Problem using MacBook Air with a data projector

Simply put, the data projector won't project an image from my new 11" MacBook Air; but with my 13" Macbook Pro there is no problem whatsoever. I can only assume that the MBA's screen resolution is not that "conventional", or am I wrong? (The data projector, by the way, is a "professional" one, a fixture, used at my university's lecture theatre, and is used with a many varieties of laptops/notebooks, and automatically adjusts to the input.)

Bump?

Many thanks for the tip, and yes I am using VGA (It worked). But took it one step further: System Preferences > Displays > Arrangement then ticked Display mirroring. Interestingly enough the Arrangement tab is not visible if there is no other display or data projector attached. Again many thanks.