great information here, thank you guys! I am debating between the 150mm and the 250mm. with the 250mm i can 'snipe' (can't think of any other word ), but 150, i need to get up close to the subject. but 150mm is still tempting because it's a classical portrait lens and cheaper too!
As I said, the 150mm is the "classic" portrait lens. I wasn't trying to suggest that it was the only lens that you should consider, simply stating the fact that it was the lens most commonly used for portraiture and that there are good reasons for this. If you want to do something a bit more funky, then go for something else : There are no rules and what I say only speaks for the way I go about my photography : Personally, I tend to 'visualise' (sorry, best term I could come up with) the kind of image I want to get and then work backwards, trying to work out the simplest (and, if possible, the cheapest) way of achieving that end .
That said, from a purely practical point of view, if you really want to "snipe" candid portraits then I'd suggest you think very hard about using a 'blad with a 250mm. That's a big lump of glass on a big lump of a camera and you may find that it attracts more attention than you anticipated. People tend to notice big cameras and this might undermine your intention of shooting candidly.
Also, you may also find that shooting candids with a 250mm and a waist level finder on a 'blad' very difficult. If you've ever tried shooting candid portraits with, say, a 300+mm lens on 35mm, then you'll know what I mean. Trying to follow action with a long lens isn't easy with an SLR - add a WLF, reversing the scene, and it suddenly gets twice as difficult...
If you are lucky enough to have a reflex viewer for your 500c/m - you'll find following a subject and framing a scene easier, because the image isn't reversed, but working with such a heavy combination (camera+lens+pentaprism) held up to your eye is tiring and hard to maintain.
Personally, if I wanted to 'snipe' with a 'blad, I'd still use the 150mm, because it's smaller and easier to handle. In fact, it's still over 12 inches long, with a lens hood, and, at a guess, over 1kg in weight. Instead of filling the frame with a just a head, I'd stand somewhat further back from the subject and frame things more loosely. This would allow the subject a little room to move about without going out of frame, enabling me to work in a slightly more relaxed manner.
Ok, I'd lose some image quality, as I'd be obliged to crop the negative for printing, but the 150mm is very sharp and the quality from 120 film so good that I'm confident I can 'lose' up to 1/3 of the negative without degrading image quality too noticeably.
Regards
Jerry