Then why isn't there a 2x wideconverter that make the lens twice as fast?
There is.
we fisheye now
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=focal%20reducer
fast is second only to sharp for worst defined word
What is speed booster? Also worth mention: A teleconverter will MAGNIFY lens abberations. A focal reducer DIMINISHES lens abberations. So, double benefit...
>>2758465
you can take half an image circle but you can't take double of it
>>2758522
fast light-gathering?!
>>2759231
in this case it's more like pouring a big-ass pot of water straight down into a water bottle with and without a funnel
>>2758470
what happens when you 2x teleconvert a fisheye?
>>2759251
You basically get a wide angle that suffers from horrible barrel distortion but not enough to be called a fisheye.
>>2758465
speed booster
>>2759246
It allows for faster shutter speeds.
