I did some testing of everyone's hopeful new wildlife lens, the 40-150/2.8 TC'ed up to 210/4, today. Since they are easiest to view in sequence, I put them on my webpage. Starting with f4, then 1/3 stop increments. There's EXIF on the right to help keep track. http://johnvanatta.zenfolio.com/p711656735/h201B1A3F#h201b1a3f All are 100% pixel crops, center frame. Focus was, of course, on the eye, though it is easier to see how things work in the "feather detail". Taken with an EM-5 on a tripod in full sunlight, target 2-2.5m away. This is pretty impressive performance from a teleconverter. Kinda mucky at f/4, but it cleans up substantially by 4.5, and by f/5 things there's a load of acuity. Beyond f/5.6, there was no further improvement (and my tests got more inconsistent, shutter speed I surmise), just diffraction gradually taking its toll. My conclusion (from other tests as well) is that at f/4 things are underwhelming: it's probably worth taking it up to 4.5 and taking the ISO hit. At lower ISOs it's probably worthwhile to further push to get to f/5, maybe <800 or so. Considering that a teleconverter is mostly taking advantage of the difference between optical resolving power and sensor resolving power, this implies that the 40-150 should continue to be a high performer for a long time into the future. Okay, if you're still unwilling to click the link, here's the f/5 sample.