Scott M wrote in post #17509153
It will depend on how much light you have to work with at the venue. The 100-400L will be two stops slower than the 70-200 f/2.8.
Yes and no. Yes, you need 4x the exposure time to get the same exposure of a blank wall as the 200/2.8 with the 400/5.6, but if you are shooting from the same spot, and have to crop the 200/2.8, you don't get any more subject light with the f/2.8. This is one of the greatest illusions that photographers face. When person A uses 200/2.8, and person B uses 400/5.6, and they are standing together and shooting the same thing at a distance, with the same shutter speed, the amount of light collected from each element in the scene is the same. f-#s only affect the light per unit of sensor area; they do not reflect the total light collected for any part of the scene. If A and B are using the same pixel density, the radius of the AA filter relative to the subject size is smaller, the coarseness of a Bayer CFA is less, and noise clumps and banding are finer with the longer focal length (bold italic text is an edit). What matters to the amount of light collected in such a scenario, with a given shutter speed is only one thing - aperture size (focal length divided by f-#). This is the same for 200/2.8 and 400/5.6, and 800/11. If you want less subject noise, get closer and/or get a larger aperture, and a longer focal length increases the detail, and makes the pixel/sensor artifacts smaller relative to the subject size.
Of course, a lower f-# can improve AF ability, and sometimes that is more important.
My model above breaks down when f-#s start to get too low. Because of the nature of the microlenses on sensors, for recent pixel sizes, an APS-C starts losing light when the f-# gets below 2.8, and a FF starts losing light below f/2 or so (the 5Ds will probably behave like the recent APS-Cs, unless Canon has come up with something new here with microlenses). So, 100mm f/1.4 would collect less subject light than 200mm f/2.8 with the same distance and shutter speed.