Way back in 2013, Canon filed a patent for a 600mm f/4 lens using the company’s diffractive optics technology. Much like the existing 400mm f/4L DO, this design promises similar image quality to the gigantic 600mm f/4L IS II USM in a smaller (though admittedly still large) form factor.
At Canon Expo 2015 in New York City, we were finally able to see a prototype of the lens in the wild. While the final design mockup was under glass, we also got to see a working optical sample mounted on a Canon EOS-1D X—a rare treat, since these rough samples almost never see the light of day.
While we didn’t get a chance to shoot with the new lens ourselves—the camera didn’t have a battery installed—we can confirm that the optical sample and design mockup were both significantly shorter than the existing 600mm f/4L USM II. It's still not something you'd want to shoot with handheld, but it's an improvement nonetheless.
Most interesting is that this updated prototype features Canon's new Blue Spectrum Refractive Optics, a tech that has thus far only appeared in the new 35mm f/1.4L USM II. BR optics involve an "organic" element placed between concave and convex elements to cut down on chromatic aberration. Canon has yet to specify exactly what this element is composed of.
Even with BR optics, it's likely that this new 600mm f/4L IS USM DO BR won't outperform the existing 600mm f/4L IS II USM. But it could give the older lens a run for its money while saving significant bulk and weight. With a design that appears to be near final and a working optical sample, it shouldn't be long before this prototype becomes the real deal.