Upload & Sell: Off
Isn't it possibly even better than a 800mm f/4 L IS in some regards?
The reasoning being that the crop sensor picks off the "sweet spot" of the MTF function for a given lens (e.g. the cropped corners are now much closer to the center of the lens; edge-to-edge resolution is improved).
As far as aperture goes, is it 800mm f/4 or f/6.4 or other?
I'm not sure I fully understand what happens with these 'speed boosters' or 'focal length reducers' or 'light concentrators' (my description to break from techno-marketing speak). Smarter people than me will have to answer the question, as I keep running into corners working the math. But here goes:
Wouldn't a 'focal length reducer' widen the effective FOV while shrinking the DOF? And if so, wouldn't it also increase the effective F-stop, thus increasing the actual exposure?
And then, given that it's being mounted to a crop sensor, wouldn't it just be making up for most of the light lost from the smaller sensor? So easy to get confused.
Aside from that, the question that sticks in my head, is just exactly what is this 'focal length reducer' actually doing. If we consider that the image circle of an EF lens with the ~44mm flange distance has to be ~44mm in diameter to clear the corners of an FF sensor, does the 'FLR' now take that full image circle and focus it onto the crop sensor with an image circle around 30mm in diameter? Or is it something different or in between?