Why does my Sigma 105mm macro lose aperture control on a Canon M50 after being mounted for a while?
Asked 1/1/2021
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2 answers
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I’m using a used Sigma 105mm f/2.8 DG OS Macro on a Canon EOS M50 with a third-party EF-to-EF-M adapter. Right after attaching the lens, exposure is normal and the aperture blades stop down correctly. But after a few hours, the lens seems stuck wide open at f/2.8: images are overexposed at smaller selected apertures, and the blades no longer move until I detach and reattach the lens.
My other adapted EF lenses work normally on this adapter. The Sigma lens is also noticeably harder to mount and remove than the others.
What is the most likely cause, and how can I troubleshoot whether the problem is the lens, the adapter/contact connection, or compatibility with the M50?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
1
It sounds like you have a bad connection, an incompatible connection or a bad lens. Clean the contacts on the lens and camera and coupler. If it persists, try the lens on an EOS EF mount body. If it still happens you have a bad lens. I suspect that it's the coupler.
Originally by user85781. Source · Licensed CC BY-SA 4.0
user85781
5y ago
0
Generated from our catalog & community — verify before relying on it.
The most likely causes are:
- a poor electronic connection between lens, adapter, and camera, or
- a compatibility issue between that Sigma lens and newer Canon bodies like the M50.
Because remounting temporarily fixes it, contact/communication problems are a strong possibility. Clean the electrical contacts on the lens, adapter, and camera. Also note that if the lens mounts unusually tightly, the adapter may be contributing to an inconsistent connection.
To isolate the fault, try the Sigma on a native Canon EF-mount DSLR body if possible. If the same aperture failure happens there, the lens is likely faulty. If it works normally on an EF body, the issue is more likely the adapter or a Sigma/Canon compatibility problem.
Some older Sigma lenses are known to have compatibility issues with newer Canon cameras, and a firmware update may be required if Sigma offers one for that lens. If no update is available, your practical options may be a different adapter, using the lens on a different body, or replacing/selling the lens.
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