Adjusting the coaxiality of the 3. lens group in an EF 24-105 IS USM lens

First of all: this is NOT an official method for adjusting the lens.

You will need this procedure after changing the aperture flex cable, or if You have a lens with not sharp image.

Changing the flex cable

If You have to repair a 24-105 aperture flex cable, take care that the aperture module is not intended to be disassembled.
It is possible to order the flex cable for it, but be careful with the dismantle of the front element of the aperture module.

Picture from here: http://www.bobatkins.com/photography/reviews/canon_ef_24-105_review.html

On the image the middle group is the aperture module, in fact it is the 3. from the left side.
This group contains the electronic aperture itself, an another mechanical  aperture and some lenses too.

If You disassemble it once, the lens will be no more aligned, which will cause aberration – for example chromatical aberration on the picture.

Here is the complete 3-4-5 group.

And here only the 3. group.

Pictures from: www.youtube.com/watch?v=hFR7ZG341kQ

Be careful – this video does not speak about the eccentric pins, and the importance of their position (they are on the left side of the table on the last picture).

The front lens of this group is not only fixed with 3 screws to the body, but it is adjustable with 3 tiny eccentric pins, when the 3 screws are untightened.

I believe that this operation (the optical alignment) must be difficult even in a professional workshop, in home conditions probably impossible – but if You look to this small adjustment pins, You will see, that they have a marking point on their head.

That point marks the thinnest (minimum) point of the excentric.
Make a sketch with the original position, then take them out from their hole. They are simply pressed in the plastic. You will need for the adjutment a hexagonal (allen) key.

 

If You put the eccentric pins exactly in the original position, you have 50% chance to get the lens in the right position: yes or no….
Ok, it depends, how precise You work too. But if the pins were forced in the past – this was last time in my case – then they will not give a well determined position to the front element. And the photo will have chromatic aberration, and not a sharp image, as this lens should give.

 

The procedure

(I dont know what is the factory method. I only know, that if we adjust the pins, the lens will occupy a lot of positions, and its axis will match the axis of the rest of the lenses in only one position. So we have to find that one) 

To control the image (the photo) is possible, when the lens is assembled.
To make the adjustment, is possible only, when You have full access to the pins.

Ouuups.

There is still a way to fulfil both conditions: to assemble the lens without the USM focusing mechanism and without the electronic board, and to turn it to the 105 mm zoom position.
This is the only position, in which You will see the eccentrics through 3 holes made for this reason.

At first, You will have to loosen the 3 screws, which fix the adjustable ring with the lens.
Then bring the 3 eccentric pins into their middle position. The have a making point on the head, that should be toward the front of the lens. (this is to start with adjusting from the theoretical concentric point with the barrels axis)
Now You have to assemble the lens without the USM module, and without the electronic board.
The focus lever is loose now, so you have to fix it for ex. with a rubber ring, or with adhesive tape (as we will se later: in the infinite focus position.)
Now put the lens on a camera. You have now a perfect manual lens, at maximum aperture.

Zoom to the tele position.

We need now a shiny regular object at the infinite

You have to make an artificial infinite object.
I used for this an old film camera, putting instead of the film a matt plastic plate, on the middle of which I glued a black foil with a hole in he middle. Important is, that the hole MUST have perfect sharp edge. Behing I placed a sheet of paper, and a very strong white LED light source.
In the camera I mounted a 200 mm / F4 manual lens (NIKON :)), rotated on infinite position.

If You look at it from the front, You will see the picture like it would be at the infinite distance.
So, if You look at it from the front with another camera, and You use the autofocus, the focusing will turn the lens to the infinite position.

Here is the focus lever, fixed with some rubber rings.

And here is the hexagonal key.

But now You have NO autofocus on the tested lens, so You have to turn manually the focus lever till You see the image very clear in the eyepiece.
With camera with liveview is much easier.
If You take a photo, You will observe, that the shine white circle has a blueish coloration on one side. This is the sign of the bad alignment.

This was the first picture, after assembling, before adjusting. (visible problem on the left side)
Now try to rotate the eccentric pins (possible ONLY in 105 mm zoom position), till the blue collar dissapears. This is possible perhaps only after adjusting all of the eccentric pins.

This is on intermediate position. (problem moved to the right side)

And this is the final picture.

When You are ready, tighten the 3 screws.

After this, try to make a shot on a distant object (200-300 meters), which has the sky behind (having  a strong contrast).
If the adjustment was OK, You will get a sharp picture, with no chromatic aberration.

The hole thing works only, I the lens is not misaligned for example on the 5- group (the lens which is on the camera side)
NEVER loosen a screw which is glued by the factory – only if You are sure, that You can adjust it correctly.

 

If You can’t do it, drop me a mail.

 

(any text, figure, photo from this article is made by me
– excepting that one which are special mentioned –
case You use it somewhere I appreciate, if you mention the source )
© canonrepair