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Yes, using an EF-to-RF adapter on your Canon EOS R with the Sigma 60-600mm f/4.5-6.3 DG OS HSM Sports (EF mount) can sometimes introduce autofocus inconsistencies. The issue you’re describing—AF shifting slightly after focusing—can be due to a few key reasons:
Possible Causes of AF Issue
1. Adapter Compatibility Issues
• Canon’s official EF-EOS R adapter works well, but third-party adapters (Viltrox, Commlite, etc.) can cause minor AF issues.
• Even with Canon’s official adapter, some third-party lenses like Sigma & Tamron may behave differently from native Canon EF lenses.
2. Sigma Lens AF Algorithm Differences
• Sigma lenses (EF) were designed for DSLR phase-detect AF, while Canon EOS R mainly relies on Dual Pixel AF (DPAF).
• The translation between DSLR-style AF and mirrorless DPAF through an adapter can introduce slight focus drift at the end of AF.
• Sigma firmware may not be fully optimized for RF mount AF behavior.
3. Lens Firmware Update Needed
• Sigma frequently releases firmware updates for better Canon mirrorless compatibility.
• You may need to update your Sigma 60-600mm lens firmware via Sigma USB Dock.
4. Focus Breathing or Slight Backlash in Focus Mechanism
• Some long zoom lenses (especially super-zooms like 60-600mm) have minor focus breathing or internal backlash in focus motors.
• This can cause a tiny shift in focus when you release the button.
5. Low-Light or Low-Contrast Conditions
• AF inconsistency in low light or low-contrast scenes can be exaggerated when using an adapter.