Phase Fresnel Lenses and the Long History of Optics

I file a lot of stories, and very occasionally one doesn't work out. The below explanation of phase Fresnel lenses is one of them. I couldn't hit the right tone for its original destination, and shopped it around, but it wasn't quite a fit anywhere. Enjoy this look into lighthouses, lenses, and the history of optics.

Cape Mears lighthouse in Oregon, now decommissioned and run by volunteers. Photo by yours truly.

Nikon Puts a Lighthouse in Your Pocket

Nikon is at the forefront of techniques to produce complex camera lenses that enable shorter, lighter barrels.

Augustin-Jean Fresnel's crowning achievement was a proof of diffraction that earned him the French Grand Prix in 1819. But it was his later work with refraction in lighthouses that secured him a lasting place on the lips of mariners and theatre stagehands alike. Nikon has taken both aspects of Fresnel's work to produce a new 300mm telephoto lens that is 30% shorter and 40% lighter than its preceding model, while improving all its capabilities.

Fresnel perfected the theory that a lens focuses light passing through it without the need of a continuous curve on the far side: the angle is all that matters. Thus, a lens can be divided into an infinite number of portions of the curve, each of which provides the appropriate focusing path for beams of light. In practice, a Fresnel lens is a set of discontiguous segments, often arrayed in concentric circular ridges, with stepped intervals at each boundary parallel to the direction of the light source.

Fresnel put this theory into wide-scale practice in his role as France's commissioner of lighthouses. Before the use of these lenses, lighthouses both made inefficient use of light, directing roughly 40% from an electric lamp of the day out to the ocean, which would carry weakly from 12 to 20 miles. Larger lenses were more difficult to cast and grind, and hard to transport and mount safely at the treacherous locations at which lighthouses typically sit.

The new lenses under Fresnel's design were not only lighter through the removal of extraneous thickness, but also were constructed in segments, which allowed the use of different kinds of lenses and replacement of individual pieces without losing the entire light. Sigils were designed by rotating a lens around a light with different colored panels, which allowed sailors to identify the origin of each light while far at sea.

Fresnel raised the efficiency to about 80%, while also collimating it long before lasers. A light could be now be seen brightly at over 20 miles (32 km) away. While Fresnel isn't celebrated by sailors the way that Samuel Plimsoll is — for his championing of a line on ships indicating their safe-loading level — he deserves it, too.

Nikon's use of a Fresnel lens also has to do with color, but in a quite different fashion. Normal refractive lenses focus light of different wavelengths at different points, with red (longer wavelength) focusing farthest and blue closest. This causes chromatic aberration, typically corrected with intermediate lenses.

Encyclopædia Britannica, 1911

Encyclopædia Britannica, 1911

However, a phase Fresnel (PF) lens introduces diffraction, as each of the concentric rings of the lens is an obstacles around which light waves bend. This diffraction inverts the chromatic aberration, putting blue closest and red farthest. The concentric rings are spaced at extremely small intervals — tinier than the wavelength of light — to produce the desired diffraction effect.

By pairing a PF element and a refractive element, the combined PF lens corrects the aberration, while reduces the number and weight of lenses and the overall focal distance needed for magnification compared to more conventional lens arrays. In cases of extremely bright spot light sources, a PF lens can cause ring-shaped color flares, and Nikon offers an in-camera software option to mitigate the effect.

The theory of phase Fresnel dates back to at least an academic paper in 1961, and has been applied to X-ray and gamma-ray observation among other purposes. Nikon first experimented with PF over a decade ago, says Steve Heiner, a senior technical manager at Nikon. In that case, the lens was a compact and lightweight small diameter telephoto add-on to a lower-end digital camera.

Nikon also uses a PF lens in some of its microscope products, and competitor Canon introduced a more straightforward Fresnel lens in 2001 that didn't carry with it all the advantages of weight and length.

This new 300mm lens, however, is aimed at the high-end consumer and professional market, where it replaces a fixed (or "prime") telephoto lens that's been a mainstay of Nikon's line for decades, and marks the introduction of phase Fresnel to a bigger, albeit rarified audience. Mr Heiner says the goal was to create a lens with all of the enhancements Nikon had developed over the last two decades as well as reduce the weight and length.

The new model, which started shipping recently as the "AF-S NIKKOR 300MM f/4E PF ED VR" and retails for $2,000, includes a three-axis anti-vibration motor that effectively increases the lens' speed by 4.5 stops, allowing for blur-free photos at slower shutter speeds, among other combinations. For outdoor photographers, the light weight and stabilisation mitigate the need for a monopod or tripod, aiding in portability.

While not a lighthouse in appearance or design, Nikon's news lens is a direct descendent of Fresnel's science — a 21st century realization of a 19th century's genius. And it should take great shots to boot.