Canon EF-S 17-85 no focus at infinite – nem tud végtelenre fókuszolni

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. 



Some theory:

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



object distance





17 0,3 18,02120141
17 0,8 17,36909323
17 1 17,29399797
17 10 17,02894921
17 250 17,00115608
85 0,3 118,6046512
85 1 92,89617486
85 10 85,7286939
85 19 85,38197198
85 5000 85,00144502

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.


About canonrepair

Ruzsa János. Amatőr fotós, Canon DSLR váz. Amateur photographer. Canon DSLR user.
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3 Responses to Canon EF-S 17-85 no focus at infinite – nem tud végtelenre fókuszolni

  1. Phil Traas says:

    Wow, amazing work! You have great skills. I have a problem with a lens, and maybe you could help me? I took apart my Canon 17-85mm USM IS lens to replace the faulty aperture flex cable. The soldering went fine, and reassembly also was no problem. Auto-focus now works again, however now I have some serious back focus issues throughout the whole focal range. Any idea what could cause this, and how to rectifiy?

    • canonrepair says:

      as the focusing is mainly done by the camera, it’s strange. the camera will determine the defocus quantity, then rotates the ring USM with the determined amount in the needed direction. this will be measured by the incremental optical sensor which is in the USM unit. It could be wrong the position sensor (dirty), but then will have typically a no focus finding sympthome. it could be another problem if You made a mistake by the reassembling of the elements. hard to say from distance. Is it too complicate to send me the lens? In this case the repair – if you didn’t destroyed something – would be approx 45-60 EUR – plus the postal fee.

      • Phil Traas says:

        Hi there, thanks for your thoughts on the matter. I also don’t understand why there is an issue now! Very strange indeed. I won’t send it to you as I’m in Reunion Island, and it’s very far and and quite expensive to ship to Hungary. Thank you for the offer though.
        I think I will take it apart again and make sure everything is right and all the sensors and FPC’s are clean. Thanks for your insight!

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