When you get aroung to shooting portraits with that lens....
Long focal lengths like the 100 mm are used for portraits because it de-accentuates the nose. If you stand close to a subject for portraits the nose, relatively speaking is much closer to the lens making it look disproportionally large. So you have to stand back as far as possible. That's why you see many fashion photographers with massive lenses.
Also, in aquarium, birds, and lots of other subject you want to get the sharpest photo you can. Tack sharp corals and feathers look awesome. But in portraits, no one wants to see the fine lines, pores, and other details in a persons face. For that you want a soft focus. This is best done in post processing.
Mike