Nikon filed a patent in Japan for a 300mm lens with a diffractive optical element (Canon has a good DOE explanation on their website). The exact aperture was not specified, but I am pretty sure it's f/4 (see this patent from last year). This is an interesting development because I expect Nikon to announce a new 300mm f/4 lens in the next few months. The remaining details from the patent application:
- Focal length: 291mm
- Lens length: 190.20mm (the current Nikkor 300mm f/4D lens is 222.5mm in length, using DOE allows to construct smaller lenses)
- Image height: 21.60mm (full frame)
- Lens design: 17 elements in 10 groups, the current Nikkor 300mm f/4D lens ($1,369) has 10 elements in 6 groups:
As I already mentioned above, last year Nikon filed another patent for a 300mm f/4 lens with VR and DOE and identical design.