1. When bracketing, won't the EXIF data show the varying exposures? All exposures are the same in the series, according to EXIF. 2. I never use auto ISO. Always fixed at ISO 100 or 50. 3. All the exposures are perfect from 24mm until 58mm, then above 62mm the images are all wildly overexposed. 4. If I start at 70mm and work my way down, above 62mm the images are overexposed, below they are perfect. That doesn't sound like bracketing to me. If it is, I really don't understand bracketing.
Bracketing seems to be the common theme, but I really don't think so. If I'm unknowingly bracketing, why doesn't it happen with my DX zoom lenses? Why doesn't it happen with my FX 60mm Macro 2.8 lens? I somehow magically turn on bracketing only with the 24-70mm lens? Possible, but highly unlikely.
I do appreciate the input. Really. I got the camera and lens back from Nikon on Tuesday and the notes said everything was fine. Next bright sunny day (effect is most apparent in high contrast situations), I'll repeat my tests and see if there is any change. And I'll try to make sure I don't somehow initiate bracketing. Promise.