I don't really have much to say about the EF 500/4L IS II and 600/4L IS II since Canon haven't really revealed much except that they are working on them and that prototypes will be on display at Photokina in September (2010). They are obviously updates along the same lines as those for the recently announced 300/2.8L IS II and 400/2.8L IS II. Expect a price increase of course!
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The EF500mm f/4L IS II USM and EF600mm f/4L IS II USM are being developed as L (Luxury)-series lenses equipped with a high-performance image stabilization system. Employing fluorite and other special optical materials to correct for a variety of aberrations, the lenses aim to realize high-resolution, high-contrast imaging performance in a lighter-weight body design.
The EF500mm f/4L IS II USM and EF600mm f/4L IS II USM are under development as successors to the EF500mm f/4L IS USM (introduced in July 1999) and EF600mm f/4L IS USM (introduced in September 1999), respectively, models which earned strong praise from professionals in the field. The new lenses target improved dust-proof and moisture-proof performance in the face of harsh shooting conditions to satisfy the demands of sports, news and nature photographers, as well as high-contrast, high-resolution imaging from the maximum aperture setting.
Canon will continue to actively and flexibly respond to the diverse needs of professional and advanced-amateur users, expanding and improving the performance of its EF lens lineup to contribute to expanding the realm of photographic possibilities for SLR cameras.
Note: Details regarding release dates and specifications for the abovementioned products have yet to be decided.