I have sold a Sigma 18-50 f2.8-4.5 OS HSM to an internet buyer. The lens worked fine, but after receiving it today, he said that whenever he turns on autofocus on the lens the camera has a glitch in the autofocus area, switching very fast between AF and M in the menu.

I have never encountered an issue like this with the lens. I believe it could be the AF / M switch broken or something electrical doesn't work on it.


  • \$\begingroup\$ With standard electronics, you can fix a lot of things by liberally spraying contact fluid. On a lens, however, this may not be possible, even dripping some fluid under the switch could possibly pollute a lens. \$\endgroup\$
    – xenoid
    May 25, 2019 at 21:05
  • 1
    \$\begingroup\$ Regardless of the cause, if you're satisfied the lens you sold is having problems, your options are to offer discount, refund, or return. Main things I would watch for are the lens is the same one you sent (serial number), the buyer isn't doing something funky with the switches (like setting them midway between their proper settings), and that the camera isn't defective (works properly with another lens). \$\endgroup\$
    – xiota
    May 25, 2019 at 21:12
  • \$\begingroup\$ vtc b/c Since OP no longer has direct access to the lens, this question is essentially a hypothetical -- impossible to resolve. \$\endgroup\$
    – xiota
    May 25, 2019 at 21:19
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    \$\begingroup\$ That image looks like a gif...There's no trustworthy way to tell how fast the switching is occurring here. For all we know, the buyer could have made this up to pull one over on you (especially if they want to keep the lens but get money back). Have them Skype/facetime you so that you can see the effect in person to confirm any potential defect. \$\endgroup\$
    – OnBreak.
    May 28, 2019 at 16:42

1 Answer 1


It's a lubrication problem, they don't put enough lubricant into the optical tube mechanism and it wears off or gets stuck. the lenses are still usable in case you would like to open the lens and lube it up properly or you simply can support the lens while autofocusing, with your finger.


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