Is a 5DII + 24-105F4L IS an adequate replacement for a 7D + 17-55F2.8 IS? Mainly in terms of concert shooting and therefore lighting, not FPS et al. I ask, because I have read about full frame cameras being able to 'catch more light', as it were. Would the first combination result in more light, thus requiring a lower iso?
Some examples that might help clear things up: suppose you have:
5D: 80mm, f/4, 1/60s, ISO 800
7D: 50mm, f/4, 1/60s, ISO 800
both shots have the same ev (if the 5D is exposed correctly, so is the 7D)
the 5D shot is less noisy
the 5D shot has shallower depth of field
Now consider this:
5D: 80mm, f/4, 1/60s, ISO2560
7D: 50mm, f/2.5, 1/60s, ISO1000
both shots have the same ev, the same depth of field, and comparable noise[edit: oops ... exposure is quadratic with aperture size, not linear. ]
So yes, the 24-105 f/4 will be an "adequate replacement", but it is a long way from optimal. As others have pointed out, a 135mm or 85mm prime are ideal for this shooting scenario.