The size of a photograph 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 a resolution of 1800 by 1600 pixels or to contain 2.88 million pixels (1800 multiplied by 1600).

Camera Resolutions
In short, the resolution of a digital camera refers to the sharpness of its pictures. CCD image sensors contain a grid of photosites—each representing one pixel in the final image. The sensor's resolution is determined by how many photosites there are on its surface. This resolution is usually specified in one of two ways—by the sensor's dimension in pixels or by its total number of pixels. For example, the same camera may specify its resolution as 1800 by 1600 pixels or 2.88 million pixels (1200 multiplied by 800).

Low-end cameras currently have resolutions around 640 x 480 pixels, although this number constantly improves. Better cameras, those with one million or more pixels are called megapixel cameras. Even the most expensive professional digital cameras offer only about six million pixels or 6.0 megapixels. Of course, the higher megapixel designation a camera has, the more it costs.

Size Isn't Everything
The larger an image's size in pixels, the larger the image file needed to store it. For this reason, some cameras allow you to specify more than one size when you take a picture. This is usually indicated by stars–one, two, three, or four. Although you are likely to get better results with a higher number of stars or larger image, it isn't always needed—especially when the image is going to be displayed on the Web or printed very small. In these cases smaller images will suffice and because they have smaller file sizes, you'll be able to squeeze more into the camera's memory. In addition, most digital cameras allow you to switch from one size image to another during the same photo session.

Resolution of Digital Devices
Although more photosites often means better images, adding more isn't easy and creates other problems. It adds significantly more photosites to the chip so the chip must be larger and each photosite smaller. Larger chips with more photosites increase difficulties and costs of manufacturing. Smaller photosites must be more sensitive to capture the same amount of light. More photosites create larger image files, creating storage problems.