Today I went out to test the 2x TC on the 300/4 and the Super Takumar Macro 50/4..
k300/4 was significantly underexposing as the in-built flash wan't enough to illumunate the owl. So used ISO 800 to get correct exposure. Shot wide open (effective f8), Handheld with the in built flash intensity at +1..
here is the result with the 300/4 + 2x TC..It is uncropped, shot in RAW, processed at the default settings of the RAW Therapee, resized in Irfanview at the default settings, 10pxl border and copyright..Have to admit that the image below looks many times better than that in camera..(though I haven't done any editing..)
Flash caused Red eyes. Don't know how to remove it..
I think Irfanview does something to the image while resizing..It's sharpness stays spot on..even noise of the ISO 800 disappeared almost completely.
Here is one taken with 300/4 without the TC..f4, flash, 1/180, ISO 200