and was replaced by a slower but more reliable 1/1000 second focal-plane shutter in the Hasselblad 1000F (Sweden) of 1952.Lothrop & Schneider, "The SLR Saga (part 2)," p. 64Jason Schneider, "Classic Cameras; The Top 20 Cameras of All-Time Countdown: Schneider’s List, The Next Five—Do You Agree?" pp. 68–70, 130. Shutterbug, Volume 37 Number 7 Issue 452; May 2008. 1948 Telemaco Corsi from Rome shows world's first working Pentaprism SLR, Italian Rectaflex, at Milano Fair in April. Production of preseries Standard 947 model starts in June. Series production of model A 1000 starts in September. Alpa Prisma Reflex (Switzerland) had a pentaprism viewfinder in 1948, but its eyepiece was angled upward at 45°. 1949 VEB Zeiss Ikon (Dresden) Contax S (East Germany): second pentaprism eyelevel viewing 35 mm SLR.Matanle, pp. 54, 69–71, 85Jason Schneider, "The 10 most important cameras of the 20th century," p. 88Marc James Small and Charles M. Barringer. Zeiss Compendium: East and West – 1940 to 1972. Second Edition 1999. Small Dole, UK: Hove Books, 1995. . pp. 16, 140–143Wade, Short History. p. 106Spira, Lothrop and Spira, p. 163 First M42 screw mount camera. (The East German KW Praktica came out at about the same time.) With earlier "waist level" SLR viewfinder systems (in which the photographer looks downward at the reflex mirror's image on the focusing screen), moving subjects are seen to track across the field-of-view in reverse direction of their actual motion, making action shooting counter-intuitive. A pentaprism is an eight-sided (only five are of significance; the other three are cut off corners) chunk of glass silvered on three sides that collects, redirects and re-reverses the light from the mirror with minimal light loss.Michael J. Langford, Basic Photography: A Primer for Professionals. Third Edition. Garden City, NY: Amphoto/Focal Press Limited, 1973. . pp. 128–131 With a proper pentaprism, all a photographer needs to do is hold the camera up to eyelevel and everything is there.Ray, pp. 314–315, 318–319 The pentaprism SLR had first been proposed in the 19th century and was used in non-35 mm SLRs in the 1930s. Similar systems (or, in the 1990s, its cheaper alternative, the pentamirrorKimata and Schneider, p. 41) became so common in 35 mm SLRs by the late 1950s that it is the characteristic pentaprism "head" atop the camera body that defines the type for most people. 1950s 1950 Ihagee Exakta Varex (East Germany; called Exakta V in USA): first