For Avalanche:
The reason we like to keep to a certain level of correctness with respect to
lens focal lengths and crop factors, is because of the differences in
DOF.
So if you shot at 35mm on APS-C and 50mm on full frame, you will certainly have very similar FOV in both images, but the 50mm
lens image will naturally provide a shallower
DOF if the image is focused at the same point and the
aperture remains the same.
Similarly with your question about cropping the fullframe shot to suit the FOV of an smaller sensor image.... the
DOF rendering is going to be shallower, so while the image may appear to be similar, it will look different to an actual shot from an APS-C camera. The output size of the respective images will determine whether you can see those differences or not.
ps. angle of view is not entirely set for any specific focal length either.
The angle of view is related to the sensor size.
That is, you can get two different
lenses at 15mm that can give you two different fields of view!! One could be set to 90° on a 35mm frame .. the other could be as much as 180° if the
lens designer wants it to be.
Also! a 50mm
lens gives you a FOV equivalent to about 50° on a 35mm frame, but that may also equate to a 60° FOV on a medium format frame .. or even a 90° on MF too! But then again the
lens may only privide a 42mm radius imaging circle, and hence only give you only 55° FOV on medium format .. irrespective of what MF size is used.... etc, etc.
It's not really as simple as you may think it is!
So this should read: the angle of view stated for this
lens on one format size will be different for another
format size!(ie. not focal length).