Morning all,
Brain has had time to refresh.
I guess that I only ever consider the change of focal length with a change in camera position. ie wide angle move closer, gives greater depth. Telephoto, move back as it compresses the scene and draws up the background.
So it is always a combination of 2 things. Camera position and focal length.
Surely if only focal length were considered then final image magnification should be taken into account as well. The crop from a 50mm lens image in comparison to that from a 150mm lens to give the same final output size, will show significant image degredation due to the increased magnification.
Don