Equipment Help with Canon 24-105L

I think he means that if they have to tweak something on the 40D to get his 24-105L to play nice with it (that makes his other lenses perform worse) he won't be happy.

Dan

 

Sorry, I know sarcasm is difficult to express via the internet. I know what he meant. There isn't much to "change" in a camera. They can't move the film plane or change the angle of the lens mount. If it's a straight front or back focus, they'll just tweak the lens. An alignment issue, which this sounds like, will mean a rebuild. They need the camera, because of the sample variation in the bodies. not every film plane is the same length from the nodal point.

 

oops

I find it helps to use [sarcasm]these tags[/sarcasm] to make sure everyone is on the same page

Dan

 

Canon wants my working 40D along with the defective lens for the repair. I don't why and all the pictures taken with it turn out great if the lens attached to it works. My worry is Canon will go the extra step of checking the camera body as well and make unneeded changes to it, effecting how my other lenses work with it.

Considering they did not bother to fix the broken lens the first time messing around with my perfectly functional camera body is an invitation for disaster. Every camera is different and even the slightest changes can lead to a re-learning period to figure out any new quirks that might come up later while in use. If it ain't broke don't fix it.

 

As has already been said, it might just be a simple problem of front/back focus. Rather than mess about adjusting the lens - can be time consuming, Canon will tweak the camera's auto focus system for the problem lens

Not sure about the 40D but many of Canons cameras have a "built-in" table that lets the camera behave differently depending on the lens, thats why they want the camera

We did not raise this before - but what is performance like when you focus manually ?

 

The auto-focus works well but manual focusing is an option for slow moving birds and landscapes. For the 24-105L it has a tendency to make close objects in focus but distant objects blurry, it almost as if the lens is nearsighted and needs glasses regardless if auto-focus in on or not. In the letter to Canon I will ask them to check the lens glass elements for defects in addition to rebuilding it for recalibration. If Canon modifies the internal focus chart to accommodate the broken lens, all my working lenses will have focusing problems.

 

They, for the last time, should not modify your camera to fit the lens (if they know what they are doing). They now that if they do, it will mess the other lenses up. The 40D doesn't (to my knowledge) have a AF fine-tune. They need the body to calibrate the lens. Out of the factory, the lenses are calibrated to spec. Sometimes there is a small variance, and need to be adjusted.

Your issue doesn't sound like a simple front/back focus to me. It sounds like an element or two is out of wack. They need to re-build the lens, basically re-aligning all the various lenses and mounts. Then they calibrate it to your body. if it was a front/back focus issue, it would just appear as a shift in the plane of focus. you have bigger issues.

Your best bet it to talk to the person working on your lens, and understand the problem and what they are doing. Good techs will talk to you about your gear and what they do.

 

Canon more often adjusts the body, as it's a quicker easier software fix for them (adjusment to af). But this is general, without seeing/testing your lens I wouldn't know if it's fundamentally a camera adjustment or lens adjustment in your case.

Canon have what are refferred to as tool lenses, these have additional adjustments on them so they can take that lens and make it neutral tolerance (in the middle of the factory tolerance) that they then should use to adjust any bodies to. Ditto for adjusting lenses, they are meant to have a neutral body to adjust lens to - unfortuantely rather than the cost/time/skill required to do this (in particular to keep those items neutral, and to buy them on the service departments budget) they find it better/quicker/easier to 'assume' one of your items the neutral standard and adjust to that. Having caught them red handed doing this, it was frustrating to hear them just squerm and wriggle to try and get out of it, they even tried to blame me claiming I had told them to adjust the system outside of factory spec/procedure. They also threw it back in my face that I had said a given test shot after an adjusment was ok - to which I had quantified that reluctant 'ok' by stating at time, that the wrong af target was used, the wrong distance, no variation in lens starting position and insufficient test shots were taken, thus that test shot was unrepresentative and should not be used to ascertain whether the system's af was working correctly.

But I feel in any case, they should not have both the lens and body. Each should have been produced in it's own right to a spec and tolerance, so that it works with other compatiable items in the canon eos system. I would not give them a body that you know/beleive to be working fine in case they should damage it - and I speak from direct experience here in that their carelessness has caused consequential issues/damage to my systems on two seperate occasions.

 

That seems a bad experience 3dheli, lets hope computer gremlin does not suffer in the same way.

If as JJ says above, the 40D doesn't have a AF fine-tune by lens, then not much point in sending the body although they might just be wanting to check for themselves that there is not a problem with the body.

 

Remember just because the 40D doesn't have firmware for AF fine tune...that's not to say that Canon does not have access to it.

 

. . . and Canon have other choices too, such as just adjusting the camera body for;
a) all af points (globally, simultaneously, one adjustment all af points) for all lenses, but having only just tested one (customer) lens.
b) only adjust centre af point (globally) for all lenses, having only just tested one (customer) lens.
c) do (a) but having tested a range of lenses, or all lenses the customer has.
d) do (b) and (c).
e) do (a) but use service centre tool lens, and not not test with other lenses.
etc etc

Hence, when/if the technician doesn't take notes, and he makes it worse in some way, then it can start to get very complicated to try and ascertain where it's gone wrong, the technician can quite literally tie the system up in knots with a + compensation on one item to balance a - compensation on another item, etc, etc. Think of multiple visits (whether different or same technician each time) and them focussing (no pun intended) on the fixing the new issue by making another change, rather than going back an undoing the error they introduced in the first place.

They do have a set procedure, a methodical method, but they just didn't understand it nor practicie it when I was their. For example, at 50x focal length they thought the factory set the cameras up to focus 40% in front, and 60% behind. This is not what the factory does, and it's not what one should do, as at the distance of 50x focal length (2.5m on a 50mm lens - Canon Inc's recommended test distance) the dof is 50% behind and 50% in front, thus the technicians approach is adding an error. The dof only gets to the 'generally held belief' of 66% rear and 33% in front at the point of infinity. Canon technicians working for CPS (Canon Professional Services) Elstree London should have known that - if they didn't know it then they should have just stuck with Canon Inc's recommended af adjustment procedure, rather than each individually making their own approach up and their managers then trying to pass it off as an officially approved method (sanctioned by a specialist).

I don't know if they still do this, but when I was there they offered pro photographers a choice of where the af was adjusted to, e.g. 50% front 50% rear, or 40% front 60% rear etc. Well this is madness - it might/could make sense to optimise the system (make the adjustments) based on tests at a particular distance as suited to the users type of photography, so for someone who only does macro maybe test closer, for someone who only does landscape test further, for someone who does tiddly winks photography professional test closer, etc. But, personally I'd still weight the adjustment in favour of the 50x focal length distance - and if required test at additional distances if they are causing specific issue. The 50mm f1.4 is/was often complained about when focussing at infinity (landscapes typically).