If You ever repaired a Tamron lens, then You know, that many zoom-lenses use as last element a special multiple element, which is in fact assembled from 2 smaller elements, which are centered to each other only by the force of the 3 mounting screws (this means, after loosening them, the lens cannot be used anymore without an adjustment), and the whole assembly is centered (adjusted) to the lens optical axis with other 3 screws (which have the same effect: if You loosen them, the lens will be unusable).
Ok, if you don’t dismount them during the cleaning, it’s no problem. But after a certain time between this lens elements You will get some dust – disassembling will be unavoidable.
I learned this after assembling the lens, and giving it back to the owner, without a exhaustive check. (That lens was a Tamron 17-50 F2,8). After repairing a Canon 17-85 for example You don’t need a special check,after changing the aperture cable in the 24-105 You have to check and adjust the sharpness)
He was not very happy with the result. The testphoto which I made after getting back the lens was this:
At that time I didn’t had the method which I will describe below, so it took me approx. a full day to find the best position for the two independently adjustable lenses.
Here is the result:
Amazing, isn’t it? After that I tried to find a more scientifical method. I have found a half-scientifical one.-
At first, here is the exploded view of a Tamron 17-35
And here is the Tamron 17-50 F2.8
On the right side is the difficult group. At the left side can You see the two screws, the higher one, which fixes the two half groups to each other, and the lower one, which fixes the group to the barrel. Under the lens elements, You have some washers too.
After taking it out the lens group, I have seen, that in fact it is a composed magnifying lens – which can be used even standalone as a lens for a camera – if You hold it to the right position in the front of the CMOS. At a right distance, and perfectly coaxial, of coarse.
You have to adjust the distance, which would be not easy – but I was lucky, in my case it was at a distance which corresponded to a 20 cm object distance, and in this way I adjusted not the focal distance, but the object distance, to get a sharp (or: ther sharpes possible) picture on the display.
If You mount a lens in the centre of a digital SLR body with liveview capability, You will get a picture on the display. If the elements in the lens are well centered, the picture should be the best. If it is not centered, you will get an unsharp picture – and a strong chromatic aberration on any line between a dark and bright transition with high contrast. If the lens is just a cheap piece of glass (like a magnifying glass), this will happen always, but always in a symmetrical form, if we are in the centre of the optical axis. If the lens is not collimated (the optical axis is not coincident with the systems axis) – You will get a non-symmetrical chromatic aberration.
So the method is simple: put the lens exactly in the middle of a DSLR, take a picture of symmetrical object with very high contrast – and adjust the two lens-halves till You will get the best picture quality, without chromatic aberration, or with a symmetrical one.
For this I made a special mount on a lathe.
It’s nothing else than a camera cap with 30 mm opening, and the front surface was machined to a perfect plane on the lathe. On that surface I mounted the last lens element whith 3 screws.
As a high contrast-object I used a fluorescent tube and a piece of a black adhesive tape, cutted to a 2×2 cm square.
The test bench was an old 40d.
Here are the pictures:
Here You can see that adjusting the elements one to the other in their separating plan, I have got different pictures, with the chromatival aberraton distributed in very different forms (unfortunatelly I don’t have the last (good) photo).
After fnding the best picture, I tightened the screws, then I mounted the element in the lens body, I repeated the measuring again, I adjusted the group to lens body screws, I tightened them – and the lens was perfect again.
Any comment is welcome.
I You repair something based on any of my descriptions, send me please a short feedback.
DIGITAL CANON repair blog 
Just used this info here to repair my old Tamron 28-200 AF Aspherical XR (A03). But instead of using a piece of tape on a luminous source, I used this focus chart http://www.bealecorner.org/red/test-patterns/star-chart-bars144-600dpi.png on full screen on my laptop and it worked perfectly. I tried to use the tape before, but I think using the chart is easier.
Thanks a lot for the info!
Thanks for the awesome post! I have the exact same problem and I’d like to ask if you could make a video on how to make the mount? Not really sure how to do that part.
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when you loosen the screws, you can move the lens a little bit on its place. that is the adjustment
Found just recently that my Tamron 17-50/2.8 is having exactly the same problem. Very blurry pictures no matter how I focus (viewfinder, liveview, manual). Not a single thing on a photo is sharp. I have no clue how it has become like that, I don’t recall having dropped or knocked the lens hard ever (even though it’s 8 years old already).
I’m interested trying to fix it myself. Your blog and photos sound promising, but I’m not sure I could create such fine mount like you had done on a camera cover. Would it be possible to get it done with naked eye? How?
Does adjusting the back element require removing it from rest of the lens assembly? What the adjustment screws actually do?
My old Tammy 17-50/2.8 appears to have got exactly this problem. It’s from 2008 so warranty has ended long ago. Taking it to service would be so costly that I could as well buy some new lens. But now I’m thinking I could as well try to fix it myself thanks to this blog.
Your method using a tooled camera mount cap sounds interesting, but I don’t have a lathe, and I’m not sure I could do it to precision to be used here. 70D has liveview though (even with 100% magnification) Is it possible to try calibrating the back element somehow else, by viewing it with naked eye?
To calibrate the back element, is it required to remove it from the lens assembly?
Is it possible to try calibrating the back element somehow else, by viewing it with naked eye?
For sure: NOT
Thanks for this article! I have a 17-50mm for my Nikon that I’ve messed up after attempting to remove dust particles from the rear-element. It seems as though I caused decentering after having removed the rear element itself. So if I’m not mistaken, it looks like in your photos you removed two pieces that comprise the rear element?
I was watching this youtube video and the guy removes what appears the be one component of the rear element:
Did you actually remove another component besides what he removed in the video in order to do the adjustments?
Here is another post I was referencing:
http://www.clubsnap.com/forums/showthread.php?t=348918
i dont quite understand what adjustments you made to the lens group. did you just add and remove shims untill it was right ?i played around with mine and got it perfect , then i took it apart again and now its worse than before and i cant get it sharp again. all i did was add and remove shims but it doesnt seem to help at all.
the holes are bigger then the screws, so you can move the element in the plane, when the screw is looseened. after you get the best picture just tighten the screws.
Thank You!
This article was the reason that I did not give up adjusting the back element of my Tamron 17-50. After several days of trying, I’ve got it right.
The story: I got a broken one, with some missing parts, and I bought an other one which was dropped by the previous owner, and had to replace the aperture module.
I replaced it from the first one, but the focusing was way off (first, I thought it was the focusing). Then I figured out (by the help of this article), that it’s not the focusing, it’s just the lens elements are not perfectly centered, so I started to mess with them, and it’s finally sharp enough.
Köszönöm János!
i’m happy to hear this 🙂
Is that…. a Tamron 17-35/2.8-4? :DDD
Yeahhhh