I recently had a portrait shoot outdoors.I would several shots and the exposure was great,and then the next 4 or 5 would be badly underexposed. I xhecked all the settings and they were the same. Any ideas? The camera was the D300s with a Tamron 18-270.
Compare the EXIF data from the correct exposure and under exposure to see if the camera is maintaining communication with the lens. My D300 would lose the connection and revert to the last used Non-CPU lens set in the menu. If the lens was an AF-S, auto focus would also drop out. AF-D would still focus but not meter correctly.
Was the camera set to Spot metering by any chance? I've got caught once or twice thinking it was on Matrix when I had forgotten that I'd set it to Spot for a specific shot and hadn't changed back to Matrix, and I was busy with the subjects and the composition and ... and the mistake was made.