If you use an interchangeable-lens camera, your camera probably has a weird symbol that looks like a circle with a line through it. If you've never learned what this symbol is, here's a 2-minute video by ZY Productions that explains the film/sensor plane indicator.
Take a look at your camera, and there’s a good chance it’ll have this symbol: Φ — a small …
London-based photographer David Wilman recently did some experiments in which he used a Canon 5D Mark II as a digital back for his MPP 4x5 large format camera. He placed his lens-less 5D at the back of the camera at the film plane and then placed a black cloth over the two cameras to prevent any light from spilling onto the sensor. Light from the Schneider Kreuznach Xenar 4.5/150mm lens entered straight into the open mirror box of the DSLR without any physical link between the two cameras. Wilman was surprised to discovered that this pairing produced quite a respectable macro setup.