Long Lenses and DOF
I like a 90 lens to be pretty fast for portraits. Maybe 1.8 to 2.8, so I can blur the background. W/ some lenses you can get just the eyes and front of the face sharpish, and have everything further back soft. I like this look. But is this necessarily true if I go to a longer focal length (within reason)?
I'm thinking of getting a 135mm f3.5 lens for my one Canon FD body, which would normally be far too slow for portraits. However, since it's a 135 and not my usual 90, maybe this isn't that important? I know that longer lenses compress space, which shouldn't really have a large effect w/ portraits, but is my reasoning sound....namely, that the longer the lens the less need for ultimate speed to get the same shallow DOF? Or is f3.5 going to be f3.5, more or less, no matter what?