Think of it this way: A 50mm f/2 lens is a 50mm f/2 lens on any system, since these are physical properties of the lens regardless of the camera format they are used with.
Filmmakers who are used to working within a Super 35 world have certain visual expectation of the image that, say, a 50mm lens will produce on an S35 frame with regard to magnification and horizontal angle of view. No one is 'cross-calculating' anything because the common visual system of reference that everyone is working in is S35, not 35mm photography full-frame.
By way of example: If you look at the Zeiss Master Prime lens data http://www.zeiss.com/camera-lenses/en_us/cine_lenses/master_lenses/maste... you'll see for example that the 50mm has an angle of view of 28.26 degrees (last column). Since a 3-perf Super 35 frame is somewhere around the size of the APS-C sensor (vendor differences aside) you'd expect angle of view to be similar - and a quick look at a cross-system equivalency table shows it is:
https://www.bhphotovideo.com/images/Angles-02z.jpg Take a look down the APS-C / Foveo (i.e. 1.6x to 1.7x) columns, and find something similar to 50mm you will see that angle of view is what you'd expect.
So when a DP asks for a 50mm they mean a 50mm and they see exactly what they expect to see on a Super 35 frame.
Think of it this way: A 50mm f/2 lens is a 50mm f/2 lens on any system, since these are physical properties of the lens regardless of the camera format they are used with.
Filmmakers who are used to working within a Super 35 world have certain visual expectation of the image that, say, a 50mm lens will produce on an S35 frame with regard to magnification and horizontal angle of view. No one is 'cross-calculating' anything because the common visual system of reference that everyone is working in is S35, not 35mm photography full-frame.
By way of example: If you look at the Zeiss Master Prime lens data http://www.zeiss.com/camera-lenses/en_us/cine_lenses/master_lenses/maste... you'll see for example that the 50mm has an angle of view of 28.26 degrees (last column). Since a 3-perf Super 35 frame is somewhere around the size of the APS-C sensor (vendor differences aside) you'd expect angle of view to be similar - and a quick look at a cross-system equivalency table shows it is:
https://www.bhphotovideo.com/images/Angles-02z.jpg Take a look down the APS-C / Foveo (i.e. 1.6x to 1.7x) columns, and find something similar to 50mm you will see that angle of view is what you'd expect.
So when a DP asks for a 50mm they mean a 50mm and they see exactly what they expect to see on a Super 35 frame.