I've been considering getting an L-series zoom lens, and the two that I am looking at are the Canon 24-105mm f/4L IS, and the 24-70mm f/2.8L.
Aside from the extra reach of the 24-105, the main difference is obviously the f/2.8 aperture versus the active Image Stabilisation.
Canon state that the IS on this lens "permits the use of shutter speeds up to three stops slower than normal". It's the "up to" in this statement that got me thinking about how image stabilisation is rated.
There is a general rule of thumb that your shots should be good (with a steady hand) at shutter speeds of up to 1/focal length, so clearly the focal length is a factor of image stability. Now, I can't imagine that Canon would publish anything but the best numbers for their spec sheets, so this leads me to assume that the "three stops" of IS they quote are at a focal length of 24mm. So my next question is, what does this mean at the 105mm end? I've done a few calculations, and I invite you to check my workings.
3 stops is a linear factor of 8 (2^3). I think this equates to a linear IS factor of 1.83 @105mm (8*24/105). Or, 0.87 stops. Less than 1 stop!
Using the same calculation but for a focal length of 70mm, the IS of the 24-105mm comes out at 1.46 stops.
Do I have my calculations correct?
PS: I know that the increased aperture of the f/2.8 will give greater background blur, better capture of moving subjects, blah, blah... so please don't feel that you have to lecture me on that. All I'm asking about is IS.