I have a Sinar P Large format camera. Currently i am using 135mm and 210 mm lenses. How do I go about converting this camera so I can take 8 x 10 (or 5 x 7) photographs? Can I still use the 2 lenses I already have? What extra items do I require (eg. back, lens board, film holders etc.) Any information is appreciated. In addition. can anyone recommend a quality polaroid 4 x 5 film with negative?