Why would an adapted Canon EF lens on a Sony body sometimes fail to stop down during a timelapse?

Question

I shot a sunset timelapse on a Sony A7S using a Canon EF 17-40mm f/4L on a Commlite EF-to-E adapter. In shutter-priority mode, the camera adjusted aperture as the light changed. Most frames were fine, but a few in the middle are clearly overexposed even though the EXIF shows the same settings as neighboring frames: f/16, 10s, ISO 100.

The overexposed frames also lack the dust spots visible in the correctly exposed frames and show stronger chromatic aberration, which makes me suspect the lens may have stayed much closer to wide open even though the camera recorded f/16.

Is this more likely to be a lens fault, or an adapter/communication issue between the Sony body and Canon EF lens?

user15871 · Answer

With semiconductor electronics anything is possible. My guess, however, would be that the adapter you're using has been reverse engineered without access to Canon's proprietary information regarding exactly how their aperture control protocol works. You've likely discovered a bug in the firmware the adapter uses to translate the instructions from your Sony camera to the language understood by your Canon lens.

A quick google search leads to a few threads where similar issues have been experienced when using variously branded adapters with mostly older Canon EOS lenses. The EF 17-40mm f/4 L is a relatively old design, having been introduced in 2003. Other lenses commonly reported to have the same issue when using adapters with Sony E-mount cameras are the EF 50mm f/1.8 II (1990), EF 50mm f/1.4 (1993), EF 85mm f/1.8 (1992), etc. On the other hand, some users report successfully using the EF 28-135mm f/3.5-5.6 IS (1998) that predates the 17-40.

I also found this tidbit at the Sony Alfa Forum:

Quite often adapters must be initialized with camera and lens.
1) Turn on camera
2) Mount adapter
3) Turn camera off then back on
4) Mount lens

Same process is recommended for Metabones adapters to initialize the adapter.

I've taken thousands of photos using an EF 17-40mm f/4 L on Canon bodies and don't recall ever having had a case where the aperture appeared to have not stopped down properly. I've taken hundreds of thousands of photos with various other Canon lenses and don't recall ever having had a case where the aperture appeared to have not stopped down properly. The electronic aperture control used by the EOS system has, in my experience, been much more reliable and consistently accurate than previous mechanically linked apertures.

The last time I can recall having had a lens with an aperture not stopping down properly was with a Konica Hexanon 40mm f/1.8 sometime in the late 1980s or very early 1990s. I simply replaced it with a used Hexanon 50mm f/1.4 from B&H. Shortly after that I started using the all-electronic EOS system and have never looked back.

Why would an adapted Canon EF lens on a Sony body sometimes fail to stop down during a timelapse?

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