Hopefully our banter is providing some entertainment for others...
The depth of field is not shallower on FX, it merely appeares shallower, because the lens will appear slower on a smaller sensor.
Let me give a real example: Sportsprotogs usually spend 6k on a d4s or 1dx and then another 30k on fast long lenses.
Why would they do this if they could get the same result by just sticking let's say a 70-200 2.8 on a nikon1 system camera with an adapter? They would have an incredible reach with f2.8!
They don't do this, because on the Nikon1 the same lens does not have f2.8 anymore, it's more like f16.
Ever tried to freeze fast action with f16?
I find it hard to explain in a forum. Watch the video I linked to below, tony explains it perfectly.
Sorry, my choice of words ("all else being equal") probably confused matters. I think the point is that if you want to frame the photo similarly (i.e., to keep an equivalent field-of-view), if you keep the camera-to-subject distance and f-stop the same, you will end up with a shallower depth-of-field on FX than DX. So if you shot the same subject from 10 feet first with a 35mm @ f/1.8 on a D7000 (DX) and then with a 50mm @ f/1.8 on a D800 (FX), the FX image would have a shallower depth-of-field (1.29 feet on the FX setup, as opposed to 1.77 feet on the DX setup, according to
http://www.dofmaster.com).
But my point is that the lens is not "slower" on a DX body with regard to exposure. I'm not familiar with Tony Northrup, but I watched the video and I think he acknowledges this at 16:25 when he says that crop factor does not affect the camera settings (while showing similar exposures from DX and FX shot with the same shutter speed, f-stop, and ISO settings).
I'm not a sports protog, so I can't speak with much (any) authority to their buying decisions, but I suspect it has to do with auto-focus speed, auto-focus accuracy, shutter delay, frames-per-second, image quality, noise levels, and perhaps depth-of-field -- not necessarily in that order).
yes, only the center portion = less total light gathered.
It's not really a matter of opinion, it's a fact. If you claim that 70mm(fx) = 105mm(dx) then f2.8(fx) = f4(dx), because N=f/D. Otherwise you are saying that basic algebra is wrong somehow. Also you can observe the results in real life.
I'm not disputing algebra, I'm questioning the inputs or underlying assumptions. The focal length and f-stop numbers in that equation are a function of the lens, not the body.
Yes, there is less total light gathered, but there is also less sensor area. With regards to exposure, if you shoot the same subject with the same lens from the same position with the same settings on FX and DX, the DX image is basically a crop from the center portion of the FX image. The amount of light hitting that portion of the FX sensor is the same as what hits the DX sensor. Resulting noise in the image may be a different issue...