Silverbased

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.