Lens comes from the same Latin root as the word lentil. And the lentil outline, bulging outwards on both sides, is the most familiar shape for a glass that focuses light—as any child burning leaves and ants with a magnifying glass can tell you.
But for focusing an image onto a piece of film, that basic double-convex shape has some flaws. One is that light passing through the outside edges of the lens gets focused at a different distance than rays going through the center. This is spherical aberration. It also means light rays entering the lens at an angle get focused into blurry comet shapes, an aberration called coma.
It turns out you can improve the sharpness of a single-element lens with a couple of tricks: First, you drop the lentil shape, and instead, use a lens with one side that’s convex and one concave—a “meniscus” lens. Second, on the concave side you add an aperture stop (think f/11 or smaller), to block those pesky misfocusing outside rays.
Meniscus lens and aperture stop—ready to build a 16×20″ Brownie
A meniscus lens can offer surprisingly decent sharpness, if you only need small snapshot prints; the arrangement I just described has been used in millions of simple cameras, dating back to the original 1888 box-shaped Kodak.
But this optical arrangement has one quirk, which is that straight lines in your subject can warp into curves. Here’s a photo from a plastic Diana-type camera from the 1970s, with a simple meniscus lens in front of the aperture stop:
The straight lines sort of suck inwards towards the middle—an effect you can also see in photos from Holga cameras, which have the same lens arrangement. The shape of this warping somehow inspired the name pincushion distortion.
If a meniscus lens is placed behind the stop, the distortion curves the opposite way—bulging outwards—called barrel distortion. Interestingly, most early box cameras used this lens arrangement. Perhaps that’s because subjectively, barrel distortion is slightly less distracting than the pincushion kind.
But a camera with no outwardly visible lens is strange-looking to consumers. Kodak actually resorted to adding a dummy piece of flat glass in front of the aperture stop for its Brownie Hawkeye Flash camera, so customers would be reassured by a more normal-looking appearance.
Other camera makers solved the problem by putting the lens in front of the aperture stop, but then curving the film gate to help straighten out the pincushioning.
(And yes, if you pair two identical lenses on either side of an aperture, it cancels out the distortion—inspiring the “Rapid Rectilinear” designs of the brass-barrel era.)
Another problem with single-element lenses is that glass bends light through different angles depending on its color (think of prisms). So a simple lens focuses different-colored light at different distances, a defect called chromatic aberration. Again, it’s most obvious with rays passing through the lens at an angle. Here’s a detail from another Diana-clone shot, showing just the red/blue color fringing in one corner of the frame.
Happily, it was realized quite a long time ago that there was a solution. Different formulas of glass have slightly different light-bending properties. Some bend light more strongly than others. Some disperse colors more strongly than others. By crunching a bit of math you can find a clever combination of two lenses, whose different glass recipes and different curvatures cancel out the chromatic aberration.
Two glass elements working together like this are called an achromat, and occasionally you’ll find a vintage camera lens named that way.
Achromats are most often a positive and a negative element cemented together into a single unit—a symbol that shows up at the bottom of this old Argus Camera logo, among many other places.
A Sticky Business?
Finally, there’s one last aberration to annoy us: Astigmatism. The word is familiar to a lot of eyeglass wearers (including one close friend of mine who jokes that he wears glasses because of his “stigmata“). But the term means something slightly different in the context of lens aberrations.
I won’t bore you with a longwinded explanation, except that it can make points of light focus into weird little seagull shapes towards the corners of the image. Might we find some optical trick to save us from this horror too?
During the 19th century, the mathematics of tracing light ray paths and analyzing aberrations were essentially worked out—although computing any particular design required a brutal equation-solving slog of many months.
Finally, in 1893 an optician named H. Dennis Taylor devised a lens of breathtaking economy, using just three elements: Two positive (lentil) lenses flanking a central negative one. He was able to show that this lens corrected all the classical aberrations to a high degree—including, notably, astigmatism—even at reasonably fast apertures of f/4.5 or so.
Rather unfairly, this design gained fame as the “Cooke Triplet,” named for his employer. I’d rather call it the Taylor Triplet. This design was the godfather of numerous of popular lenses, some of whose names may still ring familiar today: The Argus Cintar. The Zeiss Novar. Yashica Yashikor. Schneider Radionar. And (with mystifying cult status) the Lomo LC-A’s Minitar.
Taylor showed the way in creating the simplest, least expensive lens free from astigmatism. And so, whenever you come across a vintage camera lens proudly labelled “Anastigmat,” the chances are good it’s a simple triplet design. Naturally, any decent lens with four or more elements will be anastigmatic too; but by then the marketing department will be off boasting about something else, like its speed or coverage.
As it happens, the triplet design is somewhat “brittle,” in the sense that it must be designed and manufactured to exacting tolerances to reach full potential. And even then, triplets can show some softness in the corners of the frame until stopped down to middle apertures.
So, If Three Are Good…
A decade later, Zeiss introduced a 4-element lens design called the Tessar—replacing Taylor’s single rear element with a cemented achromat. This incredibly successful design set the standard for affordable, highly-corrected lenses for the next three-quarters of a century. Virtually every lensmaker created their own version: the Leitz Elmar, Schneider Xenar, Yashica Yashinon, Agfa Solinar, and countless others. Then, newer 6-element formulations made fine lenses possible with unheard-of apertures, like f/2 or even wider.
So, where does this leave the lowly Anastigmat today? As a solution to an engineering problem, triplets have a certain thrilling elegance—if you’re a total geek about these things, like I am. But they will never be counted among the world’s elite optics, the ones boasting hyper-acute resolution from edge to edge. On the other hand, triplets are not nearly bad enough to rank as true “toy camera” lenses, giving clearly distressed, lo-fi imagery.
Yashica 44 image; triplet Yashikor lens
But in the past few weeks, I’ve been shooting with a nice Yashica 44 (a TLR taking 127 film), featuring a Yashikor triplet. And I’ve been unexpectedly delighted with the results. There’s no heavy-handed vignetting or blur. Yet there’s just a slightest touch of “old world” softness to the edges of the frame, which works nicely with some subjects. Stop down a bit further and the crispness comes back. Somehow the images have a certain warmth which agrees with me.
So, for a lens that’s not laughably blurry, yet not over-clinically sharp—sometimes an old triplet is just the thing you need.