[Marc Sabatella]

I'm sorry to take this thread off-topic, but can I ask why tungsten lighting would make a lens front-focus?

I don't know the exact science behind this, but the basic deal is that different wavelengths of light behave differently in terms of how the lens and the prism in the AF sensor bends them, and apparently the system is calibrated to work better when one end of the spectrum doesn't dominate so much.