I'm wondering if anyone has experienced anything similar. With the 100-400 and 1D2N, about once every 1000 shots (roughly, maybe more) in the middle of shooting I noticed that the viewfinder readout went very dim. Metering seemed far too dim as well, showing something like 1/50 wide open outdoors in the sun at ISO400.
Restarting the camera fixed it everytime, but I'm wondering if there might be something wrong with the lens. I don't recall getting an error, and it has only happened with the 1-4.
I had a similar thing happen with a 135mm USM Canon lens right after I pushed the DOF Preview button. I cycled the lens through AF to MF (focused it back and forth through the full range), took one shot at MF, then back to AF and things were fine.
I am not sure how old your lens is, but with the newer Canons the blades are well sealed against dirt -- but they are controlled electronically. More times than not, the controller circuit is failing. Any Canon Certified repair facility should be able to bench test it and tell you the status of the circuit. Good luck!
