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.