Originally Posted by Gary Briggs
needed to know that the camera would recognize the lens and give you the accurate view.
21 lens would be 42, showing you the correct shape/size
Actually, the camera doesn't "recognize" the lens at all. It just shows you what the sensor sees. The format will be whatever format you set the E-M5 to record ... 3:4 is native for FourThirds and nets the whole sensor area.
The Field of View will be the field of view resulting from the focal length and the format you choose. The reference multiplier is 2x for a focal length on FourThirds format compared to 35mm format, but that's really just the diagonal. Since 35mm format is shaped with different proportions, the horizontal multiplier is 2.1x and the vertical multiplier is 1.85x. So while a 21mm lens provides approximately 42mm equivalent field of view, the effect is really a more square proportion image with a bit more vertical FoV and a bit less horizontal.
Bottom line for a simple rule of thumb, on FourThirds:
25mm = normal
18mm = wide
35mm = portrait
50mm = tele