The Canon EF 200-400mm f/4L IS USM lens with built-in 1.4x teleconverter has been a wildly popular topic over the past year, with virtually everyone who has used it giving the lens nothing but praise. For a $12,000 lens, it seems to get around quite a bit.
It really shouldn’t come as much surprise that Canon may be leaning toward other optical formulas that use a built-in teleconverter. Specifically, Canon has submitted a patent application for a 300-600mm f/5.6 with a built-in 1.4x teleconverter, which produces a 420-840mm reach at f/8 when the teleconverter is applied. Additionally, a 600mm f/5.6 with built-in 1.4x teleconverter is also patent pending.
Of course, being a patent application, we may or may not ever see these lenses. If we do, I would wager the 300-600mm option would be priced in the range of $15k or higher.