Zooms—The Silent Killer
May 4, 2010
Despite endless technological evolution, one photography convention has endured for more than a century: Our numbering system for lens apertures. That is, the familiar “f-stop” scale:
1.4 2 2.8 4 5.6 8 11 16 22
This sequence is admittedly peculiar-looking, and it always confuses beginners. Why do larger numbers mean smaller-diameter lens openings?
But as many of you are aware, these numbers are actually “f/ratios“—that is, they’re the ratio of a lens’s focal length to the aperture diameter. Setting a lens to f/4.0 means its aperture opening measures one-fourth of the focal length.
(I am ignoring a slight complication here. In camera lenses, those sizes are distorted by the refraction of light through multiple lens elements; so it’s the “effective” diameter and focal length which apply. Still, this simple ratio idea will help us understand some essential truths about lens design.)
Opening a lens up by one full f/stop doubles the light it collects. To do this, the area of the aperture opening must double.
Doubling the diameter of the aperture would actually quadruple its area. Instead, to open by one f/stop means widening the aperture by a factor of √2—which equals 1.414. So the √2 the origin of those odd-looking f/stop numbers, 1.4, 2.8, 5.6, etc.
When the aperture blades are fully retracted, it’s the clear diameter of the glass elements themselves which defines the f-ratio. Lenses are always labeled with this maximum opening, as this is what limits our ability to take photos under dim lighting:
To create a lens that is one stop “brighter” means that the area of its elements must double. This doubles the glass surface that a manufacturer must grind and polish to exacting tolerances. And the volume of glass required will roughly quadruple—thus so will its weight.
Wide-aperture lenses are challenging to design while still keeping image quality high. Actually, the designer of a fast lens might need to jam in even more additional elements—juggling their extra surfaces to help cancel lens aberrations.
So, lenses of the widest apertures are always prized, and are sometimes exotically expensive.
Creating a lens with a variable focal length—a zoom—also means extra chunks of glass are required. A wide-range zoom might require 10 or 14 lens elements; while you can create an excellent single-focal-length (“prime”) lens using only 5 to 7.
Camera makers know that bulk, weight and cost are major turn-offs for consumers. Zooms already have a problem, needing all those extra elements. So to keep things reasonable, typical “kit zooms” must compromise heavily on their maximum aperture.
The focal length/aperture ratio holds for zoom lenses too. The widest aperture of the diaphragm can’t be bigger than the glass itself; so as the zoom’s focal length increases, its wide-open f/ratio must decrease. That is, zooms are brightest at their short end; dimmest at the long one. Again, this is indicated on the lens barrel:
The only way to maintain a nice bright maximum f/stop while zooming in is by using oversized lens elements. You’ll see this in some f/2.8 “constant aperture” zooms. Such lenses tend to be the province of professionals or hard-core enthusiasts only, because the penalty in cost and weight is significant.
With an everyday vanilla zoom, in the middle of its range you forfeit about 2 f/stops compared to an equivalent prime.
What this means is that for available-light shooting, you might have to accept a 1/15th second shutter speed rather than 1/60th. Even on a camera with an anti-shake system, a subject in motion can appear blurred at such a slow speeds. Or alternatively, you might try cranking up the ISO setting—but at the cost of higher noise and poorer dynamic range.
An interesting aspect of optics is that a particular lens design can be scaled up or shrunk down in size, while still having all the computed ray paths remain the same. Hence you can take any design and reduce it to compact size—as long as you shrink down the sensor format by the same amount.
You can see this shrinkdown happening in today’s superzoom cameras.
Consumer-level models now battle to outdo each other in “zoomsmanship,” boasting ranges like 15x, 20x—even up to 30x. But there is no physical way to achieve those ultra-telephoto settings except by shrinking the entire system. This implies using a teensy-weensy sensor chip too. And those tiny sensors (with tiny pixels) are the source of the many ills which this blog regularly protests: Excess noise, and resolution degraded by diffraction.
A more promising direction would be to take wide-aperture primes from the film era (and many excellent designs exist), then shrink those down—presumably with a nice savings in weight and cost. For APS-C, their dimensions should reduce by 2/3rds; on Micro Four Thirds, by half. Yet those sensor sizes still permit large enough pixels for high image quality.
This brightness potential becomes obvious when you look at C-mount lenses designed for the 1″ video format (a 16mm image circle, slightly smaller than µ4/3). These are physically small lenses, and apertures of f/1.4 are routine.
One complication is that legacy lens mounts derived from film SLRs all have long-ish flangeback distances, of about 45mm, to leave room for the reflex mirror box. You can’t just shrink down a film-camera 50mm f/1.4 into a svelte new 30mm version—it would sit too close to the sensor, and get whacked by the mirror.
But with “EVIL” cameras, there are no such restrictions. So, shouldn’t we be seeing an explosion of exciting new lenses today—fast yet compact? Yet to date, only Panasonic’s 20mm f/1.7 pancake for µ4/3 has delivered on this promise. Mostly, manufacturers are still wallowing in zoom-think.
If our ideal is to create cameras with great image quality, but at a convenient size to carry, it is clear that we need to reconsider the zoom.
Reviewers become giddy at small improvements in sensor quality—ones amounting to a fraction of an f/stop. Why does everyone forget that any zoom sacrifices two or more stops right off the bat?
Zooms have become ubiquitous in today’s camera market. Yes, they give us convenience in framing. But shoppers have somehow forgotten what their true compromises are.
They steal light from us.