Canon 70-300 DO Autofocus has become inaccurate

[Mel_P]

Hello - I am not aware that I have dropped this or damaged it but I went out with this lens to test my new Kenko teleconverter on my Canon 5D MkIII and my 7D MkI and the images were all slightly blurred. I haven't used my 70-300 DO f4.5-5.6 IS USM for a couple of months.
(PS it's not the teleconverter - this is perfectly sharp with my 100mm f2.8 IS USM macro lens).

At home afterwards I scratched my head as to why my images of the Egret were blurred so I did some testing at home.

- if I do an autofocus and then switch to live view the image is not quite sharp - I have to manually focus slightly nearer to get sharp.
- if I do a manual focus in live view and then switch off live view and change to Autofocus the focus changes to slghtly further away but is not sharp
- manually focussed images are sharp.
- same results on 5D and 7D so it's definitely the lens.
- auto focussed images with my other lens are in focus.

So it seems an issue with my 70-300 DO lens - I have tried the AF microadjustment on the 5D and adjusting to -20 improves the focus but is not quite there. If I send it back to canon can they do some adjustment / repair (can't see how damaged - works perfectly in other respects)?

Anybody with knoweldge about these things please?

 

[Mel_P]

Perhaps I haven't explained enough.

I just happened to notice the problem when I was using the teleconverter - my first reaction was to blame the teleconverter - but it's not that causing the issue. It seems to be the 70-300 lens itself.

- The same behaviour is observed with or without the teleconverter
- the 5D Mk III happily autofocusses at f8 - I have the new firmware

HOWEVER, whether it autofocusses at f8 is not the issue.

Auto focus works both with and without the teleconverter but the autofocus is inaccurate in both cases. After auto focus is achieved, with or without the teleconverter, the image is not in sharp focus but needs an extra manual "tweek" to focus the image sharply. Not a big tweek but enough for the captured image to be slightly out of focus. Same effect seen on 2 camera bodies.

PS - the 7D is not supposed to autofocus at f8 but with the kenko it actually does - I didn't expect it to!

Canon teleconverters are very restrictive in their use - only on expensive L lenses - As I suspect only then can they guarantee no drop in quality - but the kenko one works fine for me and others (from a web search before buying).

 

[Mel_P]

He said that the same problem happens with or without the TC? I suspect there's more going on here than tolerances.

I fear you are right - but it's only this lens - I don't really want Canon to change either body as they are both spot on with 4 other lenses! I hope I can just return the 70-300 lens. I have emailed canon - omitting an mention of a TC which has only served to confuse the issue - the problem is there with both bodies and the lens on its own.

Perhaps I will return the TC!!

Mel