When shooting a studio photo, the image size you use to capture it has a big effect on how large it can be displayed on the screen or printed. Generally, the best approach is to shoot at the largest available size. You can always make an image smaller in a photo-editing program, but you can never make it larger and retain the original quality.

The relative size of a digital image is determined by the device used to display it. However, the absolute size of the image is determined by the number of pixels used to create it (sometimes referred to as resolution). More pixels in an image add detail and sharpen edges.

The size of a digital image is specified in one of two ways-by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 3008 � 2000 pixels (where "�" is pronounced "by" as in "3008 by 2000), or to contain 6.016 million pixels (3008 multiplied by 2000). Since the term "megapixel" is used to indicate 1 million pixels, an image with 6 million pixels can also be referred to as a 6 megapixel image.

The number of pixels in a digital image is important because if you enlarge an image enough, it begins to loose sharpness and eventually the pixels begin to show—an effect called pixelization. This is not unlike traditional silverbased prints where grain begins to show when prints are enlarged past a certain point. The larger the image is to begin with—the more pixels that it contains—the larger it can be displayed or printed before pixelization occurs. However, with even inexpensive cameras capturing 2 and 3 megapixel images, most images will never bump up against this limit.

One advantage of larger images is seen when editing. Changes to such aspects as color balance, hue, saturation, contrast, and brightness are more effective on larger images because there is more image data to work form. After making these adjustments, you can reduce the file to the needed size.

Here are some rules of thumb about what image sizes you need for certain outputs.

• On the Web, images are displayed on screens that have resolutions of 1280 x 1024, 1152 x 864, 1024 x 768, 800 x 600, or 640 x 480. A few years ago, a 1024 x 768 monitor was unusual so most people in the industry settled on assuming that the lowest common denominator for screen sizes was 640 x 480 or, at best 800 x 600. For this reason, images should be of similar or smaller sizes- no more than 800 pixels wide (on eBay's Picture Services the maximum allowable size is 800 pixels wide). This ensures that images will display correctly on the vast majority of computers. If an image is too large, users will not be able to see it all at once and will be forced to scroll around it. If too small, details will be lost. Size also affects the speed with which images travel over the Web. Smaller (and more compressed) images travel faster so people see them more quickly.
• For laser and inkjet printers you need between 200-300 pixels or dots per inch. If your camera can capture images that are 2400 pixels wide, you can expect good results when printed at 8 to 12 inches.
• When images are printed on a printing press, as they might be for a catalog, the pixels in the image are printed as dots on the page. Photographic prints that are to be printed on a press are first "screened". To do this, a clear glass plate with scribed grid lines is laid on the photograph and then a picture is taken of the photograph through the screen. The scribed lines on the glass plate break the image up into dots called a "halftone screen". The negative is then used to create a printing plate used on the press. Today this process is usually done digitally, but it has the same result. The fineness of the screen used determines the quality of the printed image. Most photographs are screened with somewhere between 85 lines per inch (lpi) in newspapers to 200 lpi in high quality art books. For the best results, your images need to have 1.5 times as many pixels per inch as the screen's lpi (confirm with your printer). For example, if the printer is using a 150 lpi screen, your image must have at least 225 pixels per inch. This means to print a 4-inch wide image in a catalog, you need to have an image width of at least 900 pixels.