Originally posted by Na Horuk Any FF lens can also be mounted on an APSC sensor and f1.2 brightness is f1.2 (when ISO and shutter are the same).
No, "f/1.2" is not "f/11.2" independently of the sensor size.
The physical properties of a lens, say that its focal length is 50mm and its widest aperture is f/1.2, does not change when changing sensor sizes, but the effect on the photo does. Surely you will agree that "50mm" is not "50mm" independently of the sensor size (the FOV changes). Likewise you need to recognise that "f/1.2" is not "f/1.2" independently of the sensor size (while the exposure is the same, the total amount of light is not).
Note that the total amount of light matters for IQ, not exposure. Exposure is independent of sensor size and hence unsuitable to be used as a sole indicator of IQ.
Originally posted by Na Horuk Suddenly EV, shutter speed blur, ISO/noise are no longer important features of the photo, only DoF is.
Exactly the opposite is the case.
All the aspects you mention are important to consider when comparing sensor sizes. When comparing different sensor sizes you need to make sure that you are not comparing apples to oranges (e.g., by using the same 50mm on both cameras as this would result in drastically different images due to the different FOV).
You can only make meaningful statements about the advantages of one sensor size of the other by comparing equivalent images, which includes the same DOF. If you are comparing images with different DOF than you are comparing apples to oranges. You would not take a photo with a Nikon at 1/1000 & f/2.8 and another one with a Pentax at 1/10 & f/28 and then conclude that the Nikon is better for shooting sports because there is less motion blur. You could do it and say "
I really don't know why in a comparison you insist on using the same shutter speed?" but it would be not be a defensible position.
Originally posted by Na Horuk Regarding some of the earlier comments, I agree that FF has its uses, especially when it comes to improved noise performance (due to larger possible photosites) and wider possible angles (this advantage is shrinking, though).
Please note that
"larger photosites" do not provide any noise advantage (if the sensor size is the same).