Now that I also have the 70-200L IS it looks like I am selling my 100-400L IS and replacing it with a fixed lens.

The 2 contenders right now are the 300 f2.8L IS and the 400 f4.0 DO IS

It would be a big help to get feedback from users of these lenses. Obviously there is a major price difference but I am more interested in useability issues. Like are there optical issues with the DO technology? And how much difference does 1.3 lbs make for handholding?

cmr, I have the 300 f4 IS, and it's quite heavy. Perhaps as a result, I find I'm not using it much. Certainly, it's not motorcycle transportation friendly. I'm aware I'm not answering your specific questions, this is more like general feedback. No point in discussing optical quality, as you're looking at different glass.

Originally Posted by wxwax cmr, I have the 300 f4 IS, and it's quite heavy. Perhaps as a result, I find I'm not using it much. Certainly, it's not motorcycle transportation friendly. I'm aware I'm not answering your specific questions, this is more like general feedback. No point in discussing optical quality, as you're looking at different glass.

We have different standards on 'heavy' as your 300mm f4L is substantially lighter (2.6lb) than my 100-400L (3.0lb) and my 70-200 IS L (3.2lb).

My 1dMkII at 3.4lb (with battery) is a little lighter than my EOS-1n based DCS520 was at 3.6lb. It is not uncommon for me to have walked around for 8 or more hours with the DCS520 in my hand (not around my neck) with the 100-400 on it for 7lb (including 2 pcmcia cards and the straps). Hey, if the camera is not in my hand, it isn't ready to use. The neck strap only gets used when changing lenses.