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.
Tamron SP 90/2.5
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.

Tamron SP 90/2.5
Step 2.
These are the parts of the rear mount. There are some washers glued to the second ring.

Tamron SP 90/2.5
Step 3.
Remove the 2 screws holding the rear aperture assembly. Note that I haven’t remove the aperture ring (B).

Tamron SP 90/2.5
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.


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Comments

brett
19/12/2011 16:55

Hi Dipo,

Thanks for sharing all this amazing information. A question for you regarding the last line in your instructions (I tried it on both my copies. and now both can stop down to f 32)
as I have a similar problem with a pentax 85 to 210 that I am planning to fix.

How do you know that it is going down to f 32. In other words how do you calibrate it to know that the smallest setting is in fact f32 and not f30 or f34 for exemple?

Thanks a million
Brett

Reply
21/12/2011 00:13

Hi Brett,

Thank you for visiting my site. Short answer : I just shoot some test picture, and compare the results :)

Long answer : In the Tamron SP90 there is a tab that limit how far the aperture actuating lever can go. So the real aperture will not exceed the marked value. eg it won't close down to f34 when you set it to f32.

The spring will try to pull the aperture actuating lever as far back as possible. It will not stop until it hit the limiting tab, or it can't pull anymore. So if the aperture blade moved when you change the aperture selector from f22 to f32, you could be confident that the f22 is correct. But the f32 might be actually be f30.

The best way will be to open the lens and check whether the aperture actuating lever hit the tab or not, but that will be very troublesome with the Tamron SP90. I think it is easier to do with Pentax.

The Pentaxes that I've worked on have different type of aperture mechanism than the Tamron. They will be easier to fix.

I hope this help.
Regards

Reply
Dung Ngo
08/04/2012 23:22

Hi Dipo,
Great post. Thanks a lot for the pictures and how to fix it. I actually did the same mistake and open up everything down to the aperture blades inside the tubes. It was a pain to put it back. It was lucky that I can put everything back but in the progress, I messed up the stop down mechanism. When you pushed the leveler (the ball!! or whatever it's called), the aperture lever supposed to to hit the limiting tab but it doesn't. Apparently, I bent the push down thing and it has a little more room to move. I put some paper in there to prevent the moving parts but unfortunately for some reason, when I screw the screws back tightly, I can't stop down. If I loosen the screws then it's fine, but then it's really loose. Do you have any idea how to fix this?

Reply
Dipo
18/04/2012 09:51

Hi Dung Ngo,

Sorry for the late reply. I can't give you a specific answer, as it is very difficult without actually looking a t the thing. Paper might not be a good idea. My first suggestion is to try slowly bent the thing back into shape. Please check whether other parts of the push down is touching the barrel or other parts.

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