I'm looking at the Image Quality Tool pages. First, choose your lens, and the 5Ds. If you're seeing something softer than what's there, then there's something else wrong. If you're seeing a similar level of softness, and it's unacceptable, then keep reading.

The 70-200 f/2.8 L IS USM you're considering was only ever tested on a 1Ds2 and 1Ds3. So, choosing 1Ds3 for both lenses, I see that the IS version is quite a bit softer at 70mm (and that softness will be magnified on the 5Ds or any crop body). At 200mm, they look about the same, but the IS has less vignetting.

Right, so the IS version I isn't going to help... at the short end it will make things worse, so that leaves "other lenses". Without much guidance it's hard to suggest other lenses. I'm assuming you're not considering the version II lens based on price, so that also rules out things like the 100-400mm version II as well.

Getting sharp, cheap, means compromising elsewhere:
  • Aperture
  • Flexibility (using primes)
  • Going non-Canon, and potentially having weaker AF
  • Used, Old lens designs... I picked up a 70-210 f/4 push-pull zoom for cheap for Bella. Seems sharp to me, but focus is noisy and slow.


You can consider the 70-200mm f/4L IS USM lens. It's sharper than the f/2.8 versions, and you'll maintain your fast focus.
The 200mm f/2.8L is supposed to be fantastic (except when compared to the f/2L!)
I thought you could go 70-300mm f/4-5.6L IS USM, but it looks like, when stopped down to match apertures, your lens is already sharper (but it has more Chromatic Aberration).