Wednesday, January 23, 2008

Just how big is a pixel anyway?

Scott Condray from Visualville was kind enough to share this lesson plan with us from his Photoshop course. This knowledge combined with the fact that our LightJet images files at the equivalent of 4,000 dpi can be a powerful tool when preparing your files.

It’s Not The Size Of Your Pixel… It’s How You Use It!

In this Lesson we’ll explore the concepts of resolution, dpi and ppi.

A pixel is the smallest unit in a digital file. Digital files consist of a series of pixels running the length and width of the file, thus creating a two dimensional image. Open an image file in Photoshop and zoom in as close as you can, (1600% is the maximum) and you’ll be able to see the individual pixels that make up the image. The higher the number of usable pixels per inch, the higher the resolution. Resolution is more about the ability to show sharp image detail and less about the physical size of an image.

Digital capture devices, (Scanner and Digital Cameras) measure resolution in pixels per inch (ppi). Output devices, (photographic printers and printing presses) measure resolution in dots per inch (dpi) they are not equivalents. Scanners and digital cameras create pixels, not dots, however the pixels will eventually be output as dots.

For example:
A 300 pixel per inch scan, output on a printer with a 300-dpi resolution will yield a print that is one inch in size (one dot was created for each pixel).

It might be better to ask, “How big is a dot?” For a printer capable of printing 300dpi, each dot is 1/300 th of an inch. If you print a file that is 2400 pixels by 3000 pixels on a printer that prints 300 dpi, and the printer produces dots that are 1/300 th of an inch then one inch of paper will be printed for every 300 pixels in the file. The size of the print output will be 8 inches by 10 inches. How big is the pixel? 1/300 th of an inch.

If you output the same file on a printing press that produces a 600 dpi output each dot will be 1/600 th of an inch. With 3000 pixels in the file, the press will produce one inch of image on paper for every group of 600 pixels. The total output will be 4 by 5 inches. In both cases there are 2400 X 3000 pixels in the file, but one output device yielded an 8 X 10 and the other a 4 X 5. Which device produces the higher resolution… the printing press.

Think of resolution in another way. Digital files do not have any size other than the space they occupy. How big is an RGB file that is 2,400 by 3,000 pixels? About 20 MB. How big is that? The answer is we don’t know until those 2,400 by 3,000 pixels are output.

So how big is a Pixel? As big as you want it to be. Wouldn’t it be great if everything worked like that!

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