Focus Issues With L-Series Lens

Question

I have a really frustrating issue with a Canon 24-70 ƒ4 L Series Lens.

I noticed that it was unable to focus at or near infinity except at the wide end (when used with my 5D Mk3). When focused manually on infinity, distant subjects were significantly blurred.

I had knocked the lens not long before, so I assumed that the impact had caused an issue with the optics. I sent the lens to one of the three Canon-authorised repairers in the UK and waited for a month to get he lens back. It was immediately obvious the issue was sill present, so I returned the lens. They apparently looked at it again and I received it back this morning. The same issue is still present.

They claim there is no issue with the lens, but the lens is effectively unusable. My first thought was that the issue might somehow lie with my camera body, but while it was being repaired, I have been using a variety of lenses on it, including a 50mm prime, a 70-200mm zoom and a 100-300mm zoom with no noticeable issues.

Just to be certain I reset all my camera settings today and tried several lenses (and the problematic one), and only the problematic one had issues focusing.

I'm at a loss as to what to do. Either the repairers are completely incompetent, or something else is causing the issue, but what could it be? I don't know what else to try.

Answer 1

When using autofocus, the camera may need to be calibrated, via AFMA, to the lens. This is something the factory service center can't check without your camera attached to the lens.

When focussing manually, be sure you are not moving the focus ring and the corresponding focus elements past infinity focus! Many modern lenses, especially Auto Focus zoom lenses have this characteristic. There are several reasons for it:

• Unless a lens is parfocal the exact point of infinity focus shifts as the lens is zoomed in or out, and so obviously there will be a point where infinity for one focal length is past infinity for another.
• As temperature and other environmental conditions change, the various materials that make up a complex lens expand and contract at slightly different rates. This affects focus position for infinity.
• Auto Focus lenses use fairly strong motors to move focusing elements quickly. By leaving a little extra room past infinity, the lens designers allow the motors to power the focus assembly all the way to infinity without bumping against a hard stop that could reduce the life expectancy of the motor and other focus components.

The best way to get clear manual focus at infinity is to use Live View at high magnification to manually focus the lens, then leave the focus set at infinity and turn off Live View.