One of the ironies in digital photography is that although it makes it easier and less expensive to display images to millions of people one on one, or small groups gathered around a TV set, it makes it more expensive to show them to room-sized groups. The days of the old lantern slide show are long gone and 35mm slide shows have recently followed them off stage. Slide shows are now given with a digital projector (sometimes called a multimedia projector) and screen. If you've ever slept through a PowerPoint presentation, that was the technology being used. Your show can be stored on any of the computer's storage devices and played from there, or fed to the projector from a TV or DVD player. Here are some things to consider when choosing a projector.

• Projector technology. Two major technologies are currently used in most projectors. LCD (liquid crystal display) projectors form the image on a panel of LCDs and a bright lamp and lens projects the image onto the screen. Digital Light Processing (DLP) projectors use a solid state chip with hundreds of thousands or millions of tiny hinged mirrors on its surface. Light hitting a mirror tilted one way reflects through the projection lens to the screen. Light hitting a mirror tilted the other way reflects to a light absorber. By placing a spinning color filter wheel in between the light and the mirrors full-color images can be projected onto the screen.
• Resolution and aspect ratio. The projector's resolution, specified in pixels, determines how large and sharp the image is. Most are either 800 x 600 (VGA), 1024 x 768 (XGA), or 1280 x 1024 (SXGA). All of these resolutions have an aspect ratio of 4:3. However, some projectors are designed using the HDTV aspect ratio (16:9) and have resolutions such as 1280 x 720 (called 720p) and 1920 x 1080 (called 1080p). Generally, higher resolutions are better for photographs. However, if your photo doesn't have the same aspect ratio as the screen, it will fill the screen in one dimension and have blank borders in the other.
• Contrast ratio is a measure of the difference between the brightest and darkest areas of the projected image. Higher contrasts bring out subtle color differences and darker blacks making images appear richer. Ambient light lowers contrast in brightly lit rooms so you need a very high contrast ratio for those settings.
• Brightness is specified by an ANSI lumens rating, with higher numbers being brighter and better for photos. With lower ratings you may have to make the room darker or project the image from closer up so it's smaller. Projectors range from a low of 700 lumens to a high of about 5000. Since the brightness of some projectors fall-off in the corners of the screen, its uniformity is given as a percentage of brightness over the area of the projected image. An 85 percent or higher uniformity indicates an even distribution of light throughout the picture.
• Lamps. Projectors normally use either Metal Halide or UHP ("Ultra-High Performance") lamps with UHP being better. They tend to retain their brightness over their lifetime and have less color shift as they age. Lamps are rated with an estimated life in hours. This number actually represents the lamp's half-life—the point at which it be half as bright as it was when new.
• Color quality. The quality of a projector's colors are determined by a number of factors and can only be determined by doing side by side comparisons. However, one indicator is the number of bits used per color. For example a projector using 10 bits per color can display 1024 levels of red, green, or blue while one with 8 bits can only display 256.
• Lenses. Some projectors have a zoom lens and manual focus capability. The range of the zoom lens determines how large or small the projected image can be at a given projector-screen distance. Some projectors allow you to correct keystoning that occurs when the projector is pointed upwards at the screen. Without this control the image will be wider at the top than at the bottom. Some screens also have a lens shift function that lets you shift the image up/down or left/right quite a distance. This allows you to project the image with the projector offset from the center of the screen in any direction.
• Throw ratio. When giving a presentation to a group, you need to ensure that the projected image fits the screen and is large enough to be seen. The size of the image on the screen is determined by the projector's throw ratio the ratio between the projector's distance from the screen (throw distance) and the diagonal of the projected image. This ratio is fixed unless the projector has a zoom lens.
• Noise. The fan used to keep the projector's lamp cool makes some noise, listed in the specs in decibels (dB). A lower number indicates less noise.
• Remote control. If you like to walk around and talk as you give a show or adjust focus and zoom from your chair, you need a remote, preferably wireless. Some remotes have laser pointers built-in so you can point to parts of the photos, and mouse controls so you can also operate the projector and the device feeding images for the show.
• Card slots. Some projectors have a memory card slot so you can show images without connecting a computer.