When I move my eye closer to a photograph, it will fill a larger part of my view. When I move further, the area filled by the image will become smaller. Apparently, there must be some point where the angle of view filled by the image is the same as was the angle of view for the lens when the image was originally taken.
I have seen the claims such as "a 50mm (35mm equivalent) lens will give the same perspective as human eye", but this seems to imply some certain combination of output media and viewing distance that I've never seen accompanied with the claims. An older question addresses the optimal viewing distance by resolution and sharpness, but those are not my current concern.
How can I calculate the distance x, where my eye should be from an image with diagonal d, shot with a lens with 35mm equivalent focal length f, to have it fill the same angle of view as it was originally filling from camera's point of view?