By now, chances are you have a digital camera or have thought about buying one. Approximately 6 out of 10 adults in the U.S. own one, according to an April 2008 report from market research firm Forrester Research. It's no surprise.

These marvels of technology make taking and sharing photos more convenient and fun than their film-based predecessors, and for most purposes, they have equaled or surpassed their quality.

It can be tricky, though, deciding when to replace your trusty old, and possibly obsolete, digital camera with a spanking new one. The following tips may help. They may help as well if you haven't yet gone digital and want to.

Digital cameras come in two main varieties, just like film cameras: compact cameras (also called point-and-shoot cameras) and single-lens reflex (SLR) cameras. Compact cameras are smaller, easier to use, and less expensive, while digital SLRs (dSLRs) produce higher quality photos and are more versatile.

People replace their digital cameras about every 2 years, much like a cell phone, according to anecdotal evidence and reports in computer and photo magazines. Over the years, digital cameras have become smaller while producing better images.

The first commercially available digital camera, the 1990 Logitech FotoMan (also sold under the name Dycam Model 1), let you take only black-and-white pictures at a grainy resolution of about 100,000 pixels. It retailed for about $900. Today you can buy a low-end digital compact camera for as little as $100 that produces color photos at 8 million pixels (8 megapixels). The average price for a name-brand digital compact camera is about $200.

All those pixels, or picture elements, mean higher resolution for sharper detail. But pixel capability is only one aspect of digital camera quality, and it's often overrated. Even more important is the quality of the lens, and it's here that dSLRs, with their superior optics, outdo compacts.

One trend in the digital camera market is the move away from compact cameras and toward dSLRs. The more expensive dSLRs made up only 8% of the market for digital cameras in 2007, according to market research firm IDC, but sales grew more than twice as fast as for compact cameras. Low-end dSLRs cost around $450, with the average price for a name-brand camera about $800.

A digital compact camera remains a good choice for a typical home user. Just stick it in your pocket or purse, point and shoot, and output the pictures to your inkjet printer. Lots of other options exist as well, including uploading the pictures to your computer where you can correct or enhance them with an image editing program such as Photoshop Elements (www.adobe.com/products/photoshopelwin for Windows, www.adobe.com/products/photoshopelmac for Macintosh) and showing them off to others through email or a photo-sharing website such as Shutterfly (www.shutterfly.com).

In Consumer Reports' latest round of digital camera testing, in its July 2008 issue, Canon's PowerShot A650 IS received the best overall score for compact cameras, while Sony's Cyber-shot DSC-W200 scored best in the magazine's subcompact category, with subcompacts being ultrathin cameras you can easily slip into a shirt pocket. If you take a lot of sports or other photos from a distance, a zoom lens is a convenience, and Sony's Cyber-shot DSC-H9, with its 15x optical zoom, received the top score for "superzoom" compact cameras.

Today's digital compact cameras typically produce images of 7-12 megapixels in size. Higher resolution cameras are better for 8x10s and larger prints and for cropping out extraneous parts of a picture.

For years after digital SLRs were first introduced commercially in 1991, they were used primarily by professional photographers. Then lower cost prosumer (professional consumers) models, targeting advanced amateur photographers, were released.

Most dSLRs today fit into the prosumer category, though the lower cost models are often referred to in reviews as "entry level" because they're aimed at users crossing over from compact cameras. With dSLRs, not only is image quality better, particularly in lower light situations, so is speed and versatility. You can take one photo after another more quickly, and you can use different lenses on the same camera.

Consumer Reports ranked the Nikon D80 as the top "basic" dSLR camera and the Nikon D300 as the top "advanced" dSLR, targeting professional photographers. The Nikon D60 is a similar but newer camera than the D80, released after the magazine did its testing. It's lighter and, at about $650, is $100 less expensive. According to reports from users, overall, it's a better choice unless you need the autofocus feature of older Nikon lenses.