Buried in Antoine Claudet's 1849 paper to the British Association is a problem that nearly every early photographer cursed at, and which almost no modern person remembers existed. Claudet listed five great mysteries of the daguerreotype process, and the fourth was this:

What is the cause of the difference in achromatic lenses between the visual and photogenic foci? why do they constantly vary?

Read that again. Claudet is telling us that a lens which produced a perfectly sharp image to the human eye would produce a blurry photograph — and vice versa. The point at which light came to focus on the ground glass was not the same point at which the picture would actually be sharp on the silvered plate. Worse, this offset "constantly varied" depending on the subject, the light, and the lens.

Claudet was no crank. He was a French-born London optician who had bought a license to Daguerre's process directly from Daguerre himself in 1839, opened one of the first commercial portrait studios in the world, and was eventually appointed "Photographer-in-Ordinary" to Queen Victoria. The paper is his attempt to drag photography out of what he beautifully calls its "mysterious darkness" into something governed by physics.

And he was right. The phenomenon is real, and it has a name we no longer use: chemical focus, or actinic focus. A glass lens bends violet and ultraviolet light more sharply than it bends green and yellow. Your eye is most sensitive to yellow-green; the silver-iodide plate of a daguerreotype was most sensitive to blue and ultraviolet. The two images formed at different distances behind the lens — sometimes a centimeter or more apart on a portrait camera. Daguerreotypists learned to focus visually and then nudge the plate holder by a calibrated amount before exposing.

When panchromatic films and modern apochromatic lens designs arrived in the twentieth century, the problem quietly vanished from daily life. But it never went away — it just hid.

• UV photographers (forensic, art-conservation, entomological) still have to refocus after putting on a UV-pass filter, because their lens hasn't been corrected for those wavelengths.
• Infrared shooters on older manual lenses focus on the subject, then rotate the focus ring to a little red "R" mark to compensate — the last vestige of the chemical-focus correction on the barrel of every Nikkor and Takumar lens of the 1960s and 70s.
• Astronomers using refractor telescopes still chase a residual version under the name "secondary spectrum," the reason apochromatic glass costs five times as much as a regular doublet.

Claudet didn't have the language of "wavelength-dependent refractive index," but he had noticed, measured, and named a phenomenon that took another fifty years to be fully tamed. His mystery #4 is a lovely reminder that the camera and the eye are not the same instrument, and never were — they just learned to agree most of the time.