The new Nikon D5000 and Canon PowerShot SX1: The new mini-S.L.Rs

Physics can be such a bummer, you know? Those doggone rules of nature make our luggage heavy, melt polar ice caps and conspire to make the toast land jelly side down.

And physics explains why you can’t take professional-quality pictures with a small camera. Big, bright, beautiful photos require big sensors and big lenses to shine on the chips. So we’re stuck with two camera categories: pocket ones that take mediocre pictures and big heavy black ones (S.L.R.’s) that take stunning photos but require a neckstrap or, ideally, a wheelbarrow.

Because both ends of that spectrum are already teeming with camera models, the manufacturers are eager to differentiate in hopes of finding new markets. This month, Nikon and Canon are going for a category that, so far, is sparsely populated — the one right in the middle.

Call it the mini-S.L.R.

Nikon’s new D5000 and Canon’s PowerShot SX1 IS look very similar. The bodies are sized and shaped identically — like mini-S.L.R.’s —and they weigh roughly the same (around 1.5 pounds).

Each camera can capture high-definition video; each even has a mini-HDMI jack for connecting to your hi-def TV. (Mini-HDMI requires a special cable, not included.)

Each has a flip-out, swiveling screen on the back. That feature permits you to shoot at high, low and side angles, not to mention allowing self-portraits. (Nikon’s screen flips downward below the camera, rather than off to the side — an arguably better design, since it keeps your view in line with the lens.)

But a flip-out screen also gets the big black camera away from your face, thereby getting more natural, relaxed expressions from your subjects, especially young ones.

Despite these similarities, though, these cameras approach the middle from opposite ends. Nikon has tried to consumerize an existing S.L.R.; Canon has tried to S.L.R.-ize an existing consumer camera.

The Nikon D5000 is a genuine S.L.R. It takes interchangeable lenses, its eyepiece lets you peer out through the lens itself, it has an enormous light sensor inside, and it has no shutter lag (the delay after you press the button).

In fact, if you just read the brochure, you might mistake the D5000 for Nikon’s more advanced, more expensive D90; the sensor inside, the specs and most of the features are exactly the same.

The D5000 has the same superb 12.3-megapixel sensor, the same sensor-shaking mechanism to eliminate dust spots, and the same 11-point autofocus system, which makes it easier to compose focused shots. It even has the same burst rate (4.5 shots a second), which is fantastic not just for sports and action but for capturing fleeting facial expressions.

Like most S.L.R.’s these days, the D5000 also has Live View mode. It makes focusing slower, but it means that you can frame your shots using the screen, if you like, instead of holding the camera to your eye. The D90 was the first video S.L.R.; the D5000 inherits that hi-def goodie, too.

But here’s where things get crazy: Nikon has packaged all of these professional features into a much smaller, lighter camera that’s obviously aimed at amateurs. For example, the new camera is loaded with scene modes to make the thing less intimidating to novices — canned presets for Sunset, Pets or Beach/Snow, each with a sample photo on the screen.

As on other recent low-end Nikon models, the D5000 also makes photography easier to understand by displaying a visual representation of the actual aperture (lens opening) inside as it grows and shrinks. The screen also indicates your shutter speed both graphically and with a fraction — “1/25,” for example, rather than the “25” that appears on most S.L.R. screens. (The D5000 lacks a top-panel status screen like the big boys have — one casualty of the smaller body.)

The D5000, in other words, is such a mix of high- and low-end that Nikon’s representatives actually asked my opinion on how to describe the thing. Here’s my suggestion: it’s a professional camera trapped in a consumer-cam’s body.