This camera is rocket-fast, too. Autofocus is nearly instantaneous, shutter lag (the delay after the button is pressed) is zero, and you can snap 4.5 photos a second for as long as you keep the button pressed.  There are some new features: a clever calendar that lets you hunt down photos by date, right on the camera; an effect that simulates the spherical view of a fish-eye lens; a jack for an external G.P.S. receiver that is coming from Nikon (for geotagging pictures); a function that straightens off-kilter horizons; face-recognition autofocus; and so on.  But none of that is the big news.  If you want a hint, consider the D90’s Live View feature, with which you can frame a photo using the extremely sharp three-inch screen.  This, of course, is how every compact camera on earth works, but until Olympus pioneered the system a couple of years ago, S.L.R.’s required you to compose a shot by pressing your eye to the optical viewfinder. The screen was used exclusively for playing back pictures.  The challenge, on an S.L.R., was figuring out what to do with the mirror inside. Ordinarily, it bounces light from the lens into the eyepiece; it flips out of the way, exposing the sensor, only at the instant a photo is taken. To display a live image on the screen, you’d have to lock that mirror out of the way, so that the sensor receives light continuously.  That’s exactly what happens when you press the Live View button on the D90. You hear a little clack — that’s the mirror flipping aside — and a live video image appears on the screen. Now you can shoot photos at angles where holding the camera to your head just wouldn’t work; it comes in handy more often than you might think. At this point, you can also zoom in to assure perfect focus.  There are compromises, though. When the mirror is out of the way, the D90 relies on a slower, contrast-detection autofocus system; it can sometimes take several seconds to lock focus in Live View mode.  But don’t knock Live View. It, after all, is the key to the breakthrough feature of the D90, the secret that turns it into a completely new kind of recording instrument. Ready?  The D90 is the first S.L.R. in the world that can record video.  High-definition video, at that. Stunning, vivid, 720p, widescreen, 1280-by-720, 24-frames-per-second video, with the color and clarity that only an S.L.R. can provide.  Evidently, it occurred to some engineer: “Hey, we’re already showing a video image. Isn’t that, in essence, what Live View is? Maybe we could figure out a way to record it!”  Now, most people’s first reaction is: “Well, duh. My $200 Canon has been capturing video for years.” Or maybe: “What a gimmick. Who’d ever use video on a $1,000 piece of photographic equipment?” Or, at best: “Well, I guess it might sometimes be useful to snag a video clip when I’m out there shooting stills.”  But there’s something much bigger going on here. Remember: any control, effect or lens that’s available to the D90’s still photos is now available for videos.  Think of all the freedom you gain that you wouldn’t generally have on a camcorder: control over focus, depth of field and exposure; special effects like fish-eye, monochrome and vivid; and excellent image stabilization when using a Nikon VR lens.  But here’s the real mind-blower: You now have a video camera that takes interchangeable lenses. Before the D90, if you wanted a hi-def video camera with removable lenses, you’d pay $7,000 for the camera alone, and another $7,000 to $20,000 for each lens.  On this camera, though, I tried Nikon’s $500 fish-eye lens, and filmed a complete 180-degree vista without having to turn or pan. With a macro lens, I filmed a bumblebee, huge and clear as though it were in a National Geographic documentary. With a huge telephoto lens, sitting in my bleachers seat at the Pilot Pen tennis tournament, I was suddenly filming what other people could capture only as still images. (You can see sample stills at nytimes.com/personaltech.) Independent filmmakers, rejoice.  Meanwhile, the zoom ring gives you far more zooming control than a typical camcorder does. For example, you can do a snap zoom, from 1X to 10X, in half a second.  Now then, before you get on the D90 waiting list, a word of warning: it’s not a camcorder.  Its screen does not flip out and swivel, so you can’t easily film yourself. There’s a microphone and speaker, but the sound is mono. And the AVI-format video, while easily edited in programs like iMovie and Movie Maker, eats up an insane amount of memory-card space — about 400 megabytes a minute. (Fortunately, the camera accommodates SD cards as big as they come — 32 gigabytes, for example, which is good for 80 minutes of video.)  An individual shot can’t be longer than five minutes in hi-def, either, or 20 minutes in standard definition; after that, the sensor heats up too much. So don’t expect to film the school play on this thing.  Worst of all, the autofocus doesn’t work for video. You can autofocus before you start rolling (half-press the shutter button), or you can use the manual-focus ring while filming. But those are kludges; if the distance to the subject changes while you’re filming, moments of blurriness are inevitable.  In other words, the Nikon D90 presents benefits and drawbacks unlike any other category of image-recording equipment. On one hand, your creativity may be challenged by the lack of autofocus, the limited shot duration and the enormous file size.  On the other hand, the D90 brings you an incredible range of video image controls; interchangeable lenses at a fraction of the usual prices for video; a gigantic sensor that takes videos in lighting conditions that would render traditional camcorders useless; and the convenience of capturing short, sharp video clips without hauling around a second machine.  It’s pretty funny that it comes from Nikon, a company with practically no experience in video cameras — and not, say, a camcorder/camera behemoth like Sony or Canon.  But who cares? Even with its 1.0 limitations, the D90 is an outstanding still camera and an eyebrow-raising video camera. Check it out, people: the era of the video S.L.R. has begun.  A version of this article appears in print on August 28, 2008, on Page C1 of the New York edition with the headline: New Nikon Holds A Secret. Order Reprints| Today's Paper|Subscribe
The Nikon D90 camera, arriving in stores next month, has the features and performance of a single-lens reflex, and can record high-definition video.