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I'm using a Canon EOS 60D with a Canon 40mm f/2.8 STM. I bought both used about a week ago. I've had no problems except for one: sometimes the autofocus only jumps between two apertures, and the manual focus doesn't work. As I've gathered, the focus on the 40mm is all managed by the camera, so I suspect that the camera sometimes sends the wrong signals to the lens.

The first time it happened was after being out in the rain for 4-5 minutes, and the other time was just random - so I think maybe moisture could have something to do with it? Maybe moisture is messing with the connector?

If I deattach and then reattach the lens, the problem disappears and the autofocus works just like it should. Is this a camera issue, or an issue with the lens itself? Is there anything I can do to fix this?

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  • \$\begingroup\$ Just to be on the safe side: When half-pressing the shutter (thus starting autofocus), your camera/lens does not focus, but the aperture closes? Did I get that right? \$\endgroup\$
    – flolilo
    Jun 18, 2018 at 9:36
  • \$\begingroup\$ 'If I deattach and then reattach the lens, the problem disappears and the autofocus works just like it should.' Does this mean the issue is fixed? \$\endgroup\$
    – Crazy Dino
    Jun 18, 2018 at 9:47
  • \$\begingroup\$ @flolilolilo Yup, the aperture closes. \$\endgroup\$
    – gloriousCatnip
    Jun 18, 2018 at 9:53
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    \$\begingroup\$ @CrazyDino If I detach and then reattach the lens the problem disappears for a while until it randomly returns, so unfortunately it's not fixed. \$\endgroup\$
    – gloriousCatnip
    Jun 18, 2018 at 9:53
  • \$\begingroup\$ It sounds like you think autofocus should be controlling your aperture because you are equating changes made by the camera reading the light meter to autofocus because they are both activated at the same time by depressing one button. ( the shutter button ). When you depress the shutter release button half way down TWO things are happening. When properly functioning The autofocus is activated AND the light meter is activated. The autofocus is NOT setting the aperture, the cameras software is reading the light meter and setting the aperture base on info it gets from the light meter. \$\endgroup\$
    – Alaska Man
    Jun 18, 2018 at 17:03

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The only way to determine whether the issue is with the camera or the lens is to use another EOS lens, preferably another EF 40mm f/2.8 STM or more generally any STM lens, with your camera and/or use your lens with another Canon EOS camera.

My guess, however, is that this is most likely a problem with the lens. Dry it out for several days and see if the issue continues.

Also check to be sure the lens is properly mounted and turned all the way until the lens release 'clicks'. You should not be able to rotate the lens more than a millimeter or two in either direction without pressing the lens release button. If you can rotate the lens more than that when the lens release mechanism is "locked", you need to have that repaired. It is highly unlikely this is causing the odd behavior you describe unless one of the camera's or lens' contacts has become misaligned or there is something physically shorting them out, because the way the lens/camera connection is designed the power contacts are the first to lose connection and none of the lens' other contacts pass over the power leads on the camera as the lens is rotated off the camera.

Just to be on the safe side: When half-pressing the shutter (thus starting autofocus), your camera/lens does not focus, but the aperture closes? Did I get that right? – flolilolilo

Yup, the aperture closes. – gloriousCatnip

One thing you might want to try is to reassign the 'Depth of Field Preview' button to another function and see if the lens still occasionally physically stops down the aperture on a shutter half-press. It might be that the 'DoF Preview' button is malfunctioning and causing the aperture iris to stop down on a half press. If that is the case, then the issue is definitely with the camera. Or it may be that you are accidentally pressing the 'DoF Preview' button by the way you are holding the camera.

Going forward

The next time you are shooting in wet conditions, unless you are using weather resistant pro-grade bodies and lenses, use a rain cover on your camera and lens.

If your camera does get wet again and starts to act 'crazy', turn it off and remove the battery immediately. Most damage to microelectronics is done by the power shorting across gaps using the impurities in the water. Wait until the camera is completely dry inside and out to place the battery back in the camera.

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  • \$\begingroup\$ @gloriousCatnip Answer expanded to include a couple of other possibilities. \$\endgroup\$
    – Michael C
    Jun 19, 2018 at 1:47
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The first time it happened was after being out in the rain for 4-5 minutes

This, to me, suggests you may be misusing your equipment. The Canon 40mm STM lens is NOT water resistant.

The same is basically true for Canon 60D: Is the Canon 60D weather proof?

Letting the camera and lens dry thoroughly and cleaning your contacts may solve your problem. Or, it may not.

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Generally speaking, a problem that goes away when you disconnect and reconnect an electrical circuit is caused by dirty contacts. If this only occurs with one lens, it could be that the contacts on that lens are dirty or corroded. Clean them. If it occurs with more than one lens, it could be the contacts on the camera. Clean them.

There's a description here of how to do this for Nikon lenses, although the procedure is just as good for other brands. One note: the photo showing how to clean the contacts in the camera doesn't make clear that you should hold the camera with the lens opening down when you do this, so that nothing falls into the camera.

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