It happened with a 17-85 which I have got to repair: focusing problems at the infinite end position.
As the lens was repaired before by somebody else, and as he was not very professional, I decided to take apart the lens at first.
In the inside there were many committed failures, the aperture cable has been changed in a very bad way (wrong soldering, the flex cable was forced, so it was absolutely necessary, to change it again. Some screws were also damaged and loose in their place.
After mounting it back, I’ve found the problem with the focusing: the front lens was loose, because the 3 screw had been not tightened. By this lens the front lens has a possibility of ca. 60 degrees of rotation, ith this You can adjust the infinite point of the lens.
Ok, I have done the adjustment (I rotated it to the former position, which had some markings on the plastic mount.). After this, I checked the focus on 85 mm, it was ok, so I gave back the lens.
(Normally I never take out the front lens by this lens, even for cleaning, exactly for this focus adjustment reason)
After 1 day, the lens came back: there is no sharp infinite by the 17 mm zoom position. Oooups. I should been checked it at all distances.
But what could be wrong?
Sometimes you must simple have luck. One day later I’ve got a 17-85, only for changing the aperture flex cable – but this item was surely never opened before.
I checked the front module (with the front lens and the focusing lens behind. No difference to the defective one.
And then, after taking off the mounting ring… I observed, that the good cameras mounting ring is a little thinner than the bad cameras ring.
On the ring there was a text: 2,046 (written by hand).
This is the thickness of the ring, in mm! The measuring precision is amazing.
OK, then how much is the thickness of the wrong one:
Ouuups! The difference is about: 2,41 – 2,05 = 0,37 mm.
My measuring precision is even more amazing: I use a Chinese gauge, the price was ca. 15 USD 🙂 I think Canon uses something better.
But is this the reason for the bad focusing? A short change – and Yes, the focusing is now perfect. Or, nearly perfect, because if I focused with the AF on infinite for zoom position 17 mm, and after this on the 85 mm zoom position, the AF made a small correction (this could be seen even on the focus distance ring.
The solution was a tuning on the front lens position. After adjusting it, the two infinite in-focus points were practically identical.
At this point the problem is solved (excepting the fact, that tomorrow I have to take off 0,37 mm from the bad ring on my good old Russian mini lathe (made in the USSR in the year 1968)). Or a little bit more or less – the two lenses are not perfectly identical.
One day later I’ve done the machining. It’s not so simple, because You have to be perfect parallel with the original planes. So the first operation is to fix the piece in the lathe using a gauge. In may case the final precision was 0,02 mm.
Now the lens is working at infinite, but at 17 mm focal length the 0,5 m is reached with ring position at 0,8 m. Anyway, the lens is workking.
It seems that by the EF-S 17-85 the focus should be adjusted at focal length 17 mm at the infinite with the thickness of the mount ring, and after then should be adjusted at focal length 85 mm with the angular position of the front lens.
But is it possible, that this tiny 0,37 mm can make such a big problem?!?!?
The answer is in the geometrical optics. I made a short calculation about the focus distances for different focal lengths and focusing distances.
f is the focal length, k is the picture distance, t is the object distance
As You see, the focal distance is in fact the picture distance if the object is at infinite distance.
And if You modify the position of the lens with ca. 0,37 mm, the picture will be sharp for the 17 mm lens at 0,8 m (!!!), by the 85 mm at 19 m. – instead of the original infinite.
So, here is the explanation, why the wrong mount ring has this big influence by the 17 mm zoom position.
The most beautiful think is to work after somebody else.