The availability of digital cameras, in both camera form and in cellphones, has nearly rendered film photography obsolete. However, the technology has introduced the task of printing your own pictures. Pictures taken on film were sent to a photography lab, and you ordered what size prints you wanted. Now, you possess the image in digital form and must it to the size you want or size it to view on your monitor. To calculate the number of pixels for an 8-by-10 picture, you need to understand pixels-per-inch and image resolution.
Pixels Per Inch and Resolution
In basic terms, a pixel is a dot. In a digital image, each pixel is a dot of color. When viewing hundreds of pixels together in a coordinated sequence, our eyes see an image. PPI is just what its name suggests – the number of pixels equal to one inch. DPI, or dots per inch, is sometimes used interchangeably with PPI, though DPI is generally used when referring to the output of the image, such as on a monitor or when using a printer. PPI refers to the image itself.
The PPI of an image is its resolution. The higher the resolution, the more pixels per inch the image has. An image with a resolution of 150 PPI will be a lower-quality image than one with a resolution of 300 PPI. The higher the resolution, the more pixels per inch and the more fine detail the image has.
The display on your computer monitor could be as low as 72 DPI or as high as 150 DPI. To display an image on your computer and have it appear 8 inches by 10 inches on your monitor, the image would have to be 576 pixels by 720 pixels for 72 DPI and 1,200 pixels by 1,500 pixels for 150 DPI. Calculate the desired size – in this case 8-by-10 – by the dpi of the monitor. 8 inches times 72 PPI equals 576 pixels.
An image 576 pixels by 720 inches that appears 8-by-10 on your computer monitor will print either very poor quality at 8-by-10 or a very small image. This is because the resolution of the screen presentation is much lower than the resolution required to produce a quality printed image.
An average quality photograph is 300 DPI. To print an 8-by-10 picture at 300 DPI, the image needs to be 2,400 pixels by 3,000 pixels. Depending on your printer, you may be able to print an 8-by-10 image at 1,200 DPI or even 2,400 DPI. At 2,400 DPI, the image needs to be 19,200 pixels by 24,000, which may be a lot for your graphics program to handle. At 1,200 DPI, an image 9,600 pixels by 12,000 pixels may be more manageable. The higher the resolution, the more pixels per inch and the higher the quality of the printed image.
You don't have to follow a hard and fast rule when you're trying to determine an 8-by-10 image in pixels; quality depends on the resolution of the image. By determining what resolution is desired for your intended purpose – screen display or printed output -- calculate the number of pixels by multiplying 8-by-10 by the resolution.