprises two optical groups.   The main group and the rear element.

There are two versions of this lens model and to a small extent the disassembly procedure varies after one has reached the point where the lens-mount module is separated fron the main focusing helix.

The standard version features just the M42 auto-iris pin at the lens-mount.

The variant adds a slider which is obviously for those Pentax models with open aperture metering-coupling.

See pertinent comments on the construction of this variant model later in these notes.  I recall that the Pentax ES is such a model so when alluding to any such variant in these notes I shall refer to such as "ES".

In brief on setting up the focus

With non SLR cameras the initial lens focus is set up by viewing an Infinity image on ground glass at the film plane and then adjusting the front element or front lens group to optimum sharpness.    Even with an SLR this ground glass methodology is best practice.  However,  if from results experience with the camera the owner is absolutely certain that  the film plane image and the viewfinder image are identical  (ie: the mirror rests at exactly 45 degrees and all the associated prism and fresnel geometry is correct)  then it is good enough to adjust the lens focus according to the viewfinder image.

The construction of the ƒ3.5 135mm Super Takumar is such that the set-up focus is the product of axial movement of the entire main lens group.    The main lens group does not itself rotate or otherwise screw in or out.  Rather  the main lens group is screwed fully hard down into the female thread of the focusing helix and it is the axial movement of the helix which is used to set up the focus.

If the lens is already partially stripped down during or following service and with only the focusing barrel and filter-ring to install - then this is the point when initial focus set-up is most easily established.  With the helix screwed fully inwards, mount the lens on the camera and whilst viewing far distant definition,  manually unscrew the helix by hand until the aforesaid Infinity image is sharp.   Then slide on the focusing barrel,  free-turn it to the Infinity Stop and then lock it in that position per three screws (each with a small washer under its head).
** Once having established the optimum focus it is a good idea to make a small index mark at 12 o'clock on the brass face of the helix so that if by mischance the helix is disturbed during installation of the focussing barrel one can use the mark as a reference and avoid repeating the focus procedure.

If the sole purpose of the service work is just to set up the focus - then the three aforementioned screws need only be loosened to enable fine adjustment rotation of the helix.  Then retighten them.

Complete disassembly procedure

Name-ring off.   (Unscrew with a rubber tool).

Remove the filter-ring per three screws.

Remove the focusing barrel - per three screws (as earlier described).

Next remove the the focusing index ring  (the one as with DOF scales at each side of the central index line). It is held both rotationally and axially by three small set-screws as with pointed business ends.

The aperture ring taps off forward.   Take care to capture its single detent ball-bearing.

Beneath where the indexer ring has earlier lain, are three countersunk screws.

Removal of these facilitates pulling off the lens-mount  - which is a deep-cup one piece aluminium machined part as containing all the auto-iris mechanism.

** If the lens is an "ES" with a slider,   then there are two additional screws to remove before the mount can be separated from the helix / iris module.  These are deep headed guide screws which ride in diametrically opposite slots as to be seen at the mating part of the helix / iris modue.   The screws simply serve to retain in the lens mount the large brass component which is the slider.  (See later notes here).

Rear element removal

The rear element is held by a slotted retainer ring but I found that in removing the similar slotted high shroud,   all came out as one module.  Very convenient.

Back to the internals of the auto-iris mechanism

The central part of the lens mount end through which the M42 iris pin (and any possible  ES slider)  protrude, is simply held in place with contact adhesive.  It is a fairing.  Nothing more.  If this is carefully prised off one can see what is going on inside the auto-iris mechanism and depending upon the reasons for the strip-down it may be possible to fix any malfunction without any further disassembly.  Otherwise observe the following.

If the lens is an "ES" version one must first remove the large brass capping.  When properly functioning the whole rotates because the "ES"slider is in fact part of it.   The whole is to be a well lubed precise fit.

To remove this large brass component one must provide a removal path by  first removing one screw and pulling  off the large iris regulator arc lever as together with its underside spring.  Note that the central securing screw has a small copper washer under its head.    The regulator arc lever will lift off together with its dedicated spring beneath.  Note that there is yet another small copper washer on its underside.

The large rotating brass component can then be lifted off to reveal the circular platform as carrying the entire auto-iris mechanism.    It is an interference-fit in the machined deep-cup lens mount housing and it can be lifted out complete.     The entire mechanism is straight-forward to troubleshoot and service.

Again ....  I mention the "ES" version of this lens.

In addition to the M42 iris pin and the aperture slider,  the ES version also features a very small spring-loaded pin which projects approximately 1mm.   When the lens is mounted on the camera this pin is pressed flush.   Why it is there could only be explained by someone familiar with the ES camera metering.

Regardless - its function is to lock the "Auto /Manual" slider in the Manual position when the lens is removed from the camera.  Mountng the lens releases the lock.

How it does this is simplistic and totally apparent when one looks into the auto-iris mechanism.

I have mentioned the above feature because it is not beyond the realms of probability that some non-aware owner of such a lens may, upon finding the Auto/Manual slider "rock solid"  - deem the lens to be faulty and in need of a strip and sleuth.     Just mount the lens on a camera and unless something is genuinely amiss  - all will come good.

About the lens-mount central outer component

This is that to which I refer earlier and as held in place with contact adhesive.

Removal of that component provides a good view in respect to the following, Auto / Manual Slider.

The Auto / Manual Slider is dust-sealed underneath by a strip of typical Japanese light foam rubber which is in turn held in place by some sort of specialist adhesive tape.    As with light seals and mirror-buffer strips,  the aging process converts this foam rubber into  black 'orrible sticky gunk.   The "specialist" tape is even worse.     The contingency of a few specks of dust  in the auto-iris mechanism is much to be preferred over complete contamination with dead rubber gunk.  So you might want to get rid of the lot and clean up the aftermath.

The main lens group

The first ring which holds the front element has a stupid slotting arrangement as with the slot passing only through half the ring thickness from  its outside diameter.  The manufacturer configured it this way because the inner edge of this retainer ring is (just) visible beneath the lens nameplate and it was obviously deemed that a slot would spoil the aesthetics.  Only a special custom-made tool could therefore loosen it.   Such poor design is most surprising from the stable of the revered Asahi Pentax.   A perfectly valid solution is to use a Dremel-Tool end-mill bit to extend the the slot  across the full thickness of the ring.  One can then remove the retainer ring with conventional , non-special, tools and touch up the shiny spots with matte black upon completion of service work.

A similar problem also with the 2nd element down but this time for no fathomable reason.

The retainer-ring slots are no wider than 1mm - albeit that in this case they traverse the full thickness of the ring.

The ring is also sealed with shellac (or whatever).     So we have a very thin-walled ring which is likely to be very stubborn;   well torqued down - and locked with varnish.

My answer was to run the tip of my Xacto knife along the screwed joint so as to remove as much varnish as possible  - and then to trickle in a drop or two of acetone and leave for a couple of hours.  This is normally quite effective but on this lens the ring slots were so narrow and the ring so tight as to bend and destroy my stainless-steel key.  Thus I resorted to widening and deepening the retaining ring slots with my Dremel end mill.     Then with a more robust tool and a lot of muscle - the ring came off.

Reassembling

Refit the lens-mount auto-iris mechanism housing to the helix / iris module.

Install the aperture ring as together with its detent ball-bearing.

Test that it rotates and detents and that the iris blades correspond with whatever aperture is set.

The aperture ring is held axially by the rear face of the focusing indexer ring.  Install the indexer ring and push it firmly against the aperture ring.   The indexer ring has now to be backed off by the thickness of a piece of photocopy paper - otherwise the aperture ring will be stiff due to friction.

Take care with the rotational positioning of the indexer ring.

Position it approximately but only very slightly nip up the set screws.   If they are tightened down hard then their points will create an indent below and those points will subsequently follow the indents - making fine adjustment almost impossible.    With the aperture set to ƒ22,   set the index  line to bisect the two 2's and then tighten the set screws (but not overtighten).

Now  screw in both lens groups and set up the focus as earlier described here. 

All that remains now is to refit the filter ring (3 screws)  -  and the lens nameplate.

Lens carcass cosmetics

If there are wear marks, shiny spots or little dings .....  

Provided the surface is thoroughly cleaned with refined petroleum or alcohol  - it is quite amazing what can be done with one of those felt-tipped  black indelible Marker Pens.

I welcome any criticisms, corrections,  shortcuts, smarter methodology, or any comments which will serve to reduce my fallibility and enhance my capabiities.

Stuart Willis.
September 3. '04.