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When an image is sent to a certain output device, such as a printer or a monitor, its resolution becomes important because each device or medium needs a certain base resolution to reproduce the image with the best possible quality. Let’s look at some typical uses of images:
Magazine printing
For images printed on good quality paper using an offset press, the general rule is that the image resolution needs to be twice the screen ruling used to print the job. Magazines are typically printed using a 150 or 175 lpi screen ruling. This means images need to be 300 dpi. My bird picture is 3000 pixels wide, which means that for best quality it can be printed up to 3000/300=10 inch wide.
Please note that a slightly lower resolution such as 220 to 250 dpi is also perfectly acceptable for most people. For high quality jobs such as art books or for images containing critical patterns such as brick walls or clothing with a pattern in it, a higher resolution is sometimes recommended by the printer. Typically 400 dpi is used for such images.
Naming your files
The above rule applies to both color and grayscale images. The notable exception is line art, pure black and white drawings that contain straight or curved lines. Cartoons or logos are examples of line art. Such images need to have a much higher resolution. Most printers recommend 800 dpi as the bare minimum. Preferably resolutions of 1200 to 2400 dpi are used.
Newspaper printing
Newspaper are printed at a higher speed on lower quality paper. This means the resolution requirements are not as high as those for magazines. Typically a resolution of 200 to 250 dpi is considered sufficient. For line art images, 400 to 600 dpi is recommended.
billboard printing
The larger a picture gets printed, the lower its resolution needs to be. The main reason behind this is that the viewing distance also increases. For large billboards, 30 dpi is often sufficient – which means that a lot of digital cameras are perfectly capable of generating such files.
Printing photos
Usually 250 dpi is considered the optimum resolution for printing high quality photos. Don’t be fooled by the fact that a photo printer has a much larger resolution, such as 720 or 1440 dpi. The printer may be able to print very small dots but it can only accurately reproduce colors by combining a large number of dots to emulate various tints. That is why a 250 dpi image offers perfect output quality on a 1000+ dpi printer.
The professional photo lab equipment used to print thousands of images per day also has a higher resolution, typically 300 to 600 dpi. The same rule applies for pictures printed using such a machine: 200 to 250 dpi offers excellent quality.
Viewing an image on a computer monitor
Most computer screens have a resolution of around 100 dpi. That means my picture of the bird is big enough for a 30 inch wide computer screen. That is 30 inch horizontally, not diagonally. Such computer screens aren’t for sale (or affordable) yet! The resolution of digital cameras far exceeds that of monitors at this point in time. The resolution of television screens is usually even lower than that of computer screens.
Resolution and print quality
If you are not sure what resolution images need to have for a certain project, consult your designer, Brent Ferraro.
If the resolution of an image is too low, this results in
- a loss of sharpness.
- Images also get a ‘pixelated’ look.
- Straight lines will show a staircasing effect.
Too much information may sound as if it is harmless but that is not true:
- Your file will be bloated, taking up more storage space, time to print or time to transmit
- Images can lose a bit of sharpness.
Other parameters that determine image quality
Don’t forget that resolution is only one of the parameters that determine the quality of images in a printed job! Image sharpness, noise, color accuracy and the composition of a picture are as important as its number of pixels. Using a lossy compression algorithm like JPEG can make images blurry. The choice of printing paper and the settings of the press also have a huge impact.