I was about to ask why the new Canon 24-70mm F/2.8L II doesn't have IS, but then I came across the answer to Why there is no Canon 50mm *IS* lens? and it got me thinking...
(The short version of the linked question's answer: a wide 50mm doesn't need IS)
Let's assume I'm shooting at 200mm and to be "properly exposed" the shutter is set to 1/250. If I frame the same image at 70mm or even 24mm does that mean that I can shoot 1/1000 or faster to achieve the same exposure?
Part of this question comes from the fact that there are multiple versions of the 70-200 with IS and no 24-70 with IS (the 24-105 has IS, but is also F/4 where as the 24-70 is F/2.8 so either the longer focal length or the smaller aperture justifies the need for IS).
Edit: before I can accept an answer I need a clarification and maybe someone should tell me if this should be a different question. Light coming from the scene is constant regardless of the focal length, but apertures are represented as a function of focal length and we usually just refer to the denominator. So if F/4 at 24mm is 24/4 versus F/4 at 200mm is 200/4, does that mean that the amount of light actually touching the sensor is different or did I screw up math again?