Astrozoid: DPReview says that " it decreases the focal length and increases the aperture."
This is incorrect and physically impossible.
The focal length is decreased which makes the focal ratio faster.
The aperture stays exactly the same.
These types of telecompressors have been around for years in astronomy with telescopes.
They were never made for camera lenses before because the physical distance between the lens and sensor has to be less with a telecompressor and you couldn't do this with a normal camera lens.
The reduced register distance on the mirrorless cameras now make use of a telecompressor possible with normal DSLR lenses.
The telecompressor may make the focal length shorter and the focal ratio faster, but I don't know about improving the optical performance. With a telescope the telecompressor usually has to be designed to match the specific optical design of the scope or lens for best performance. I doubt it's going to work great on fast lenses.
I understand what they are saying, and they have an excellent reputation with the Coastal Optics 60mm f/4 UV-VIS-IR APO Macro lens, and I'm sure they know infinitely more about lens design then I do.
I'm just skeptical by nature and I want to see what the coma looks like in the corners of a star field shot with a fast wide-angle lens.
Well, we'll see how it performs with lenses on a star field, the toughest test there is for a lens.
Perhaps my photographic education was more rigorous than yours, but I did not learn that aperture was synonymous with focal ratio or f-number.
To use the term aperture in this way is not only incorrect, it is confusing and misleading.
The formula that has been around in photography (for quite a while) is:
Focal Ratio = Focal Length / Aperture
You can not substitute focal ratio for aperture in this equation.
DPReview says that " it decreases the focal length and increases the aperture."