For a focal length of 70mm I'd use about a .275mm pinhole for optimum on-axis sharpness, or slightly larger to improve corner sharpness. If your pinhole is drilled directly in the body cap, it will illuminate only a small spot in the center of the image area. If so, the hole can be enlarged to accomodate a pinhole made in very thin material. Properly made pinholes have good wide angle coverage: 70mm should be no problem on your camera. Two sites with much information on pinhole photography are and