Wow.
Features:
- Lens construction: 13 elements in 10 groups
- (protective glass and drop-in filter included)
- Diagonal angle of view: 2°-5°
- Focus adjustment: Inner focusing system with USM
- Closest focusing distance: 14m / 46ft
- Filter size: 48mm rear drop in
Canon’s most powerful supertelephoto lens
The world’s largest interchangeable SLR AF lens, in terms of both focal length and maximum aperture. Two large fluorite elements eliminate secondary spectrum, resulting in extremely sharp, high-quality images. With Extender EF 1.4x or 2x. a whopping focal length of 1700mm f/8 or 2400mm f/11 can be obtained respectively.
Sources:
Williamo's Blog 