This patent uses a different combination of gratings then I've seen before. They are said to reduce the diameter of the lens. Its a trade off, reduce length or reduce diameter, the goal of this one is to reduce diameter, which might make a lens less expensive, but longer. Here are some snippets from the patent which talk smaller diameter.
"In the optical system containing a diffraction optical element, the object of this invention is small-diameter-izing a diffraction optical element, correcting several aberrations including a chromatic aberration satisfactorily."
The front diffraction grating is a Blaze type Grating, the second is just referred to a diffraction grating.
BTW, lengths are specified as whole length, every lens patent is specified like this, its nothing new. But, they still tell you this in the patent. Its the length from the front optical element to the sensor plane. BF is the length from the last lens element to the sensor plane, it is not the flange distance.
From what I see, the most likely lens to appear is the 600mm f/4 DO. Its a lot shorter while there appears to be no length advantage to the 800mm, and the 400mm is longer than the existing DO lens.
In the first example, we have a 600mm f/4 with a 394mm whole length and a back focus of 63mm, the back focus being the distance from the rear optical element to the sensor plane.
The existing 600mm f/4 is 448mm (457mm fully extended), so the new design is a lot shorter once you add the 44mm flange distance.
Various data
Focal distance 582.00
F number 4.12
Half field angle (degree) 2.13
Image height 21.64
Whole length of the lens 394.25
BF 62.79
In the second example, we have a 400mm f/4 with a 330mm whole length and a back focus of 50mm, the back focus being the distance from the rear optical element to the sensor plane.
The existing 400mm D) II f/4 is 233mm (240mm fully extended), so the new design is longer once you add the 44mm flange distance. Its pretty certain that it won't appear unless its lighter and cheaper.
Various data
Focal distance 405.00
F number 4.12
Half field angle (degree) 3.06
Image height 21.64
Whole length of the lens 330.16
BF 50.46
In the third example, we have a 800mm f/5.6 with a 490mm whole length and a back focus of 105.13mm, the back focus being the distance from the rear optical element to the sensor plane.
The existing 800mm f/5.6 is 461mm (472 mm fully extended), so the new design is only slightly shorter once you add the 44mm flange distance. I doubt we'd see it either unless its a lot lighter and cheaper.
Various data
Focal distance 795.00
F number 5.60
Half field angle (degree) 1.56
Image height 21.64
Whole length of the lens 490.24
BF 105.13