My customer has found it for a bargain price (no, in fact was not so cheap…) – but the lens has been dropped before.
I had to disassemble it, repaired the USM motor (see: refurbishing a USM) – and after cleaning and mounted the lens has a terrible resolution.
This lens has two adjustments, which could be done in the service: the shift adjustment on lens 13 from the group 5., and tilt adjustment on group 1.
The problem is, that you dont have access to group 13, as it is covered by an external tube, and by the USM motor.
Let try to adjust the lens with my favourite method: to get a symmethrical chromatic aberration , which means, case you look at a very bright light source on a black background, in the form of a triangle or a circle, with the lens in a slight defocus position, the sharp black/white transitions will be coloured in orange or blue. Case The lens is centered, the colour margins will be symmetrical. As a light source I use a cheap LED-lamp with a magnetic base (sold by Lidl), plus a mask made of a metal sheet or whatever you have with a perfect triangular or circular hole (3-5 mm).
As a first method I tried to adjust separately ONLY the group 5 – so I disassembled the lens – but to put it in the front of a camera is relly hard with the horizontal axis…
So, I had to make a holder of aluminium (covered with plastic tape), and the camera was positioned with the bayonet upwards. It’s a great help in ths my good old Zeiss lentgh measuring stand – 80 kg of cast iron, with movable and adjustable table. Is a green beauty.
The DSLR Canon was connected to my PC, so I could follow the picture in live mode.
By the way: at first I hade to determine the optimal distance to the cmos, as the lens element is NOT a full photografic lens, as a lucky thing, it is a positive lens (magnifier), and by playing with the cmos – lens distance I have found the correct one to get a picture from a target, positioned at approx 100 … 200 cm , as this is the usable hight in my workshop.
The target lamp was positioned on the top of a pole (yes, should be something more elegant and better, and…. I hade no time for cosmetics now), on a steel sheet, so I could position the lamp exactly above the camera.
As Yo see, works. The 3 screws which hold the element no. 13 (as I understood) are only to hold it in plane – but the fixing of the xy position is done by… a glue, or paint.
So, adjustment has been done, monted is the camera…. image was terrible.
Ok, let’s do it with the whole lens. But how?!?!?
And when I took away the covering tube and the USM motor, I observed, thet the 3 screwheads are visible !!!
So, I repeated the whole thing with the half disassembled lens.
So, now I can access the element 13 – but the lens MUST BE focused, and I have no USM..
I took a focusing lever from an another USM, monted with a rubber on the correct place – and set it to the slight defocus.
This time I used the circular target.
The 3 screws areNOT tightened, so it is possible to move the lens to get the optimal image.
After this, just fixing the position with some paint (nail varnish).
After mounting together the whole lens, made the test shooting – the lens is perfect!
Try it! (and drop me a mail, if workes)