I recently purchased a Sony A100 and made the jump to digital. I have a Sigma 28-300 that I used on my old Minolta 5000. Here is the dilemma...when I take a picture using the Auto setting, with the automatic flash up, and with the zoom fully extended, the picture is completely dark. If I don't use a flash at all, the picture is dark, but much lighter than with the flash.

Is this a lens thing? Do I need an adapter because it isn't a "digital" lens? One of the reasons I purchased the Sony was to use this lens, so I would prefer not to have to purchase a new lens. Any help would be appreciated.