Why do sunny photos from my FED 2 show a light leak/halo in the corners?

This pattern is most consistent with a damaged shutter curtain/blind rather than a lens problem. On rangefinder cameras like the FED 2, strong sunlight can be focused by the lens onto the shutter curtain when the camera sits pointed toward the sun, especially if no lens cap is used. Over time that can burn or weaken the shutter blind, causing bright leaks or flare-like halos that show up most clearly in sunny exposures.

Because the issue appears mainly in bright outdoor shots and in consistent frame areas, the shutter curtain is the likely culprit. Inspect the shutter blinds carefully for pinholes, thinning, or damage. Using a lens cap when not shooting can help prevent further damage, but an already damaged curtain usually needs repair or replacement.