Silverbased

Occasionally some self-appointed guardian of language purity will scold photographers that they’re misusing the word telephoto.

It is true that in optical design, the term “telephoto” originally had a specific technical meaning. Through the use of a negative component closest to the film, the overall length of a lens can be reduced, so it becomes shorter than its optical focal length.

Rear negative element makes this Zeiss design a true telephoto

The purist would have us refer to longer focal lengths lacking this special configuration as long focus lenses. The distinction can be relevant for view-camera users, because it affects the bellows extension required to focus. But should the rest of us become angst-ridden about using the “wrong” word?

Recently I’ve been reading Rudolf Kingslake’s A History of the Photographic Lens. He makes a few interesting observations about telephoto lenses.

The size reduction of true telephotos becomes increasingly valuable the longer the focal length required; however they demand careful design to avoid pincushion distortion. A classic telephoto configuration can yield a lens that is physically about 80% as long as its focal length—called its “telephoto ratio.”

Yet a mirror lens using a folded light path can do even better: its physical length can be less than one quarter of its focal length. With such impressive telephoto ratios, don’t these designs also deserve the name?

The lenses for 16mm or 8mm cine cameras cover a small film format; accordingly, their focal lengths are proportionately shorter. In the days before zoom lenses became ubiquitous, movie-makers would switch between individual wide, normal and telephoto lenses. Yet because of their shorter focal lengths, cine “telephotos” rarely needed to be true telephoto designs.

Yet the term telephoto has been firmly entrenched in movie usage for at least 50 years. Thus, the zoom controls for subsequent cine cameras were labeled “wide—tele” and not “wide—long.” And of course this convention was carried forward to all motorized zooms in point-n-shoot still cameras, whatever their optical designs.

Another problem with this fussbudget distinction about telephoto is that most of us don’t know the exact optical configuration of the lenses we use—and have no reason to care.

A curious example of this is the Olympus XA, which includes a 35mm f/2.8 lens. Any sane standard would regard this as a moderate wide-angle. Yet in fact, the XA’s optical design is a true telephoto—using a large negative element close to the film, to permit the camera’s impressively compact clamshell body. (Kingslake cites the Kodak Disc Camera lens as a similar example, although the XA design preceded it.)

Would the pedants insist we call the XA a telephoto camera?

In its official literature, Olympus listed my beloved OM-System Zuiko 85mm f/2.0 alongside other telephotos, while noting its telephoto ratio as 108%. In other words, it’s not a true telephoto at all—it’s merely shorter than competing brands. (In a cross-sectional diagram, its rear lens group appears to be positive.)

Zuiko 85mm: Too tall for tele?

Today of course, zoom lenses are ubiquitous on all types of cameras. It’s rarely clear how to pigeonhole their complex optical designs. (Canon’s basic DSLR kit zoom uses 11 lens elements.) Should we name the long end of their zoom range telephoto or not?

Photographers will always need SOME term to describe a narrower-than-normal field of view; or equivalently, focal lengths longer than the image diagonal. And at this late date, telephoto has become the term most universally understood.

Steven Jay Gould once wrote a column in Natural History, “Bully for Brontosaurus” (which lent its name to a book collection of essays). In it, he bemoaned the 1970s nomenclature revision in paleontology which (on narrow grounds of priority) replaced the familiar name brontosaurus with apatasaurus instead.

Not only did he find the reasons for the change questionable; but he also felt that the brontosaurus—as one of the “rock star” dinosaurs—had done much to fuel public interest in his field. So he encouraged his readers to continue using the old name, without guilt.

In the same spirit, I say “bully for telephoto.” Unless you design lenses for a living, ignore the finger wagging of prune-faced killjoys. Use the word telephoto freely, and be happy.

And remember, the images you take with your lenses are more important than the names you use for them.

You don’t need me to repeat the litany of complaints about compact digital cameras. Autofocus lag. Poor viewfinders. Image noise in low light. Mike Johnston half-jokingly concluded that the entire class shares so many inherent flaws that you shouldn’t even waste your time comparison-shopping between brands.

Recently I’ve posted my own grumbles about digitals’ lithium batteries, and their excessive depth-of-field. But I’d like to take a moment to discuss another subtle failing of digital point-and-shoots, one I rarely see mentioned: Aperture range.

Usually, the range of available apertures on a compact digital is a scant 2 or 3 f/stops. I’m serious: Go check out the “aperture range” listed in some typical specs …I’ll wait for you to come back.

Now, trading off aperture versus shutter speed—to control motion blur and depth of field—is a cornerstone of creative photography. So how could digital camera manufacturers permit such a crippling flaw?

Remember that f/ numbers are defined as the ratio of the lens focal length to the aperture’s diameter.* So as you change to longer focal lengths, the size of an “f/8-sized” hole must grow larger too. But teeny digital sensor chips demand ultra-short focal lengths; on a digi compact, f/8 could mean an opening only 1 millimeter across.

A hole that tiny almost starts behaving like a pinhole. And as any pinhole photographer will tell you, if your hole’s too tiny, you run into a problem: the limits of diffraction.

Diffraction: The Optical Wild Card

Optical calculations generally assume that light rays follow mathematically straight lines, until they’re bent by some air-glass surface.

But it’s not that simple. Because of its wave nature, light grazing the edge of an obstacle can veer off-course. These light waves diffracting in random directions can reduce the sharpness and contrast of an image.

As the aperture stop of a lens becomes smaller, an increasing proportion of the light grazes the edge of the iris opening—rather than passing unaffected through the middle.

Rather than focusing to a single point, this stray light forms a fuzzy bulls-eye pattern instead.

How all this affects image sharpness gets into some choppy waters, technically. With digital cameras, the diameter of those bulls-eyes can can even grow larger than the spacing between sensor pixels. You might have paid for an 8 megapixel camera—but if each point of light is smeared over several pixels, you might effectively get only 2! (For a more technical discussion of these complications, I recommend this helpful page.)

Note that zoom lenses add another wrinkle: An opening of a given diameter equates to different f/stops, depending on what focal length the lens is zoomed to.

The diameter of the lens elements determines the widest possible aperture; but the corresponding f/number changes as you zoom (this is the reason zooms list a range like 2.8–4.0 as their maximum aperture). And if the smallest-possible iris diameter remains constant, this equates to dimmer f/stops (higher f/numbers) as you zoom towards the telephoto end of the range.

Anyway, the upshot of all this is simple: There is some limit to the smallest f/stop which can be used, before diffraction damages sharpness too severely. And tiny image formats suffer the worst from this effect.

A digital point-and-shoot’s lens might stop down to only f/5.6 at the wide-angle setting; and f/8 at the telephoto end. (Contrast that with with lenses for 35mm cameras, which can close down 2 to 3 stops further.)

What about opening wider?

This explains why compact digitals must limit their smallest f/stop. But could we extend the f/stop range to wider apertures? This would also help with our image noise and depth-of-field headaches too.

But the problem here is not optical; rather, it’s a function of what today’s camera marketplace demands.

You can’t increase a lens’s maximum f/stop without adding to its diameter, weight, and cost. And with compact size being a highly-desired camera feature, that’s a tough sell today. Thus, it’s rare to see maximum apertures greater than f/2.8 (and remember, that only applies at the wide end of the zoom range). Compare this to 35mm film compacts of the 1970s, where affordable models often sported excellent, faster-than-f/2.0 lenses.

Actually, the tiny sensor of a digital compact would permit the design of an ultra-fast lens—let’s say f/1.4—that would be smaller and cheaper than its 35mm equivalent. But the snag is, it would not be a zoom. Zoom lens design has made enormous strides in the past decades; but we still can’t avoid a speed penalty of a couple of f/stops, compared to the best single-focal-length designs.

And zoom range is a major tick mark on today’s camera-shopping checklist. With a few notable exceptions, camera makers consider it commercial suicide to offer a zoomless camera (aside from their cheapest and tiniest models).

Both lenses open to f/2.8—the difference is in their focal length. Smaller image formats allow more compact lenses of the same speed.

Between a rock and a hard place

So lens design for small-sensor cameras is hemmed in at both ends. The smallest f/stop is limited by diffraction. The widest is limited by market demand for zooms—but compact and inexpensive ones. Thus the range of available apertures only covers 2 or 3 stops—seriously limiting creative exposure choices. (With primes for 35mm, the range can be 7 or 8 stops instead.)

As new digital models are introduced, some oft-heard complaints—like autofocus lag and poor viewfinders—are gradually being addressed. Other problems like image noise in dim light might be tamed eventually, using better sensors or savvier image-processing.

But any lens is still bound by the laws of optics. No amount of technological whiz-bangery can change that. Compact cameras imply tiny sensor sizes; tiny images imply short focal lengths.

And with those limitations, using aperture in the way a creative photographer demands becomes impossible.

*Note: This explanation of f/numbers ignores a few optical complications, which do not affect the main point here.