This pattern is more consistent with a shutter problem than lens flare. On the A7 III, a dark semicircle or partial shadow at fast shutter speeds can be an early sign of mechanical shutter curtain damage or mis-timing. Community reports mention the shutter curtain edges becoming rough or “serrated,” which can cause uneven exposure and may eventually lead to full shutter failure.

A useful test is to enable electronic front curtain shutter (EFCS). One user reported the artifact disappeared with EFCS on, which points to the mechanical shutter as the likely cause.

What to do:

• Inspect the shutter curtains carefully for any rough, bent, or damaged edges.
• Test with EFCS on and compare results.
• If the issue persists or you see physical damage, have the camera serviced promptly, especially if it may still be under warranty.

Because the effect shows up at higher shutter speeds and across multiple lenses, the camera shutter is the most likely culprit.