From the December, 1998 issue of PC Alamode
Holiday Buying Guide:
New Year Resolution
by Susan Ives

The main factor in the quality of any digital image, whether printed or displayed on a screen, is resolution, or the number of pixels used to create the image. Whether you view your photographs on a computer screen or print them out, the images are composed of small dots called picture elements pixels. More and smaller pixels add detail and sharpen edges.

 There are numerous ways to express the resolution of an image. Cameras can describe their optical resolution by their dimensions in pixels (for example, 1152x864 or 480x640) or by the total number of pixels they are able to capture (for example, 1 million.) Both of these are objective standards, arrived at by counting the number of photosensors embedded in the camera. 

A third way of describing resolution, by the size of the photorealistic output, is more subjective. When Kodak says that its DC220 can create a 5"x7" photorealistic picture, it is a matter of individual judgement how realistic that photo really is. The final way that digital cameras measure their resolution is by their file size (for example, a file size of 5.7mb) which, in my opinion, is not terribly useful. 

To complicate matters, scanners and printers measure resolution in dots per inch (dpi) or pixels per inch (ppi); they mean the same thing. Monitor resolution is typically measured by the height and width in pixels, as in 480x640. The first number is the horizontal measurement, the second the vertical. Monitor resolution is approximately 72dpi. Consumer-grade printers can range from a low of about 150 dpi to a high of about 1200dpi.

 To further confuse you, cameras and scanners might also list an interpreted (also called interpolated) resolution. Some cameras and scanners can add extra pixels not captured by the hardware by inserting them based on the colors of the neighboring pixels. In your buying decision, the optical resolution, an absolute count of the number of the image sensor's photosites, is the only number you should pay attention to. Interpreted resolution is marketing hype. Interpolation can also be accomplished by imaging editing software, a better solution.

 Digital cameras are generally divided into three classes, depending on their resolution:

     VGA: VGA cameras produce 640x 480-pixel (or lower) images, a little over 300,000 pixels per image. Low-cost VGA cameras are fine for images you view on-screen, such as pictures sent via e-mail , posted to a Web site or used in a Powerpoint presentation, but the images will "pixelate" if you try to print them and larger than 3" x 5".

     XGA: With a resolution of 1024 by 768 (about 800,000 pixels), XGA images are excellent for on-screen work will print an acceptable picture up to about 5"x7".

     Megapixel: Megapixel cameras capture images that contain more than1 million pixels. Used with a special photo printer and coated paper, a megapixel camera produces snapshot-size prints nearly indistinguishable from 35mm prints and can be acceptably enlarged to about 8"x10".

To put this in perspective, your television set has a resolution of 320 x 525, or a total of 168,000 pixels. The human eye has a resolution of 11,000 x 11,000, or 120 million pixels. A color slide has about 20 million pixels.

 But the resolution of your camera is only half the story. The ultimate size and resolution of your picture will depend on how it is going to be used. Results will vary depending on whether your output device is a computer monitor or a printer. How big can you make a photograph before it starts breaking down?

 The formula for converting pixels into inches is to divide the number of pixels by the resolution of the output device. Computer monitor resolution averages at about 72 pixels (or dots) per inch. Let's say you buy a digital camera that has a resolution of 640x480. If you e-mail a photo to a friend for on-screen viewing or post it to a web page, the size of the photo will be:

     Width: 640 pixels / 72 ppi = 8.88"
    Height: 480 pixels / 72 ppi = 6.66" 
Now let's assume that your friend wants to print this picture. Clarke prints the magazine at a resolution of 300 dpi, so we'll start with that:
     Width: 640 pixels ö 300 ppi = 2.13"
    Height: 480 pixels ö 300 ppi = 1.6"
In other words, the higher the resolution of the output device, the smaller the photograph that can be reproduced without compromising on quality.

 The calculation for scanning a photograph is a variation of this formula. If you know that you will need a 3" x 5" photograph for a catalog, and that you will print the catalog at 600 dpi, you will multiply the number of inches times the dots or pixels per inch (ppi) of your printer:

     Width: 3" x 600 ppi = 1800 pixels 
    Height: 5" x 600 ppi = 3000 pixels 
If you try to stretch this image to make it larger in a word processing or desktop publishing program, you will still have the same number of pixels spread out over a larger area. To make an image larger or smaller for a particular output device, it must be resized in a photo-editing program. This interpolation can only take you so far; in my experience, you can interpolate a picture to make it about 20% larger. After that, the degradation becomes obvious.