Zoom lenses are usually slower than the same lengths in fixed lenses. The Bronica 45-90 is a f4-5.6 lens, compared to the 50/2.8 and the 75/2.8. So at the long end you're two stops slower, and at the short end, one stop slower. The disadvantage is that the viewfinder is somewhat dimmer (harder to focus) and you can't open up as wide to compensate for light levels or film choices. For me, lens speed is always at a premium. The advantage of a zoom is that you don't have to change lenses as much to compensate for where you are when you take a shot. That can be a big advantage.
Another advantage of having the 50 and 75 as opposed to the 45-90 is that you have two lenses. If one breaks the other is still working. With a zoom, if it breaks, that's it.
Personally I think the 40 and the 75 are a better combination. The 40 is not much more money than the 50, and it's wider. 50 and 75 aren't that far apart.
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