I know this is an old thread and under 35mm, but it's the only one relevant.
Originally Posted by Jorge Oliveira
I have been scratching my head over this since I did lens test.
I took my Hassy out with the 250mm and a 2x converter. I shot a distant brick wall using a tripod, mirror lock-up and a cable release.
With the converter I shot frames at 5.6 (11), 8 (16), 11 (22), . .up to 45 (90)
Without the converter I shot frames at 11, 16, 22, 32, and 45 to coincide.
Magnifying the bricks to where they are the same size, the 2x converter shots show better resolution and accutance.
Can anyone explain this?