Digital Camera Image Dimensions and Resolution

A digital image is made up of points called pixels . The brightness and color of each pixel is expressed in numerical terms. The most important data of an image is it's size in number of pixels . The number of pixels that make up an image depend on it's real size (the dimensions of the image once printed) and it's resolution , which is the number of pixels contained in a unit of measure and may be variable. The resolution is expressed in pixels per centimeter or per inch (this last is - ppi- is the most generalized unit of measure, which we will use here). The number of pixels of an image therefore depends on it's dimensions and resolution. For example, with the same resolution of 300 ppi contains the double of pixels that an image of the same physical dimensions and a resolution of 150 dpi.

Establishing a parallel with film, the resolution of an image is comparable to the grain. The greater the resolution, the greater the detail and richness of the images tones. We can produce an image with high resolution were the pixel is invisible, or one with low resolution were the pixels are so large that the are visible ( pixeling ). The same as with film's grain, the pixeling may be invisible for images of reduced physical dimensions, and even unacceptable from a quality point of view, when the size of the image is relatively larger. The problem is that the grain in film doesn't have a defined structure, while pixeling is a matrix of adjacent squares which calls attention to itself and creates a much greater interference, a noise . A print of a negative were there is grain can be acceptable, while a printed digital image which appears pixelized is something we should avoid with sufficient resolution. When producing digital images with a camera or scanner and manipulating them in the computer with a program we constantly encounter this variable.

300 ppi or more is high quality resolution. The results can be acceptably compared with a film image from 300 on. If we have a good printer, prints on quality paper have an excellent quality, without the pixel matrix being visible. This is true as long as the original photograph has been produced at that resolution in the camera or scanner.

Between 300 and 150 ppi we obtain an acceptable quality image, although the appearance on paper isn't very detailed, and around 150 ppi it is pixeled. It is an acceptable resolution to view our photos on the TV screen or computer screen. We can print smaller images at that resolution, drafts or documents that don't require maximum quality.

Between 150 and 72 ppi is the resolution to see photos on the computer screen. If we print images of this resolution the result will be very poor, far from the quality of a film image.

Starting with a original data resolution, with a digital photography editing program we can always obtain a second image, a lower resolution replica or of smaller dimensions. In both cases we eliminate part of the information contained in the original image. For example, if we have made or scanned an image of 18 x 24 cm at 300 ppi we can maintain the dimensions and reduce the resolution in half (the information is reduced in the same amount) or we can maintain the resolution and reduce the dimensions in half. These two new versions of the image contain the same amount of pixels (half as much as the original image). This reduction is usually made when we want a version to see on the screen, send by e-mail or store on a web page and don't want it to take to much memory.

Starting with the original dimensions we can obtain an image of higher resolution and lower dimensions. If, for example, we have and image of 18 x 24 cm at 150 ppi, we can reduce it's dimensions without eliminating information and obtain an image at 13 x 18 and 300 ppi. When printing the first image we will get a poor copy, because the resolution is too low. Nonetheless, if we reduce the dimensions without eliminating information, the number of pixels doesn't vary, it increases the resolution at the cost of a smaller size.

What we can't do is obtain more information were there is none. With an editing program we can make the given image go from the dimensions of 13 x 18 cm to 18 x 24 cm, but then the resolution will be reduced in half. If the original resolution of this case were enough, for example 600 pp, we would still be able to get a larger image with good resolution, since doubling the size would make it half of the original, meaning, 300ppi. Nonetheless, such a high resolution isn't frequent and we usually can't make large prints from a digital image because the resolution suffers. On the other hand, if we have an image whose dimensions don't allow us to increase the nominal resolution, but that doesn't increase the sharpness of the image, it only multiplies the number of pixels without adding more information.