Originally posted by Class A Your "1.414" is a factor, which corresponds to one stop. I don't see where the invalidity of the Kenko claim is.
Hi Class A,
Sorry I was so cryptic - the readers of this thread are so into the TC vagaries that I made assumptions and failed to fully explain. Here is what Kenko claims
THK Photo Products, Inc.:
An Example:
The Tokina AT-X 100 PRO D 100mm f/2.8 macro lens will focus as close as 11 inches. At that distance the lens has a life-sized reproduction ratio of 1:1. This means that if the subject is only half an inch long it will be half an inch long on the film or CCD. The K1.5x SHQ DG converts the lens into an equivalent 150mm lens with a reasonably bright equivalent f/4 aperture with the same 11 inch minimum focusing distance.
Further down:
If using manual lighting or metering it is import (sic) to remember than the K1.5 has an exposure factor equivalent to a 1 stop light loss.
Also noteworthy is the claim in the text that this is a "5-element design," but the specifications clearly indicate the construction is 4 elements in 4 groups.
Obviously, the emphasis has been on the IQ of the product, not its proper marketing.
One more thought: The fact that the Tamron MC4 no longer provides this TC in Pentax mount, and Hoya/Tokina/Kenko has shifted the Pentax mount entirely to the Pentax brand hints further at the commonality of the MC4 and SHQ.
By the way, there's a betting line in Las Vegas on how long this thread can keep going, but don't place a bet as you'll never see a pay out. Of course, I'm the one who restarted it.