Karine,
I'll take another shot at one of your questions:
It sounds like your are trying to fine tune your mm's a little too sharply.
In my opinion the best situation is to have two zoom lenses. One on the short side which includes 50-85mm within it's range, and one longer at around 70-200mm. I think this satisfies most every contingency for a portrait photographer / painter.
A good argument can be made for a 50mm(+-) fixed lens of the f1.4 - f2.8 variety. These lenses are wonderful in low light and are relatively inexpensive because they lack the zoom.
My entire glorious career as a painting photographer has been conducted with the two above mentioned zoom lenses. I love the zoom. Neither are particularly fancy. One is a Nikon D70 kit lens (on the short side) and the longer lense is an antique from my old Nikon film cameras. Expensive, fast lenses are great if you can afford them. Don't let yourself think that just because you don't have one you can't expect great results from your efforts.
Keep in mind the "Focal Length Multiplier" mentioned above.
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Mike McCarty
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