I saw on ebay a back to replace a standard back to a horseman 4x5 to which a canon digital camera can be attached. In essence if you had a 5d you could have a large format camera with a 12.5 MP digital back. Is this as straight forward as it seems? What adjustments in calclulating an exposure need to be made to make this work. I would also imagine you would only capture a fraction of the field you otherwise would get shooting standard 4x5 Can you meter through the camera view finder.