The vast majority of SLRs now sold are digital models, even though their size, form factor, and other design elements remain derived from their 35 mm film predecessors. Whether a dedicated digital design such as the Olympus Four-Thirds system, which permits equivalent performance with smaller and lighter cameras, will ultimately supersede the film-derived designs from Canon, Nikon, Pentax, and Sony is as yet unclear. Additionally SLRs are facing a threat from the rapidly expanding mirrorless interchangeable-lens camera segment among all types of camera user. Chronology Significant SLR technology firsts (including optics peculiar to SLRs and important SLR evolutionary lines now extinct). Pre-19th century 1676 Johann Sturm (Germany) described first known use of a reflex mirror in a camera obscura. The camera obscura was known to Aristotle as an aid in observing solar eclipses, but its use as an artist's aid was first expounded by Giambattista della Porta (Italy) in 1558. The reflex mirror corrected the up-down image reversal that could make using a non-SLR camera obscura disconcerting – but not the left-right reversal. 1685 Johann Zahn (Germany) developed a portable SLR camera obscura with focusable lens, adjustable aperture and translucent viewing screen. These are all the core elements in a modern SLR photographic camera – except for an image capture medium. It would not be until 1826/27 before Joseph Nicéphore Niépce (France) made the first permanent photograph using a bitumen photosensitized pewter plate in a non-SLR camera. All advances in photographic technology since then – mechanical, optical, chemical or electronic – have been convenience or quality improvements only. 18th century SLR camera obscuras popular as drawing aids. Artist can trace over the ground glass image to produce a true-life realistic picture. 19th century 1861 Thomas Sutton (UK) received first patent for SLR photographic camera. An unknown number made but very few; no known production model; no known surviving examples. The manually levered reflex mirror also served as the camera's shutter. Used glass plates. 1884 Calvin Rae Smith Monocular Duplex (USA): first known production SLR. Used glass plates (original model 3¼×4¼ inch, later 4×5 inch); many were adapted to use Eastman sheet film. Large-format glass plate or sheet film SLRs were the dominant SLR type until circa 1915. However, SLRs themselves were not commonplace until the 1930s. The Duplex's name was a reference to the SLR's one lens performing both viewing and imaging duties, in contrast to the two separate viewing