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Adjusting infinity on a Tamron BBAR 05A 4.5/70-350 mm
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PostPosted: Sat Apr 18, 2015 7:56 am    Post subject: Adjusting infinity on a Tamron BBAR 05A 4.5/70-350 mm Reply with quote

Hi,
maybe someone can help me with this issue. I got a Tamron BBAR 05A 4.5/70-350 mm. I have now 2 adaptall adapter, one for Nikon and one for C/Y. Both allow me to reach infinity with other lenses on my NX30. I also tried my Nikon adapter for mFT but I am also unable to reach infinity. In my attempts to find a screw, I checked the 3 rubber bands of the focus, zoom and aperture rings. I even tried to check at the aperture level and now have a declicked aperture (which was not my intention). I have the found the small ball, but have no idea how and where to insert it. But that is not a big issue.
The most important question is how to adjust infinity. Does anybody know how to do it?


PostPosted: Sat Apr 18, 2015 8:40 am    Post subject: Reply with quote

Tamron does kind of a strange thing with at least some of their zooms. They actually hold barrel sections together with tape. And the way the barrel sections are positioned can affect infinity focus. With the two Tamron lenses that I'd dismantled to that level, it was possible to adjust infinity focus by untaping the sections, adjusting their positions and then retaping them. Sounds strange, I know, but it works. The first Tamron lens that I dismantled where I discovered this was an AF 24-70 that I bought new in 1994. I dismantled it probably seven or eight years ago, for some reason that I can no longer recall, and replaced the tape, since it lost its adhesive properties after I peeled it off. And I didn't replace it with anything fancy either. But the tape I used is good against shear, which is the sort of movement it must resist. That was seven or eight years ago, and the lens is still functioning normally. I don't recommend electrical tape, which is not good against sheer. The tape I used was actually the blue 3M masking tape, believe it or don't.

Now, I assumed that this was something done with just a later type of zoom, especially AF, but when I dismantled an earlier adaptall-2 Tamron zoom last year, I ran across the same tape situation. This time, though, I left the tape alone.

If, upon inspecting the lens closely, it shows evidence of being previously dismantled, there's always the possibility that the person who did the "work" may have separated the helical's threads and put it back together one thread off. I've done this quite a few times before and ended up having to put things back together trial and error -- double checking infinity focus after rethreading the helical each time. This might be all the problem your zoom has. But sometimes, especially with zooms, getting to the point where you can unscrew a helical can be rather involved.

If you''re gonna try and do the work yourself, my first suggestion is that you document your disassembly every step of the way with your digital camera. It'll really help when it comes time to reassemble things. My second suggestion is that you make sure you have the proper tools for the job of lens repair. Often you'll need a set of jewelers screwdrivers, a lens spanner, and an assortment of large rubber stoppers. Good, fine-pointed tweezers is often essential too. That'll be enough to get you started, leastways.

Now as for that ball that fell out that you found, first of all, I'd like to say "congratulations," because when this happens to me, I'm almost never fortunate enough to find the ball after it drops out. If you look at the portion of the barrel and aperture ring that the ball ran against, you will most likely find a small hole, usually in the aperture ring, with a very tiny spring inside of it. That's where the ball goes. The spring is what pushes it against the click stops. Good luck putting it back in place without losing it.


PostPosted: Sun Apr 19, 2015 7:55 am    Post subject: Reply with quote

Hi and thanks for your helpful reply. Indeed, that sounds strange and also not very precise. I don't know if I am going to completely disassemble the lens, it seems like an awful lot of work. Additionally, I could not find any schematics on that particular lens and I hate flying blind.
I just thought that many MF lenses would allow to adjust infinity with just a screw and this would be the case too for the Tamron.


PostPosted: Sun Apr 19, 2015 1:45 pm    Post subject: Reply with quote

Under the rubber on my 05A is this heavy metal tape. i only peeled it back a bit but it looks like the focus ring is split as Michael said (this is actually a common design for lenses of the era) and there was a scratch mark crossing the join which I have ringed in red. Not sure what the screws are for, on my 28A there was a similar fitting but in a slot and that was definitely to allow some micro adjustment of the focus.
The trouble with focus adjustments on lenses like this is that there are normally separate adjustments for opposite ends of the zoom range.
How much are you out of hitting infinity? If it's substantial then your lens has major problems. If its only just missing eg only really noticeable at low f, ask yourself if you can live with it. if not, then take the metal tape all off (make sure you make a scratch to indicate starting position of the split ring if one isn't there) and ther should be some way of making micro adjustments by altering the relative positions of the two halves of the split focus ring.



PostPosted: Sun Apr 19, 2015 3:03 pm    Post subject: Reply with quote

Thanks, that is super helpful! Both at NX and mFT, infinity is not too far when maximally focussed. The lower end is not really interesting to me, but 350 mm is. I also don't care if there is focus beyond infinity, as long as I can reach it.
Marcus, I will try your approach.


PostPosted: Sun Apr 19, 2015 9:22 pm    Post subject: Reply with quote

Miles, you may be aware of this already, but it is not uncommon for lenses with focal lengths of 300mm or more to be able to focus well past infinity. They build in this flexibility to allow for exapansion and contraction of the lens due to temperature extremes, which can affect infinity focus. You might want to check around and see if that was the case for you lens.