Tamron SP 90/2.5 aperture repair

This is a very nice lens, both optically and mechanically. This lens is a joy to use, especially the way image snap into focus.  I’m not sure what caused this, whether the high contrast & resolution, or the large aperture.  My other fast lenses, the Pentax 50/1.4, or CZJ Sonnar 200/2.8 don’t have this effect.

I happen to own 2 of these fine lenses, both are unable to stop down more than f4 or f5.6 when fitted with PKA adapter. It appears to be a common problem,  one that has been bothering me for a couple of years or so.

This is my most expensive project so far. On my quest,  a lens element decided to stuck itself in the lens tube. I have to use  heat to convinced it to come out. It worked, the element comes out, real fast, straight into the table. Please contact me if you’re looking for a slightly broken copy of this very fine lens. After all this trouble, the fix was embarrassingly simple. Lessons learned, next time try the simplest solutions first.

Step 1.

Remove the 3 screws (A). After this you could lift the both the rear mount (B) and aperture ring (C).  Again, be careful of the spring loaded ball bearing behind the aperture ring. You could also remove the aperture ring later.

Step 2.

These are the parts of the rear mount. There are some washers glued to the second ring.

Step 3.

Remove the 2 screws holding the rear aperture assembly. Note that I haven’t remove the aperture ring (B).

Step 4.

This is the metal hook (A) that hold a spring. In both my copies it is just glued in place. Use a metal hook to remove the glue, then move it so the spring has less extension. You have to experiment on the correct position. Fix the part with glue once it is in the correct position.

I tried it on both my copies. and now both can stop down to  f 32.