Your meter should tell you how you're underexposing. Put the camera in either shutter priority or metered manual and see what the meter says. With modern TTL metering there's no reason for accidental underexposure.

By the way - I'm also curious about what you're shooting with that behemoth. Could be cool. I have a friend who shot the motorcycle races at the Isle of Man with a Nikon 500mm mirror lens. The photos were great - when they were in focus and the shutter speed was adequate.