Rudolf Kingslake, A history of the photographic lens, Boston : Academic Press, , 354 p. (ISBN978-0-12-408640-1, lire en ligne), « "8. Meniscus Anastigmats" », p. 117
Rudolf Kingslake, A History of the Photographic Lens, San Diego, Academic Press, 354 p. (ISBN0-12-408640-3, lire en ligne), « 6. The First Anastigmats », p. 81-102
Rudolf Kingslake, A History of the Photographic Lens, San Diego, Academic Press, , 354 p. (ISBN0-12-408640-3, lire en ligne), « 4. Early Double Objectives », p. 49-68
Rudolf Kingslake, A History of the Photographic Lens, San Diego, Academic Press, , 354 p. (ISBN0-12-408640-3, lire en ligne), « 10. Reversed Telephoto Lenses », p. 141-152
US Patent 2549159A, Ludwig Bertele, "Highly corrected photographic objective having four axially air spaced components", published April 17, 1951, assigned to Wild Heerbrugg AG
US patent 2730016A, Ludwig Bertele, "Photographic wide-angle objective", issued January 10, 1956, assigned to Bertele, Ludwig
US patent 2781695, Gunter Klemt, "Optical system with large effective image angle", issued February 19, 1957, assigned to Jos. Schneider & Co.
US patent 2897725, Günter Klemt & Karl Heinrich Macher, "Optical system with large effective image angle", published August 4, 1959, assigned to Jos. Schneider & Co.
[5], "Improvements in wide angle objectives", published July 6, 1955
[6], "Dreilinsiges Weitwinkelobjektiv [Three-element wide-angle lens]", published June 1, 1967