I have a few pinholes. For my 8x10, it is "rated" at 300mm or 12 inch focal (working) length and f/333 aperture. So, figuring that the aperture is the focal length divided by the f/stop, it works out to 300mm/333=0.90091mm diameter. If you want that in inches, it is 12 inches/333=0.0360 inch diameter.