THE one-liner from which the headline above was borrowed comes from a review of “I am a Camera”—a stage play and subsequently a movie (and later still the musical “Cabaret”) based on Christoper Isherwood's stories about Berlin before the second world war. The witticism has been attributed to countless critics, from Dorothy Parker to Kenneth Tynan. But most now accept it was Walter Kerr who coined this widely plagiarised putdown in 1951.

Last week, while passing the house in Santa Monica where Isherwood lived for many years and ended his days, your correspondent's mind wandered to the well-worn Leica IIIf sitting on a shelf beside his desk. Like Isherwood's masterpiece, the Leitz company's III series of Leica range-finder cameras had their gestation in Germany during the 1930s, and reached their pinnacle in the 1950s.

With its screw-mounted interchangeable lenses, virtually silent shutter, synchronised flash and programmable delay, the compact IIIf was widely copied by camera makers around the world. Painstaking ripoffs by Canon in Japan and Reid in Britain have become collectors' pieces in their own right.

So long, old soldier

But the rugged IIIf remained the photojournalist's camera of choice during the turbulent post-war years until it was replaced in 1957 by the ultimate range-finder, the Leica IIIg. No one since has built a more trustworthy companion.

What finally relegated your correspondent's Leica to the top shelf wasn't having to carry a separate light-meter all the time, but a pair of scissors as well. In their bid to simplify loading 35mm film, Kodak, Fuji and other photographic firms squared off the 35mm roll's lead-in tab—so a modern SLR's motor-driven loading mechanism could grab the film's sprocket holes and wind it automatically onto the take-up spool.

Unfortunately, the Leica's take-up spool needs a long tapering tongue of film, which users must wedge under a spring clip on the spool's barrel before manually wrapping a few turns of film around it.

That meant yanking four inches of unexposed film out from the 35mm roll and trimming it to shape each time you loaded the camera. Doing that while running to keep up with the crowd could be a bit of a drag.

While your correspondent takes his little Leica with him less and less these days (and his bulky old Hasselblad 500C even less so), he hasn't abandoned film entirely. He has, however, finally begun to waver.

Ever since Casio introduced the first digital camera for the masses back in 1995, he has toyed with practically every new model to arrive in the stores. He keeps a tiny 10 megapixel wonder in a drawer for taking e-mailable household snaps.

No matter how dismal the images, the satisfaction of being able to see the result on the camera's little screen within seconds—and zap the many disappointing pictures—never fails to amuse. Meanwhile, printing the digital images with an ink-jet printer at home brings back fond memories of darkrooms under the stairs with trays of developer, acetic acid and hypo.

The question now is whether to go the whole hog and buy a serious digital camera for work as well as play. Two things have prompted this act of blasphemy.

One is the arrival of more attractively priced digital SLRs with “full-frame” image sensors—that is, with the same dimensions (24mm by 36mm) as a full frame of 35mm film. The largest sensors used previously in “prosumer” SLRs had the same APS format (16mm by 24mm) as disposable cameras. The sensors used in pocket-sized digital cameras and mobile phones are truly minuscule (5mm by 7mm) by comparison.

Bigger image sensors are better because they allow the pixels embedded in their surface to be larger. And larger pixels capture more photons—something that's especially useful when lighting conditions are poor.

Bigger pixels also emit more current, allowing their signal to swamp the electrical noise in the system. Just as grain size in silver halide film ultimately limits the amount of detail you can see in a photograph, the background noise produced by the sensor does much the same in digital imaging.

Another virtue of full-frame digital SLRs is that, provided they have similar mounts, you can use lenses designed for 35mm cameras. You can't do that with lesser digital SLRs because of the so-called “crop factor”.

With a smaller image sensor, a lens designed for a 35mm camera captures only the inner part of the image, cropping off the edges that the sub-size sensor chip can't see. So special lenses have to be designed for less-than-full-frame digital cameras. To keep the price down, that often means their optical performance is compromised and the choice of focal lengths limited.

But larger sensors allow another neat trick that serious picture-takers appreciate. They offer the chance to use a much shallower depth of field.

Photographers like to isolate the subject matter by focusing tightly on it and blurring out the background. They do that with a conventional camera by opening up the aperture of the lens and selecting a faster shutter speed.

That's impossible with a little photographic gizmo. The detail in a photograph taken by a pocket-sized digital camera is roughly in focus from a few feet away to almost infinity—just like a pin-hole camera children make in school.

Given the same field of view, a typical digital SLR will show a far shallower depth of field, but still 1.6 times larger than that of a full-frame digital camera. Professional photographers who specialise in studio work have started using cameras with “over-size” (30mm by 45mm) sensors that provide still shallower depths of field—so they can make the subject matter “pop out” against the background even more.

Apart from the arrival of less pricey full-frame digital SLRs, the other recent development that has impressed your correspondent is the way imaging chips are migrating from CCD (charge-coupled device) technology to CMOS (complementary metal-oxide semiconductor), used widely in the computer industry.

The best thing about CMOS is that, unlike CCD, chipmakers have poured billions into shrinking the components on such devices and making them run faster, cooler and smarter. Because the power consumption of a CMOS chip is so much lower to start with, full-frame sensors can be made that run cool, generate less noise and offer longer battery life.

What to buy? When released later this year, the new full-frame, 21 megapixel Canon 5D Mark II will doubtless fly off the shelves, despite the $2,700 sticker price for the body alone.

But don't get caught up in the megapixel race—a marketing ploy if ever there was one. The key thing is the size of the sensor, not the millions of pixels on your plate. Indeed, a six megapixel SLR camera like the Nikon D-40 that has room for decent sized sensor will blow a 12 megapixel pocket camera away.

With the arrival of full-frame CMOS sensors, digital photographers are about to have a ball. Your Leica-loving correspondent is finally ready to join them.