After a few months shooting with the recent Canon EF 70-300mm f/4-5.6L IS, my conclusion is: Not bad at all if not wow! I have had a love-hate history with the 100-400mm f/3.5-5.6L IS; in fact I had 3 in the last 8 years. I was searching a good long range zoom that could be light and allow good to very good accuracy in terms of sharpness and IQ.
But the 100-400mm was a deception in all occasions: too slow in terms of focus, first generation IS witch was barely 1 stop advantage, very ordinary sharpness, forget about wide open, and the infamous pump zoom...
But the 100-400mm range was excellent and so was the portability. Then comes the 70-300mm f/4-5.6L IS. I first tried it at Vistek in Toronto when chatting with my friend Henry who takes care of the PROs for Vistek. My first impressions were wow! The lens was mounted on a 7D and the focus was lightening fast in store.
I already have a 70-200 f/2.8L IS II so why bother with a 70-300? Each situations requires the proper tools. Sometimes it is not possible to bring the 300mm 2.8 in some shooting or sometimes I just want a smaller gear to feel incognito in the city. And, even though the 70-200 II is an outstanding workhorse, and the new 1.4X III extender is of better quality (contrast), history tests have proven that a L lens + extender is always less sharp than a "native" focal lens.
I won't present you a lab test: first this is not my cup of tea and second, I do not have time to do this since I prefer to be out shooting. So my review is based on field impressions.
My review on the EF 70-300mm f/4-5.6L IS is as follow:
1. A well balanced and fairly light lens
2. Very sharp even at 5.6
3. Small enough length to fit in a back pack in an upright position (like my wide angles)
4. Latest generation IS (at least 4 stops)
5. Rapid focus
Drawback: You have to get used to the fact that Canon has switched the focus and zoom rings position (focusing ring is the farthest so near the front lens); and you get rapidly used to it...
In terms of accuracy, I could easily compare the 70-300 f/4-5.6L IS lens at par with the 300mm f/4L IS and even better wide open. The benefit over the 300mm is the fact that you have a zoom and the latest generation IS.
You cannot physically put a Canon extender on the new 70-300 L. The glass on the back is placed at the very end of the lens and there is simply no space (like with the 100-400) to put the extender. You have to remember that this is a 5.6 aperture lens at 300mm so putting an extender would slow focus and tracking focus pretty much.
But after some reading on the web, I have decided to give a try to the Kenko PRO 300 GDX 1.4X on the 70-300 L. The results were surprisingly good, but not outstanding and I could not say that this couple gives ultra sharp results. But sharpness and contrast is good with a slower focus. It won't be usable to follow a sujet moving fast like a bird in flight but for slower moving subjects (a person walking) or non moving subject, it would be a good bundle. Remember that the type of camera you are using is mostly driving the focus; the lens is responding to it. I did my tests on the 1D MK IV.
My score for this lens is 4 stars.
(I give a 5 stars to a lens like the TS-E 24mm f/3.5 or the 300mm f/2.8L IS)
Best of Light !