50mm "Normal" Lens... Ok, but "Normal" on FF or APS-C Sensor?

Yes. Lee is correct, (as is usual).

It means that if we choose a different viewing distance for the image, then a different focal length will appear to be the more normal one.

Or, put another way, we can impose the appearance of wider or more tele lenses having been used to shoot with, merely by holding the picture at different distances..

• Viewing the picture from CLOSER (than nominal "normal") creates the compressed perspective associated with having shot with a telephoto lens.
  
• Viewing the picture from FURTHER (than nominal "normal") creates the expanded perspective associated with having shot with a wide-angle lens.

GRIPE at DPR :-

WHY is the line-space I put between bulleted points irradicated when posted?
BOY am I sick of this damned editor!

--
Regards,
Baz
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"