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tamron SP 28-80 diaphragm problem
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PostPosted: Wed Oct 06, 2010 8:42 am    Post subject: tamron SP 28-80 diaphragm problem Reply with quote

Hello. I have recently bought this lens (quite cheap), and it had a strange problem: when you zoom from 80 to 28, the diaphragm closes about 3/4 point, and this without actuation the closing mechanism. So if you have the lens mounted on a camera and zoom to 28, the apertura will not be 3.5 any more, but something like 4.5. I could be live with it, but it will of course disturb the auto exposition in my pentax MZ-5. After disassembling the lens, I have reached the conclusion that the close down lever has not enought travel to take the diafragm from wide open to fully closed (at f32) so you have to make a choose. I have tried all sort of adjustements, and finally have left the iris working as it should for wider apertures, but now it only closes to about f11 at 28 mm and f16 at 80 mm. Not a big problem, but somehow disturbing when using aperture priority. Of course, if the camera "choose" f16, it will close only to f11 so overexposing.

Any clues??. TIA

Javier


PostPosted: Wed Oct 06, 2010 5:15 pm    Post subject: Reply with quote

Just a throw away comment, but I know that some of the longer zoom lenses are designed with a different maximum aperture when at the longest focal length compared to the shortest. Could this be true of this lens? Perhaps the aperture is designed to automatically adjust to give constant setting throughout the zoom range EXCEPT when set on minimum aperture.

I don't know this lens, so just a thought.


PostPosted: Wed Oct 06, 2010 5:50 pm    Post subject: Reply with quote

Mine has the same behaviour and I assume it is a feature instead of a fault.
I guess the max aperture @ 28mm must be around f2.8 and the changing aperture is used to give a constant f3.5 at all focals.


PostPosted: Thu Oct 07, 2010 7:52 am    Post subject: Reply with quote

Thank you to both. I was suspecting something like that. Anyway there was some haze in an inner element and the zoom was very stiff, so I had to dissasembly it.

Would it be possible to measure the exit pupil with a caliper or so just to see if the aperture is the correct one?. I haven't tried it but I guess the accuracy will not be enought to get a conclusion.

As I have already "repaired" the diaphragm, I will try it with my pentax to see if the exposition is right. If it is not, I will restore it to its previous condition. By the way, this is a very easy lens to work with. Great building quality also.

Javier


PostPosted: Thu Oct 07, 2010 3:13 pm    Post subject: Reply with quote

OK, I have measured the exit pupil at 28 mm and it is exactly 10mm,so:
28/10=2.8. You were right, the diaphragm should be partly closed in the "full open" position. For 3.5: 28/3.5= 8 mm. I suppose this feature is intended to avoid a probably bad quality image at 2.8. Curiously at 80 mm the stated 4.2 is optimistic. More like 4.8 in my lens, supposing the focal length is really 80 mm.

Now I have to undo the "adjustment" I did. Next time I will ask here before starting playing around Embarassed . Thank you again