Kenko will join Tokina (and perhaps even Canon) in releasing a modern version of the catadioptric lenses that once provided cheap long focal length lenses decades ago. The lenses are still sought after on used equipment sites, providing a big bang for the buck, even if the contrast and color aberration capabilities are limited.
There is opportunity today to add technologies that weren’t previously available when the original designs were popularized in the 1980s – the big one being the coatings revolution that spread through modern lens designs around 2012. Tokina’s existing 400mm lens sports anti-reflection coatings, which is a small start. The great benefit of that type of coating is to reduce light transmission losses, which are especially important for lenses with many elements, where each element might suck 5 percent of the light out of the optical path. The mirror-based lenses typically have very few lens elements.
The lens will be f/8 and come in mounts for Pentax, Nikon , Canon EF and M, Sony E, and Fujifilm.
Catadioptric lenses are common in telescopes, where light gathering is all-important. They use a relatively cheap, curved mirror surface spread within a very wide lens barrel to collect much more light than is reasonably acquirable by a normal lens. Instead of allowing that light to pass through toward the sensor, it is reflected back toward the subject, but focused on a smaller, central mirror in the center of the lens that then reflects it back toward the sensor. This means that the smaller the catadioptric lens, the greater the proportion of light that is blocked by that central mirror used to reflect light back to the sensor. The wider the lens, the more efficient is its design from a light perspective, but the more light bending the individual lens elements must accomplish, which can multiply aberrations.
Using modern lens materials with different refractive indices, coatings and perhaps even lens stabilization could radically improve on the decades-old models, but we are likely to instead see designs that are optimized for the cheap-and-cheerful market of photographers looking to spend less than $500 for a supertelephoto.