Originally posted by JPT Adam, thanks for fixing the post. I wasn't sure where the image was originally from, but I thought it deserved its own thread.
Can someone explain what happens when you put this between a (hypothetical) FF body and a DA crop lens like the DA21? My understanding is that you get almost the same FOV as the DA21 would produce on APS-C, you lose a stop of light, but the apparent depth of field is similar in the final image. Am I right?
In a perfect theoretical world, if you put a 1.55x teleconverter on a APS-C lens, the results would be the same on FF as they would be on a 1.55x APS-C body.
It works. Kinda. A lot of teleconverters 'interfere' with wide angle lenses - the teleconverter puts optical elements where there's elements in the lens, so they don't physically fit together.
The optics in a teleconverter are usually designed with more telecentric behaviour in mind so putting them in a wide angle lens doesn't always work the best, either.
You are kind-a correct - the DOF will be the same. I don't know what you mean by 'losing a stop of light'. You'll have exactly the same amount of light hitting the sensor, it's just spread over a broader, 'shallower' buckets rather than a narrower, 'deeper' buckets. You'll get the same results in theory.
If you're looking for this + FF to use your old lenses... I'd guess it won't work out as well as you hope.