I have the Canon 50mm f1.4 which had the common focus issue due to deformed inner focus ring. I have straightened the ring and the lens now had full range of manual focus, but Autofocus cannot find a happy spot, but instead moves back and forth around the correct position a coupled of times and gives up with the red focus square.

I have inspected the autofocus brush and all 3 prongs are making good contact.

Professional service is not available where I live and sending overseas with combined postage approaches the cost of a new lens.

Is the motor assembly responsible for this kind of fault? Or the electronics PCB?

Ordering replacemtent parts seems my only realistic option.

  • \$\begingroup\$ Can the lens be focused manually so that you get a sharp image at the distance focused? \$\endgroup\$
    – Michael C
    Commented Jun 10, 2020 at 9:37
  • \$\begingroup\$ Yes the manual focus works fine \$\endgroup\$
    – Kez
    Commented Jun 11, 2020 at 12:56
  • \$\begingroup\$ @Kaz That eliminates lens alignment issues as a cause of your problem. \$\endgroup\$
    – Michael C
    Commented Jun 11, 2020 at 13:56
  • \$\begingroup\$ @MichaelC Ok Thanks! \$\endgroup\$
    – Kez
    Commented Jun 12, 2020 at 3:28

1 Answer 1


If you've touched the focus position sensor brushes with your bare fingers you're very likely going to need to replace them. Please remember to wear gloves and grounding straps when handling them.

  • \$\begingroup\$ I did notice one of the tiny prongs on the autofocus brush was bent. I've removed the brush, realigned the prongs with tweezers and magnifier, and cleaned the brush and travel with isopropyl but it didn't change anything. So do you think this brush is the responsible part? \$\endgroup\$
    – Kez
    Commented Jun 11, 2020 at 12:59
  • \$\begingroup\$ @kez It's hard to say at this point. You could have also damaged a flex cable, or fried the unit that reads the brushes, which can be very sensitive to static electricity. Put it back together and see what happens. At worst, there are some great deals right now on the EF 50mm f/1.8 STM, which I wouldn't hesitate to use if I needed to replace my EF 50mm f/1.4. The newer STM version eliminates pretty much all of the major disadvantages of the EF 50mm f/1.8 II and is just as good optically as the EF 50mm f/1.4 at the apertures they have in common. \$\endgroup\$
    – Michael C
    Commented Jun 11, 2020 at 14:03

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