The Short Answer To Resolution

Digital photography is in flourishing and is blossoming right before our eyes.  The digital benefit is speed.  Everything happens near instantly.  Unfortunately, misinformation and mere opinion travel with the same speed.  I have heard a lot of partially informed discussion about and received many questions about image resolution.  So here is the rapid answer.  The long answer can be found here.

To fully understand resolution I have provided some background information on image sensors “Pixel v Pixels" found here.  This will be a strong foundation for the resolution discussion.

All digital images are made up of tiny squares called pixels, and each pixel in your image has a specific location, size and a very specific color value.  Digital cameras capture images at 72 “pixels per inch” (ppi) multiplied by the number of pixels present on the image sensor.  Scanned images and image editing software allow you to choose your resolution.  The pixel size is determined by the resolution of your image and is actually inversely proportional to resolution.  In a 24-bit (RGB) color image, there are 256 possible shades of red, green, and blue mixed to represent the color of each individual pixel.  256 shades of red, green, and blue can be combined to produce a total of 16.7 million different colors.  16.7 million colors are sufficient for photo-realistic output.

only 72 dpi

Printer resolution is given as the number of “dots per inch” (dpi).  Dpi is the maximum number of ink dots that the printer is capable of placing on your paper per inch.  Photo-quality ink-jet printers spray ink through multiple nozzles to produce various sized droplets with variable droplet spacing.  When the droplets are sprayed onto paper the drops overlap, combine, or are spaced so as to reproduce the color of the individual pixel being printed.  The printer may have to spray a drop of all six colors on top of each other to generate a dark color.  In other words, it takes multiple drops of ink from the printer to create the color in each square pixel.  So even if the printer resolution is given as 1440 dpi it does not mean that there are 1440 dots lined up in a row in each inch of the print.  Much to the surprise of most people, the printer’s resolution numbers have nothing to do with the size of, or the number of pixels in your image.  Pixels per inch (ppi) and dots per inch (dpi) refer to totally different things.

You can now be advised that you would not want to send your printer a file with the camera or input resolution being the same as the stated printer resolution.  In fact, you only need to send a file closer to 240 - 300 pixels per inch.  This solution is ideal, as this allows the printer maximum flexibility in combining the ink colors to generate the color of each pixel.  If you were to send an image sized at 1440 ppi to your printer it would simply throw out most of your data.  The worst part is that in the printer driver interpolation, you have no idea which pixels the printer (and its software) will decide to get rid of, and your printed results would be totally unpredictable.

Images that will be viewed on a computer monitor such as those destined for e-mail or display on the worldwide web should be sized to a resolution of 72 ppi.  This is because image pixels will be translated directly into monitor pixels.  The size at which the image will actually be displayed is dependent on the size of the viewer’s monitor, the monitor’s resolution setting, and the actual pixel resolution of the monitor.  When the image resolution is higher than the monitor resolution, the image will be displayed on screen at a larger size than the print size.  Therefore viewer would have to scroll all around to view the image, which would be inconvenient. For example, if you want to e-mail an image and be certain that it does not exceed the size of the recipient’s monitor you would size it for 4x6 inches and set the resolution to 72 ppi.  If the recipient’s monitor is a 72-ppi monitor (Mac) the image will display at 4 x 6 inches, and if they have a 96-ppi monitor (PC) the image will display at 3 x 4.5 inches.  Both image sizes would easily fit on a small monitor without having to scroll to see the whole image.

Image resolution may be set, changed, or viewed in any editing software program, such as Adobe’s Photoshop.  Photoshop is expensive and it requires a long learning curve to operate, but it is the worlds most powerful and most often used editor.  Editing software is where all color, cropping and repairs decisions are made.  It is also where resolution is determined for web use, e-mailing, or printing.

Remember to keep you “ppi” separate from your “dpi” and life will be much smoother.

From askRodger@pictureline.com