Silverbased

Many photographers got a good chuckle this week, when the dating site OkCupid posted an analysis of which camera settings make you romantically attractive.

OkCupid harvested all this camera information from the EXIF data embedded in their members’ digital photos. This means, unfortunately, that we can’t get numbers proving that being photographed on film is the sexiest of all—but I assure you it’s true.

Since I often sputter about how harsh and unflattering on-camera flash is, I’m tickled to finally see hard numbers spelling out how bad the damage really is. Having your picture taken with flash is equivalent to adding 5 or 10 years to your age, as far as your attractiveness goes. (Hmm, plotting my age on that graph, I fear I’d better be photographed in total darkness.)

The analysis also showed an effect where viewers preferred photos taken at wide lens apertures—notice all the green boxes for f-numbers f/2.8 and below? Of course, taking shots at wider apertures means shallower focus, allowing a photographer to de-emphasize extraneous background clutter.

But I think OkCupid misunderstood something about the f/stop effect. Point-n-shoot cameras and DSLR kit zooms generally don’t offer apertures of f/2.8 or larger. Only a photographer who went out and bought a different, more advanced lens will ever have “f/1.4″ show up in their EXIF data. Such people are more engaged, and presumably more skilled, as photographers—thus, they probably shoot somewhat more flattering portraits.

Another problem is that without knowing the sensor size and the subject distance, the f/number alone will not tell you how blurred the background actually was.

Selective focus—will it get you more dates?

Nonetheless, when Christian Rudder comments, “because the photos with the low f numbers feel more intimate and personal, they get a better viewer response” I do agree. A portrait where twinkling eyes are sharp, but beyond is a soft halo of blur… it does look very stylish and appealing—even romantic.

So will running out and buying that f/0.95 Leica Noctilux convert you into an irresistible sex bomb? (Albeit one with an empty bank account?)

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Imagine taking two disks of glass and rubbing them together, along with a slurry of abrasive grit. As glass is ground away, what happens next? If the grinding motion is completely even, both surfaces remain flat. But with even a slight change in pressure, the grinding surfaces begin to take on a curve.

A moment of thought should convince you that the curve must be a section of a sphere. That’s the only shape where the two surfaces will always stay in contact as they move. Any high points that deviate from a sphere would eventually be ground away.

This is the reason why a spherical surface is the easiest (and least expensive) curve to manufacture glass lenses to.

This insight is well understood by many old-school amateur astronomers, the ones who go through the long process of grinding their own telescope mirrors. But there’s a problem: the figure actually required for a telescope mirror is not a section of a sphere—it’s actually a parabola.

Although the difference between the two is infinitesimal, the telescope-builder needs to hand-polish subsections of the mirror to reach the right final shape. And the polishing and testing to reach this special curve accounts for a large fraction of the total labor required. The effort turns the optical surface into an aspherical one—simply meaning, any curve that deviates from a simple sphere.

Camera lenses are made using multiple glass elements (at least three are needed for a reasonably aberration-free image). Those surfaces are typically all spherical. But an “aspheric” camera lens includes one of these specially-polished aspheric surfaces (or in rare cases, a couple of them).

Why Aspherical?

But since aspherical surfaces are harder to manufacture, they cost more. So why use them?

A recent aspherical lens

There’s a misconception, often repeated in internet discussions, that aspherical surfaces are needed to “correct spherical aberration.” This statement is very misleading.

We should back up and explain that spherical aberration is when light rays passing through the edge of a lens focus at a different distance than ones from the center. This is mostly undesirable, because then the fine details of your photo subject will lack contrast. (We should note, however, that a bit of uncorrected spherical aberration can improve a lens’s bokeh.) Spherical aberration is particularly hard to cure at fast f/ratios, since the ray paths must bend so steeply at the edges of the lens.

A lens designer needs to balance many factors when creating a new product. There are multiple aberrations to correct, across the whole image, including spherical aberration. But there are also issues of weight, size, and manufacturing cost to consider.

It’s easier to design a good lens when you have more “degrees of freedom.” The more different glass types you can add, and the more surface curvatures you can tweak, the more flexibility you have to cancel out aberrations.

So if a lens design has enough complexity, it can give perfect correction of spherical aberration—even using only spherical surfaces. But it may prove impractically large and expensive to manufacture.

What aspheric surfaces offer is simple: More degrees of freedom.

An aspherical surface can improve a lens design without adding extra glass. Computer ray-tracing can adjust the curvature across an element’s width, reducing all aberrations (spherical being just one). If an aspherically-polished surface can replace three or five spherical ones, it can justify its cost—yielding a smaller, lighter lens that may be be less prone to flare.

Long ago, aspheric lenses were pretty exotic: Polishing the special shapes required lots of extra labor. However technology has moved on, and now there are automated processes that can produce good-quality non-spherical surfaces. As a result, aspheric optics have gone mainstream.

A compact 35mm f/1.7 lens with one aspheric surface

So aspheric lenses aren’t made from some crazy unobtanium, and they don’t have magical powers. They’re just a way to build a better lens using fewer elements, and that’s all.