Originally posted by Pompous Moronox I am not sure what you mean here. The issue was a high-crop-factor f/2.8 versus a large sensor (APS-C or FF or whatever), right? The f/2.8 on the small sensor has no light gathering advantage at all over the large sensor camera stopped down to equal DOF. The large sensor camera will of course be set to shoot on a higher ISO. The key here is that the large sensor camera can do everything the small sensor camera can do, but also more.
i think you are mistaken. small sensor iso 400 at 2.8 will have quite a wide DOF (large in focus zone) and if the numbers above are right crop factor of 1120 mm equivalent.
same 200 2.8 on apsc will have FOV equivalent of 300 mm so much wider FOV, DOF will be much narrower (smaller in focus area) so to get the same FOV and DOF you would need to stop down the apsc to approx f16 and crop by 75% losing resolution advantage. if you are looking for deep dof and fast shooting the small sensor wins by a lot (see surveillance for example)
If you are shooting wildlife #1 you get narrower FOV, #2 you can shoot at a higher shutter speed while maintaining deep dof. if you want a narrow dof then apsc will be the better (sacrificing the FOV unless you want to carry one of the old a* 1200mm bazookas pentax made (and give up AF of course along with a good chunk of your salary if you can find one) or you can get a sigma 500 4.5 and a doubler for about 12 grand
so there is a use. will it produce as sharp and high quality an image as a well equipped apsc. nope not a chance, but it will have it's markets as will the kenko (there will be an adaptor for other lenses already announced for the kenko, i imagine the same will arrive for the pentax if the iso performance is halfway decent i can see it becoming a standard bit of kit for guys doing surveillance and as a secondary bit of kit for wildlife shooters. this is of course in addition to the real target market teenage japanese girls who want cute
coming soon baby robot cam lol