The Brand Resource & Digital Asset Management experts

Low Res or High Res?

The resolution of an image refers to the number of pixels used to represent its graphic detail, measured in the number of pixels per linear inch (ppi). A pixel is the smallest element of a raster image.

The resolution of an output device like a printer, image setter or a computer or TV screen is measured in lines per inch (lpi), and it is the relation between the resolution of the image on the one hand and the output device on the other that determines the quality of the image.

For example, the resolution of a computer monitor is 72 ppi. If an image has the same resolution, it will look fine when viewed at actual size (100%), as is the case with images on a web page like this one. However, if the image were enlarged by 200%, the quality would deteriorate, because the monitor would display each pixel of the image spread out over 2 pixels of the monitor's resolution, resulting in a so-called 'pixelated' image - the naked eye can now distinguish the pixels that the image is made up of.

Laser printers can produce acceptable quality output at a resolution of around 400 lpi. Scanners and image setters can handle much higher resolutions of up to 1200 lpi. This is important when producing output of line art like text and vector artwork, which will be pin-sharp even when printed at large sizes.

It is important to note that raster line art images like JPEG files should not be reproduced larger than their actual size, because doing so will result in a pixelated image (see above). Vector files however, can be reproduced at any size because they are resolution independent. Since these files use mathematical coordinates to describe curves and lines on a page, the quality of the image is determined by the resolution of the output device and not by the size of the original image. Also, the size of an image in computer terms (the number of bytes) will be much larger when using a raster format like JPEG than when using a vector format, especially when printing at large sizes.

When dealing with so-called continuous tone or halftone images such as photographs, the rules regarding resolution are quite different. Any image that contains colour or shades of grey will be screened when it is printed in full colour (using the four process colours cyan, magenta, yellow and black) or in black & white. This means the image will be represented on the page by collections of halftone dots, small dots producing light areas and large ones producing dark areas in the image.

The quality of a printed halftone image is determined by the screen ruling - the number of dots in the image, again measured in lines per linear inch. Most newspapers are printed with a screen ruling of 65 lpi, whereas the screen ruling of magazines and brochures will be around 150 lpi. For an image to reproduce accurately, it will need to have a resolution of twice the screen ruling. A scanned photograph printed at actual size with a screen ruling 65 lpi must have a resolution of 130 ppi, and the same photo printed at 200% requires a resolution of 260 lpi. A higher resolution would be overkill and constitute a waste of disk space. A common misconception is that a photograph output on a 1200 lpi image setter should be scanned at 1200 ppi: the image setter will in this case use a screen ruling of max. 200 lpi, which means the photograph's resolution need only be 400 ppi. The resolution of 1200 dpi becomes relevant only when dealing with line art or vector images.

Common Problems

Because Visual ID provides a web-based solution, we often encounter the common misconception that media files need only be "low res" for inclusion in our system. If you are a customer of Visual ID, keep in mind that the system is designed to deliver high quality, reproduction ready material. Demand that your suppliers give you high resolution imagery only, to ensure that the Visual ID system meets everyone's expectations.