100-400mm on a 50D - not enough MFA Adjust

[GoldenSpark]

I've been having issues with my 100-400mm. I knew it back focussed when I got it, and I have to dial in some MFA on my 50D, but now I have done some more accurate tests I find that even when dialed in at -20 (maximum forward adjustment) on the 50D, it is still back focussing slightly.

This happens most at 400mm, and whether focussed near MFD or at infinity, if I just give the manual focus a slight nudge closer after an AF, the shot gets clearer.

I am surprised that the MFA seems to give so little variation, but that's a question about the 50D.

On the lens, has anyone else found their 100-400 back focussing this much?

I am annoyed that I have a camera that is supposed to be able to correct this problem, but the lens appears to be too far out for it to do so.

I am also not very happy to just send it in for calibration. Postal strikes aside, I know my lens is pretty good when I just nudge it, so several hundred pounds for a 2 minute software adjust grates somewhat!

 

[Paris Tsantis]

I'd send it to Canon for calibration. If you have reached the end of the MA scale and it is still off it means its waaaaay off! Plus, if you give it to Canon you will end up getting better results IMO. MA does not work as well as calibration by Canon for zoom lenses.

 

[GoldenSpark]

Why do you think it is way off? Do you have evidence for that? I've heard it said that the MFA adjust only makes very small changes. This certainly seems to be the case when I do tests. The change from +20 to -20 is really quite small.

 

[Green_Thark]

It means very small ;-)

P.S. I think I read somewhere the postal strike is cancelled

 

[Nigel Bolton]

On my 5D Mark II the MFA only requires -2 adjustment if set up at 100mm and -3 if set up at 400mm (as opposed to for example +13 on my 200mm F/2.8L prime).

I suggest that based on it not achieving even at 20 then calibration would seem the solution, though I do appreciate the position regarding the cost.