Talk:Petzval lens

Reference Focal Encyclopedia of Photography
https://books.google.se/books?id=g4Wx9yKrDS0C&pg=PA159&lpg=PA159&dq=petzval+portrait+lens+1840+etting&source=bl&ots=QqwRCgRvrm&sig=n__XY5gkFkI_hSdwnQNbqCXd9A4&hl=sv&sa=X&ved=0ahUKEwihpJ22s63PAhWC1RQKHTKhBwMQ6AEIVTAG#v=onepage&q=petzval%20portrait%20lens%201840%20etting&f=false  RPSM (talk) 16:02, 26 September 2016 (UTC)

https://books.google.se/books?id=Kd5cAgAAQBAJ&pg=PA1067&dq=andreas+von+ettinghausen+petzval&hl=sv&sa=X&ved=0ahUKEwiq6NLYtK3PAhVlJsAKHX9KAoQQ6AEIJTAB#v=onepage&q=andreas%20von%20ettinghausen%20petzval&f=false  RPSM (talk) 16:08, 26 September 2016 (UTC)