how do you shoot 50mm with RX1?
As far as I know, Digital Zoom is not what you are looking for. The feature you are describing seems to be "Smart Telecon./Zoom."
I have managed to activate it by setting a designated button to it.
You can try this by going to Menue->Setting (gear shaped icon) page 3. When you get there, try assigning one of the function key to the Smart Telecon/Zoom.
After that by pressing the assigned key, you can change the perspective from 35mm -> 50mm -> 75mm -> 35mm so and so forth.
Hope this helps.