Mamo. Welcome to the world of flash. To answer your question: with macro, many times you will be very very close to your subject. So close that the lense may actually cast a shadow from the flash and you subject will not be illuminated. Plus with general flash photography moving the flash off camera to the side generally produces a much more pleasing image. I would suggest that you read Syl Arena's book on flash photography. It's and easy read and very, very informative.
7D, 50 f1.8, 17-55 f2.8, 70-200 f2.8 II, 100-400 f4.5-5.6L, 2x TC III, 580EX II, 430EX, 568EX II, 622C
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