It was the curved shape of the lens that led to its name being borrowed, in the late 17th century, from the Latin name of the lentil plant, lens culinaris. In French, another word for lens is objectif—suggesting truth and impartiality. While objectif had been used since the 17th century to describe the optical glass of scientific instruments like telescopes and microscopes, one of its earliest uses as a name for photographic optics was in Jules Verne’s 1874 novel The Mysterious Island. A group of Americans stranded in the South Pacific take a photograph of the horizon. One of the castaways, Herbert, discovers a speck on the photographic plate. While he first assumes the speck to be a defect in the lens, he realizes that the photograph reveals a ship on the horizon of their deserted island. Unfortunately for Herbert and the other castaways, they soon discover that the ship is crewed by dangerous pirates. The lens purports to show the world as it really is, but that’s also a goal it can never reach.

One lens in particular—the 50-mm lens—is often seen as the most objective of objectifs, and it is said to be the lens that best approximates human visual perspective. For example, the precision-lens manufacturer Zeiss states that its Planar 50-mm lens is “equal to the human eye.” Many artists have taken up 50-mm lenses to render ordinary, everyday experience. Yasujirō Ozu, whose films subtly depict the daily life of 1950s and 1960s Japan, used a 50-mm lens almost exclusively. The French humanist photographer Henri Cartier-Bresson also used one. Underlying its popularity is a promise of shared perspective and common understanding.

But the concept of “normal vision,” let alone the 50-mm lens’s ability to reproduce it, is hardly a given. The idea that a 50-mm best approximates human sight has more to do with the early history of lens production than any essential optical correspondence between the lens and the eye.

In the 19th century, when Verne wrote, an increased scientific study of perception had resulted in a profound suspicion of vision. Investigations of color, motion, sight, and light challenged existing beliefs in the stable relationship between perception and reality. Renaissance perspective, the dominant form of representation, no longer correlated to a scientific understanding of vision. Artistic movements like impressionism, cubism, and naturalism reflected a growing distrust of the eye’s ability to see and know the world. Many of these anxieties were tempered by the growing ideal of objectivity in scientific practice, which emerged as a belief in habits, techniques, and practices of seeing that were accepted as credible due to professional training and daily repetition. Ever since, photographic records have made powerful claims to objectivity, but objective perception often amounts to a belief in conventions of measurement.

Most arguments about the equality of the 50-mm lens with the human eye rely on the scientific tradition of quantitatively measuring perception. For one part, 50-mm lenses reproduce the proportions of faces, depth, and perspective at roughly the same size as we see with our naked eyes. For another, a 50-mm field of view roughly matches the human angle of vision. However, lenses aren’t measured by perspective or by angle of view—they’re categorized by focal length, the distance between the center of the lens and the surface on which the image will be focused.

That wasn’t always the case. Like Verne’s character Herbert wondering about the speck on the photographic plate, 19th-century physicists found it difficult to conclusively prove what made a lens defective and what made a lens work. Early lens production was an artisanal craft. Optics were produced mostly through trial and error, and their quality relied on the intuition of an individual optician. The comparison of normal vision with focal length, rather than other forms of measurement, persists because of how lenses came to be produced and used in standard practice.

The mass production and standardization of photographic lenses was largely pioneered by the German optical-instruments maker Carl Zeiss, whose eponymous company still makes optics today. Zeiss began as a small microscope company in 1846. Frustrated by the idea that the physics behind improved microscopes was purely theoretical, Zeiss began to design microscopes according to scientific theories, breaking lens production down into discrete, repeatable tasks. Thanks to these methods, Zeiss became the first company to mass-produce precision lenses reliably. New types of optical glass developed for their microscopes also led the company to branch out into telescopes, projectors, binoculars, and photographic lenses.

Although physicists were developing theories that enabled a clearer understanding of how lenses depicted the world, it was Zeiss’s advertising that created some of the strongest links between lenses and reliable measurements of vision for both the professional and popular imaginations. Zeiss promoted its scientific production of lenses widely at exhibitions, trade fairs, and in catalogs. Its advertising often emphasized the scientific principles of its manufacturing processes, carefully explaining the importance of glass materials, refinement, and testing. While its lenses were not necessarily more reliable than instruments produced by competitors in England and France, Zeiss’s advertising cultured a belief in the value of standardization for lens quality.

Standardization, it turned out, was essential to Hollywood film production. W. K. L. Dickson, the developer of the Edison Kinetograph, established 35-mm film as a standard format for motion-picture films in 1889. As Hollywood began to develop into a studio system during the 1910s, standardization became increasingly important to coordinating the multiple technical roles involved in industrial film production.

Photographic lenses were initially measured by the size of the photographic plate they could cover and the width of the lens in inches. Over time, though, lenses came to be measured by the distance of their focal length. The shift from lens width to focal length likely arose in Hollywood for reasons of precision. Ensuring a constant measurement between the lens and the film stock was more important than the width of a lens, which might vary significantly at a given focal length. Lenses could vary, but they needed to vary in relationship to the central object of the motion-picture industry: celluloid film.

For 35-mm motion-picture production, a 25-mm lens was an effective approximation of the focal length necessary to fill the diagonal dimensions of the 35-mm celluloid frame, as it was used in a cinema camera.* Some still-photography conventions were adapted from these motion-picture standards. Oskar Barnack, who left Zeiss for the optical company Leitz in 1911, began experimenting with an apparatus for testing the motion-picture film stock and lenses. Barnack wanted to make a “Lilliput camera” that would be lighter and easier to travel with than the heavy photographic cameras of the time. Around 1912 or 1913, Barnack adapted this instrument into a prototype 35-mm photography camera, the Ur-Leica. Eventually, the Ur-Leica became the Leica I 35-mm camera which, after its release in 1925, quickly became a popular camera with professionals and amateurs alike, including Cartier-Bresson.

The still camera used 35-mm film, but oriented it differently than a film camera, such that it exposed about twice the space on the negative for a single shot. The 25mm cinema lens standard became a 50-mm normal lens for photography, because it was a reliable lens for completely and sharply filling the frame of a 35-mm photographic negative. The Leica I came with a fixed, nonremovable 50-mm lens, and while the 1932 Leica II introduced interchangeable lenses, its built-in viewfinder was specifically designed to work with a 50-mm lens. Zoom lenses became standard for SLR cameras in the 1960s and 1970s, but popular consumer cameras like the Pentax K1000 and the Canon AE-1 continued to be advertised and bundled together with a 50-mm. A 50-mm ensured that users, especially amateur and first-time photographers, could capture sharp photos of their normal lives in the widest range of conditions without needing a great deal of technical knowledge.

What constitutes a “normal” lens shifts and changes over time. Today, as images are increasingly captured on digital sensors rather than 35-mm film, the relationship between a 50-mm lens and normal vision has become more of a concept than an ideal physical correspondence. The sensors on digital cameras are generally not the same size as on film cameras, and thus, their focal lengths are calculated differently. Due to digital cropping and the presence of a mirror on contemporary DSLRs, to get the same kind of perspective found on a 50-mm, the most “normal” lens for a DSLR is actually closer to 35 mm. For medium-format cameras that use larger 6-by-6-centimeter negatives, like the Rolleiflex or Hasselblad, an 80-mm lens provides a “normal” angle of view. At another extreme, the rear lens on an iPhone X is a 4-mm lens (in the Exif data for an iPhone photograph, it is still measured in its equivalence to 35-mm film, which ends up being close to a 28-mm lens). While the 50-mm lens was the optimal design to reduce visual distortions and maximize resolution on 35-mm film, modern sensors no longer require this particular arrangement.

The technical reasons for a 50-mm lens best approximating human vision break down when celluloid film or its digital-sensor equivalent fall into disuse. Yet, the 50-mm anecdote persists—in part because of the history of lens manufacturing, but also because it taps into the latent fears, anxieties, and imaginations that surround the use of technology for seeing. It’s comforting to believe that there is a standard view, and that photographic apparatuses can reproduce it.

Today, the lens represents a struggle between objectivity and relativism. Metaphorically, people look through critical lenses, cultural lenses, political lenses, and historical lenses. We zoom in and out on things, we frame them, we change lenses, we focus. The metaphor highlights how people adopt multiple viewpoints that, in turn, change how they see and think about the world.

Perhaps the 50-mm communicates an anxiety about whether an individual can understand someone else’s vision. Under the right circumstances, a 50-mm lens does create a perspectival relationship that, more or less, approximates the ways the majority of people see their everyday world. But it’s still relative. Mechanically, it’s relative to the specific apparatus to which the lens is attached. And metaphorically, it’s relative to all the social, emotional, or economic conditions that shape the everyday lives people inhabit. Perhaps we should be skeptical of the whole idea of a shared perspective. Rather, it is the difference between our lenses that leads to a better understanding of how machines, and people, see in the first place.

* This article previously misstated the way cinema cameras used 35mm film and the standard focal length of optics for those apparatuses. We regret the error.