Silverbased

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.

The word “bokeh” is the Japanese word for blurry; and based on skimming Flickr comments recently, it seems to be the buzzword of the moment in photography circles. There’s a puzzle, though. Since English already has the word “blur,” why did anyone feel a need to start using the Japanese one? It’s also odd for the word to be transliterated with a final ‘h’; after all, we write sake and not sakeh.

I had been taking photographs for over 30 years before ever hearing the term; and at first it confused me too. However it turns out that bokeh refers to quite a specific aspect of lens blur—calling attention to subtle phenomena that might otherwise be overlooked.

Unfortunately though, the meaning of “bokeh” has been getting rather blurred itself lately. We ought to make a stand to preserve its specific technical meaning, before this useful term degenerates into just another name for “fuzzy.”

A corny flower shot shows nice bokeh from a rotten plastic lens (on a vintage Diana)

Now don’t get me wrong—photographs which use selective focus to give nicely blurred backgrounds can be very pleasing. I like this effect, and have written about how to get it. And because many of today’s digital cameras limit your ability to achieve this look, a photo with shallow focus and a creamy blurred background will often attract many admiring comments about “great bokeh!”

But bokeh is NOT a synonym for “blurry background,” or “shallow depth of field.” It actually has little to do with the amount of blur. The degree of blur you see in out-of-focus areas is essentially a function of geometry—the relationship between the aperture’s diameter and its distance from the subject. Lets say you’re taking a portrait from 4 feet away using a 50mm lens at f/4. Every brand and every design of 50mm lens will render the background with the same amount of blur. But to the connoisseur, two different lenses may yield violently different bokeh.

Bokeh refers to the subjective quality of the blur. Is it “jangly” and busy-looking, or creamy and smooth? Do out-of-focus highlights have odd, distracting shapes, or are they unobtrusive circles? Does the blurred area seem to “swirl” around the center of the photo in arcs? These are some of the factors which might be mentioned as aspects of the bokeh for a particular lens. And these may be the reasons why a serious bokeh geek would chose one particular lens over a different brand with otherwise identical specs.

The word “bokeh” officially entered the English language in 1997, in an issue of the magazine Photo Techniques—whose editor Mike Johnston decided to add the final ‘h’ to make the pronunciation less ambiguous. He tells the story here, and includes some interesting photos showing different subjective effects in various blurred backgrounds.

Where does Bokeh come from?

But WHY might different lenses have different bokeh signatures? Well, there are two effects.

Each point of light from an unfocused area of the subject forms an extended bright patch at the image plane. Conventionally we call this a ‘blur disk,’ as if these were always circular; but really the blur spot takes on the same shape as the lens’s aperture stop. If the diaphragm blades form a 6- or 8-sided “stop-sign” shape (as SLR lenses typically do), so will the blur spot.

A most extreme example of this happens with mirror telephoto lenses, which have a central obstruction: Their blur disks are fuzzy doughnuts. This creates exceptionally distracting bokeh, if there are pinpoint highlights to accentuate it.

This crop from an Olympus XA shot shows busy diamond-pattern bokeh, matching the shape of the camera’s simple 2-blade aperture stop

Also, if a lens’s barrel design obstructs the more oblique light rays, the effective aperture opening becomes progressively more football-shaped towards the corners of the frame. This often leads to a “swirly” background effect if the lens is used at wide apertures.

The other issue has to do with a subtlety of optical design; namely, whether the blurred light ends up more concentrated at the middle of the blur disk or at its edges. A bright rim to the blur disk generally leads to distracting, jangly-patterned bokeh. But note that this effect often reverses depending on whether the subject is in front or behind of the focus point.

Both these effects are discussed in much detail in this excellent article (it is actually one of the original 1997 Photo Techniques articles mentioned above).

A blur disk with the light concentrated more towards its center will generally lead to smoother, creamier bokeh—and ironically one way to achieve this is to create a lens design which leaves some uncorrected spherical aberration. That compromises overall sharpness, so lens designers usually avoid it.

But there have been some specialized soft-focus lenses manufactured that exploit the effect; and it’s the reason why a plastic piece-of-junk camera often gives such dreamily smooth blur where the subject is out of focus, like in the Diana daisies shot I posted above.

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Update: This page from Rick Denny compares the bokeh from several lenses of similar focal lengths; it illustrates very well how differently each renders out-of-focus highlights (scroll down the page to the photographs).