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.