Canon 600mm f/4L IS II USM EF Lens

I rented both the 500mm f4 II and 600mm f4 II for a week before deciding on the 600mm. They are both excellent lenses in every respect and I carefully 'weighed' this choice. The decision maker for me was the extra reach with the 600 and the fact that (at that time) newer canon bodies would only AF at a minimum of f5.6. This eliminated adding the 2XIII and having AF, which is just about essential for lenses of this focal length when shooting moving subjects (e.g. - birds). The other factor I weighed was the mass of the lens (not that funny, but true). The 500 is the right choice if you can't heft the 600 around on a tripod with a solid gimbal head - it really feels much lighter. After trying the 600, I was satisfied that I could manage the weight. Both of these lenses give excellent sharpness alone and with the 1.4XIII, but I found the 840mm focal length with the 600 + 1.4X just about perfect for small perching birds and handheld action shots of hummers. For small birds you just need the extra focal length. I use the lens frequently w/o a converter for high school sports and larger wildlife (deer etc...) and I'm delighted with the sharpness and contrast at all apertures - though I shoot wide open 90% of the time w or w/o the converter. If you invest in one of these super tele's and do not already have a solid tripod and a gimbal head, I would strongly encourage you to budget for one. While I can handhold the lens in certain situations when I need maximum tracking mobility, a tripod or monopod with a gimbal head enables a full day of shooting w/o risking your back. I really don't have a negative thing to say about the performance of this lens - the AF is super fast even w tele's, the extra choices of focus distance limiters really helps when shooting birds in trees (even a great AF system is challenged here), the IS modes are super (I get good images at 1/100 sec handheld most of the time and 1/30 occasionally on a monopod), weather sealed, nice hood, solid case. The price is a bit of a shock, but I can't see a better value if you love avian & wildlife photography. My wish list for the next generation of super teles (if I live that long) is a built in 1.4X like the planned 200-400 f4 zoom. It would be wonderful to quickly swing that in and out of the optics w/o fiddling around - there just isn't time w wildlife. The added weight and cost would be OK w me.