When trying to find ways to fix my Tamron SP 90 f2.5 aperture, I had to disassemble the whole lens. This tutorial cover the rest of the lens.
After removing the 3 screws (A), unscrew the cover (C). Then remove the 3 screws (B) to remove the metal part.
This is how the lens lok after removing part (C) in th step 1. Note the aperture detent ball bearing & spring. Remove the spring for safe keeping.
Now you could remove the focus scale. The adhesive tape that hold the 2 plastic rings determine the infinity focus adjustment.
Remove the 2 screws and plastic housing on each side of the lens. After this you could take out the plastic shell. Note I took this picture before removing the focus scale.
On installation, do the infinity focus adjustment as the last step, just before attaching the rubber ring.
Attach the lens to a camera. The idea is to make the focus scale shows the infinity mark when the lens reach infinity focus. Focus the lens to infinity, aim at an object 300 meters away.Then carefully move the focus scale so it point at infinity. Stick a short (~1 cm) adhesive tape so these tube won’t move against each other. Double check the focus. If everything is fine, then wrap the longer adhesive tape around the tube.
Moving to the front. Remove the front ring with a friction tool.
The screws (A) hold the front lens group. remove these and then remove the front lens group. Be careful, if these screws are removed, nothing will hold the front group. The position of the group will affect the aperture size. On installation, set the aperture at f2.5, then adjust the position so it just at the largest opening.
Removing screws (B) will enable you to access the helicoids. There are 3 of them. I wouldn’t recommend messing with these unless absolutely necessary.
I took this picture when I need to readjust the aperture size. If you need to access the lens elements or aperture blade, remove part B. The lens elements are all in A, attached with rings. Be careful when removing the middle elements, do it slowly. Mine got stuck in the tube, and I have to use heat to dislodge it.
If you need to access the helicoids, remove the 2 screws on each side. Note the position. On reassembly, you have to position the helicoids into this position again ie. they all reach the minimum position at the same time. Mark each release point, and count how many turns needed for each helicoid.
These helicoids should be assembled so they reach their shortest length at the same time.
Reassembly of these part is a spiritual experience.
These are the front groups. The grease on both my copies are still in good condition.
When installing, arrange the rearmost helicoid so the slot for the aperture lever are opposite the ball bearing position.
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.
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.
These are the parts of the rear mount. There are some washers glued to the second ring.
Remove the 2 screws holding the rear aperture assembly. Note that I haven’t remove the aperture ring (B).
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.