AFMA 70-300L

[kirkcha]

Just picked up a refurb 70-300L and was wondering if it seems odd that when testing 300 at a normal testing distance it needs about +4 for the DOF to be centered but at minimum focus distance with the shallow DOF it needs -3 to for the subject to be in focus.

[neuroanatomist]

That's a pretty big shift, although it's normal for a lens to require different AFMA at MFD vs. 25-50x (although my 100L does not).  The 70-300L has floating elements to improve sharpness during close focusing, and that might affect AFMA. 

My 70-300L requires +2 throughout the range (70mm, 100mm, 200mm, 300mm at 25x and 50x focal length distance), but I haven't checked it at the MFD.

[kirkcha]

The only reason I decided to experiment with MFD was it wasn't quite as sharp as I thought it should be while shooting close up and it looked like it was back focusing a little and it turns out at -3 it looks great.

[Mt Spokane Photography]

Its normal to see a focus change near mfd, if its too much, Canon can probably adjust it for you.
 
Assuming its under warranty, send it in for adjustment, telling them which body, and what problem you are seeing.  They have accurately calibrated bodies they use to adjust it with.  Then, if its still a issue, your body might have a focusing error.

[kirkcha]

Thanks for the input everyone, talk to Canon and they said might as well swap it out since it has only been a week. Unfortunately they are out of stock of refurbs and even new. Might have to grab a new one from B&H before the rebates expire.