Doing lots of research on my next camera...
Most of what I read says that DX is better for wildlife do to extra reach (crop factor). Here is what is puzzling me...
If a full frame camera receives more light at the same f-stop because you have to apply the crop factor to that as well, wouldn't a full frame image taken at the same f-stop and cropped to DX size actually be able to be shot faster and at lower ISO?
False premise. The FX light is NOT greater. Exposure is the same on FX or DX. Because the concept of fstops is that f/4 is f/4, same exposure regardless of focal length or sensor size, be it a tiny compact, or an 8x10 inch view camera. There can be other differences, but NOT exposure (not due to FX or DX).
When we use a handheld light meter, we are NOT concerned with sensor size or focal length (other than of course, we do desire to meter the actual captured area). If using an incident meter, we are totally unconcerned about the camera.
or...
If you shoot a DX sensor camera at 600mm f-6.3 (effectively 900mm @ approx. F8) or shoot a FX sensor camera at 600mm f-6.3 and crop to 900mm still at f-6.3 yield better IQ?
I think I have stated the question correctly, please help me understand if I haven't.
Thank You!
It is not an easy question, and there are various different variables, but the way to bet in general is on FX (because it is already larger). However, FX cost will be substantially greater, and be heavier and more bulky, etc.
Greater enlargement is a detrimental factor, and the smaller DX has to be enlarged half again more just to view at same size as FX.
DX is simply cropped smaller.
However, cropping FX to simulate the same DX crop reduces the FX frame to 41% of the pixels it originally had.
Where the DX would retain all the pixels it was designed with.
The best and fair comparison would be the 600mm on DX and 900mm on FX, to retain all pixels on both.
Greater sensor size could also imply larger pixel areas and lessor ISO noise, but of course this varies with megapixel counts. Not necessarily always true, for example a 36 megapixel D800 and a 12 megapixel DX. But if same megapixels, FX wins on pixel size and low noise.
The 600mm DX reach would only be considered a plus if we are too cheap to buy the 900mm for FX.
But DX does have to be enlarged more, and might have smaller pixels and more noise.
And of course, the uncropped FX (with same lens on both) provides more field width, half again wider, which is a HUGE plus if you want wide angle.
A 14mm on DX acts like 21mm on FX, but 14mm on FX acts like 14 mm. Seems fair to say there is NO WAY to get that same view on DX.
DX has "more reach" only because it is cropped smaller and then enlarged more to same size. It is the additional enlargement that provides any "reach". Any telephoto effect is only because of the additional enlargement needed for DX. But images can suffer from excessive enlargement.
You can see that this way: Zoom any existing image in your editor, which crops the view you see to be larger size, and you see the exact same "reach" increase. It is the crop and enlargement that does it (simply because DX necessarily has to be enlarged half again more to view at same size). But as with cropping the FX, this zoom and crop also reduces the pixel count, which the DX crop could be designed to retain.