Digital Photography Tutorial: Resolution Explained

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What is Resolution?

Resolution is a measurement of how much detail an image holds. It is also a measurement of output capacity for printers, scanners, monitors or other digital devices. Film images used resolution to describe the detail of an image. Digital images also refer to resolution, but use different terms to describe the detail. Resolution can be confusing because so many applications and devices use different terms for resolution. Adobe Photoshop uses pixels per inch (ppi) while other applications use dots per inch (dpi). Each term refers to a different type of resolution measurement, but both are measurements of resolution. Resolution can be used to describe print size, size of a scanned image or the size of an image viewed on a digital display.

Resolution Measurements

The key to understanding resolution is to know each term and how it is used. There are four main types of measurements of resolution.

1. SPI (samples per inch) is the terminology used to describe how a scanner measures resolution. This is the number of individual samples taken in an area of one linear inch. The more samples the scanner takes, the more detailed the scan will be and closer to the original image being scanned.

2. DPI (dots per inch) is used by printers and refers to the number of dots of ink used by the printer to reproduce the image. The higher the number of dots per inch, the sharper the image. DPI is the resolution of the printer or other digital output device.

3. PPI (pixels per inch) refers to the number of pixels displayed on a computer monitor, the display resolution. It also refers to the image resolution and the reproduction size. For example, an image size of 1600 x 2500 pixels at 300 ppi will be 8 by 5.3 inches when printed. A higher PPI gives greater detail and sharpness.

4. LPI (lines per inch) is how printers print a picture using halftone screening. This refers to how close the lines are in a halftone grid. Higher LPI means more detail and sharpness.

Some people use many of these terms interchangeably, but they do not refer to the same measurements or reproduction sizes. Books, tutorials and manufacturers use many of these terms to encompass a variety of output uses. Consider the context of how the image is going to be reproduced to help you figure out which of the above terms actually applies. Understanding these terms will help to understand the different digital devices from camera resolution to output printing resolutions.

How to Use Resolution

Resolution is extremely important for quality output. The end use or output device will determine the type of resolution used. For example, an image that is to be printed would not look sharp printed at 72 dpi, but this same image would be appropriate for use on the Internet. Here is a breakdown of how to determine the resolution needs of your image.

  • Print Images should be sized using dpi and matched to the printer being used. The maximum dpi of a printer will be the largest resolution that can be printed. Use the resolution that best fits the output needs. If the image is to be printed on a flyer, 300 dpi will be sufficient resolution. For an image that is going to be used in a brochure or other advertising promotion, 600 dpi should be enough resolution to look good. The average consumer photographer may want to use 1,200 dpi and professional photographers would use 2,400 dpi. Printing an image that is going to be used in a small hand out or other low-resolution output at a high dpi is a waste of ink. Try to match the use of the image and the resolution used.

  • Printer resolution will limit the image quality of the final print. If the printer has a resolution of 600 dpi, it will not print an image larger than 600 dpi. If an image is sized to 1,200 dpi and printed on a 600 dpi printer, it will only print at a maximum of 600 dpi. Printers have a resolution that they are rated. A common printer resolution is 1,200 dpi. Some printers will have two numbers such as 720x360 dpi. This means more dots are placed top to bottom giving the printer a higher vertical resolution. Some printers are optimized, meaning they layer dots of varying colors in multiple passes over sections of the print. This type of printer uses more inks, but the image is richer.

  • Display image resolution is typically 72 ppi or 96 ppi, anything larger will not have more detail because of the limitations of the display. The ppi size is what actually appears on the display screens. An image that is going to be used on the Web should be sized to 72 or 96 ppi.

  • Display resolution is another form of resolution for computer displays that lists the number of vertical and horizontal rows of pixels, such as a display resolution of 1920 x 1200.The display resolution does not tell anything about the resolution of the display, but rather the size or number of pixels per given area of the display.

  • Scanner resolution is necessary to determine how detailed an image will be scanned from an original. The rule of thumb is to scan a larger resolution than the final image. This is done for two reasons, one is the image may need to be cropped and the other is because there is always some loss when converting an image from print to digital.