The distance from the focal point to the image plane is known as (indeed, defined as) as the 'focal length'. If the object is at infinity then a 50mm lens will focus on the image plane at 50mm from its optical centre. Nothing at all to do with a camera. 1/v +1/u = 1/f and all that. A 50mm lens is a 50mm lens - nothing more, nothing less. Cast an image from a 50mm on to a wall, and you'll get exactly the same image from a lens designed for a m4/3 camera, a crop-sensor camera, a 'full-frame' camera, a 120 camera - indeed any camera, or a 50mm magnifying glass, a 50mm enlarger lens. Some lenses will vignette more than others, but that's nothing at all to do with the focal length of the lens. You need to understand that the 'distortion' (the 'big nose' or 'flat features' effect) has got nothing at all to do with the focal length of the lens, it's entirely due to the distance between the optical centre of the lens (or, near enough, the camera) and the model. Using a bigger or smaller sensor/film, or shorter/longer lens has no effect on the distortion PROVIDED THE DISTANCE BETWEEN THE MODEL AND THE CAMERA DOES NOT CHANGE. Of course, with a 4x5 camera you'll get the frame filled differently than a crop-sensor camera, but that's a different consideration. A 120 film camera usually had a 'standard' lens of 75mm or so, a 35mm SLR would have a 50mm as standard, and an APS would have a 35mm lens as standard, because all three would fill the frame to a similar amount, so you'd tend to stand the same distance from the model to take the same shot - but it's the distance that determines the perspective, not the length of the lens. If you still don't understand, try drawing some ray diagrams.