Saturday, August 13, 2005

Deep View: Resolution - PPI/LPI/DPI

Originally from Photoshop for Astrophotographers

Resolution: PPI, LPI, DPI

It is important to understand the relationship between the resolution of the image file and the resolution needed by the output device.

Image file resolution is specified in pixels per inch (PPI). Output devices are usually specified in either dots per inch (DPI) or lines per inch (LPI).

In some cases, such as with computer monitors, film recorders, and LightJet photographic printers, there is a 1:1 relation between the PPI of the digital image file and the DPI of the output device.

In most other cases, especially with desktop printers that advertise 720, 1440 or 2880 DPI, there is a vast difference between DPI and PPI.

If we have a scanned or digital original image with 3000 pixels of true optical resolution in one inch, we can send this to an output device and make a 10-inch print with 300 pixels per inch of output resolution. The exact same file, output as a 20-inch print will have a resolution of 150 pixels per inch. In both cases, only the output resolution has changed, and no data has actually been lost. In other words, the optical resolution remains unchanged.

For a device like a film recorder or LightJet printer that outputs in pixels per inch, each pixel is a fixed size with the edges of the pixels touching each other. The pixels vary only in brightness and color.

For a device like a printing press that prints halftone dots in lines per inch, the dots are printed into a grid of fixed size. The dots inside the grid can vary in size and are equidistant from each other.

For a device like a desktop inkjet printer, the dots of ink are a fixed size and the number of dots vary to give the appearance of different tones.


10 Pixels Per Inch (not to scale)
Pixels per inch specifies the real optical resolution of the file: the more pixels per inch you have, the greater the true resolution.

PPI terminology is used in specifying real resolution in image file sizes during scanning, and for output to devices such as film recorders that output a digital file back to film as a slide or negative.


Newspaper halftone dots
"Lines" Per Inch
Lines per inch is the terminology used in offset printing, which uses a line screen to break up continuous tone material into halftone dots.

Any type of printing device that uses ink is faced with a fundamental problem - the ink is only one color. Originally, printing presses only printed images in black and white. The ink they used was black ink. The ink had only one tone, or shade, and that was black. The paper was white. In the early days of printing, only line drawings could be reproduced - black lines on white paper.

In the late 1800's a method was discovered that used tiny dots of black ink to simulate different tones or shades of gray to create the illusion of a continuous tone image. If a lot of large dots were printed in a given space, it would look like a very dark tone. If only a few small dots were printed, there would be hardly any ink and it would appear as a very light tone, essentially the color of the paper the ink was printed on. A 50 percent gray, for example, would have half of the available area of the square in the grid printed with dots, and half of the area left blank.

This trick was accomplished by photographing the original image through a screen that was divided into a grid, much like a window screen. This was called a line screen because lines defined the grid. Inside of each square of the grid were printed a number of tiny dots of black ink.

A halftone gets its name because you either had a dot (one-half of the tone) or no dot (blank paper, the other half of the tone).

Newsletters usually reproduce images with a coarse line screen of 65 lines per inch. Newspapers use halftone screens with 85 lines per inch. High quality magazines use halftone screens with 133 lines per inch. Very high quality art books may use a line screen of 177 lines per inch.

The number of dots inside of a grid on the line screen can vary. A 133 line per inch screen may have 2540 dots per inch.

A rule of thumb in the printing industry is that you need 1.5x to 2x the number of pixels for a given line screen. So for a 100 line per inch halftone screen, you would need from 150 to 200 pixels per inch.


Inkjet printer dots
"Dots" per inch
Dots per inch is the terminology used by laser and inkjet printers.

Laser printers use a single size dot, but in a group of dots in a regular pattern to simulate a halftone screen. Laser printers usually work in the 300 - 600 DPI range but with a line screen of only 50 - 100 lines per inch.

Inkjet printers use smaller dots of ink and usually work in the 720 - 2880 DPI range.

Depending the sharpness of the image, and the quality of the printer, anywhere from 150 to 360 pixels per inch are required for excellent prints on today's photo quality inkjet printers. Experimentation is helpful, because you can usually get away with a lot less pixel resolution than you would think.

Inkjet printers that print with 1440 or 2880 dots per inch do not really provide much more resolution than 720 dots per inch. To appreciate this much printing resolution, you would need a large format original scanned at a very high optical resolution which would produce a very large file. Regardless, you would probably not need more than 360 pixels per inch of output resolution.


TV - Pixels per inch
Color monitors use a triad of dots or stripes, one each of red, green and blue, to produce a colored pixel.

To calculate the resolution of your monitor simply divide the resolution of your display by the actual size it appears on your monitor screen. For instance, I run my monitor at 1024 x 768 pixels, and the 1024 pixels on the horizontal side are spread out over approximately 12.8 inches, so 1024 / 12.8 equals 80 PPI.

For an image that is 1024 x 768 displayed on a monitor that is running 1024 x 768 resolution, there is a one to one relationship between the pixels in the image and the pixels on the monitor.

Rules of Thumb

OutputOutput FormNumber of Pixels needed
InkjetDotsNumber of dots divided by 2 to 8
Offset Presshalftone dots1.5 to 2 times the number of dots
Continuous Tone
(Film recorders, LightJet, Chromira)
Web GraphicsPixels1:1


Inkjet printer:

  • 10 x 8 inches at 720 DPI (720/5 = 144 PPI) = 1440 x 1152 pixels = 4.75mb file.
  • 10 x 8 inches at 720 DPI (720/2.4 = 300 PPI) = 3000 x 2400 pixels = 20.6mb file.
  • 10 x 8 inches at 2880 DPI (2880/8 = 360 PPI) = 3600 x 2880 pixels = 29.7mb file.

Offset press:

  • 10 x 8 inches at 1.5x for a 150 line screen = 225 PPI = 2250 x 1800 = 11.6mb file.
  • 10 x 8 inches at 2x for a 150 line screen = 300 PPI = 3000 x 2400 = 20.6mb file.

Continuous Tone:

  • 10 x 8 inches at 305 pixels per inch on a LighJet printer = 3050 x 2440 pixels = 21.3mb file.
  • 1 x 1.5 inches at 4000 pixels per inch on a film recorder = 4000 x 6000 pixels = 68.7mb file.


  • 10 x 8 inches at 800 x 600 resolution on a 17 inch monitor with 12.8 inches on the horizontal side = 62.5 PPI = 625 x 500 pixels = 916K file.
  • 10 x 8 inches at 1024 x 768 resolution on a 17 inch monitor with 12.8 inches on the horizontal side = 80 PPI = 800 x 640 pixels = 1.47mb file.

Web Graphics:

  • 10 x 8 inches at 800 x 600 resolution on a 17 inch monitor with 12.8 inches on the horizontal side = 62.5 PPI = 625 x 500 pixels = 916K file.
  • 10 x 8 inches at 1024 x 768 resolution on a 17 inch monitor with 12.8 inches on the horizontal side = 80 PPI = 800 x 640 pixels = 1.47mb file.


Epson suggests using 240 pixels per inch for high quality printing on good photographic papers with their photographic inkjet printers. You may be able to get away with as little as 150 pixels per inch, depending on how critical you are. Some people use 300 PPI or even 360 PPI. But this means that you have to have that much true optical resolution in your original. Using interpolation on a small file to reach these high resolutions will not provide the highest quality output.

Lets take an example of a sharp fine-grained 35mm transparency original scanned at 2700 pixels per inch of true optical resolution. This produces a 2700 x 4050 pixel file that is 31.3mb.

If we want to print this image with a print size of 10 x 15 inches on a photographic quality inkjet that prints with 2800 dots per inch, we would not multiply 10 inches times 2880 dots and think that we needed 28,800 pixels by 43,200 pixels to make a print. Remember, dots do not equal pixels in this context.

Instead, knowing that 300 pixels per inch will produce excellent output with these printers, we would divide the true optical resolution of the original file by 300 pixels per inch on the short side of the print and see that we had enough true resolution to output 9 inches (2700 PPI / 300 PPI). We could output 10 inches on the short side at 270 PPI, and this would probably also produce an excellent print.

No comments:

Post a Comment