Blender camera lens 90?

Is choosing a camera lens of 90 the same idea as having a flat camera lens? Would it need to be 180? Or what about 0

By default the Blender camera does not work in degrees, but in milimetres, assuming the size of the sensor, or film, is 32mm.

I have added blue text and arrows showing sensor size (32mm) and the focal length (x) to this diagram.

You can set it to work in degrees… but I’m not sure if I understand the concept of a flat camera lens. An orthographic or non-perspective camera is also possible.

Yes, those sort of numbers are suitable for telephoto type len’s, which help flatten out perspective.