High and low resolution refer to the concentration of dots or pixels in a digital display or printed images and documents. Generally, the higher the resolution the more crisp the image. Because the image is made up of tiny pixels, the size of an image is determined in large part by its resolution. Resizing an image beyond the limit of its resolution causes it to be grainy or pixilated.
The way the brain works to interpret images plays a role in the difference between high and low resolution. Gestalt psychologists explain that the brain perceives dots that are very close together as a single, continuous image. Monitors and printers exploit this phenomenon to render images.
Resolution is measured by the number of pixels that appear in an inch (ppi) or by the number of pixels that make up a line. A digital camera with a resolution of 4000 pixels x 3000 pixels would have a 12-million pixel sensor and would be marketed as 12 megapixels. The size of the photograph determines the ppi. A 20-inch monitor with a resolution of 1920 x 1080 can display a maximum of 110 ppi.
Input vs. Output
The resolution of the captured image (input) sets the upper limit of the resolution of the photo when it is printed or displayed on a screen (output).
A photo captured with a 12 megapixel camera can produce a 13.3" x 10" image at 300 ppi. The camera captured the image with a maximum of 12 million pixels, and resizing the image reduces the number of pixels per line. After the image has been captured, you can't increase the number of pixels available to the image.
"Digital SLR Pro Secrets," by David D. Busch, explains that some programs will allow you to resample the image to inject new pixels. "But there is no way to add detail that wasn't captured in the first place," he says.
The method used to save the file to your computer also impacts the resolution. In some cases, saving your file with the file compression set too high will degrade the resolution of the image beyond repair. Since most computer monitors display more colors than the eye can see, Joint Photographic Experts Group (JPEG) file format deletes pixels that you will not miss. The process allows for a balance between file size and image quality.
Rick Mathews of the Wake Forest University website explains that the terms ppi and dpi (dots per inch) are used interchangeably, but do not technically mean the same thing. Printers must use multiple dots to create a single pixel, so many more dots are required per inch than pixels.
Higher resolution is not always better. Images displayed on a computer screen at 72 ppi look just as good as images at 300 ppi. A 300 ppi image file takes significantly more memory and would take much longer for you to download. Most professional websites display images at 72 ppi.
Printed images require a much higher resolution. An image at 72 ppi would need to be set to quite a small size to be printed without obvious distortion or pixilation. Most images are printed at 150 to 300 ppi.
- Wake Forest University
- Digital SLR Secrets; by David D. Busch;Thomson; 2006