Image resolution

Image resolution and associated abbreviations such as dpi and ppi confuse a lot of people. This page tries to explain what resolution is and how it needs to be taken into account when creating pages or printing pictures.

Since resolution has to do with digital images, let’s first look at such an image. We’ll use an image which I shot a few years back in a local zoo.

If such an electronic image is enlarged a lot, you can see that it consists of a matrix of picture elements (or pixels). Below are the pixels that make up the eye of the bird. The original image was shot using my Nikon D70 camera. It contains around 2000 rows of pixels, with each row containing 3000 picture elements. Sometimes resolution is used to describe the image size in pixels. The image of the bird has a resolution of 3000×2000 pixels or 6 megapixels.

To confuse matters, resolution can also refer to the actual size of the picture elements. That is its traditional meaning in graphic arts. The problem is: how large is a pixel? Since a digital image is not physical, pixels don’t really have a size. They can be as big or small as you want them to be. In the first image, each pixel matches the size of the pixels of your monitor but it is no problem to imagine that all those pixels could also be big enough to fill the football field of Chelsea.

The moment that such an image is sent to a physical device like a computer screen or a printer, you need to define the size of those pixels. For some reason the size of pixels is not defined in millimeters or inches, but rather by stating how many of them fit in an inch. This is even common practice in countries where the metric system is used. The abbreviation for pixels per inch is ppi although a lot of people refer to it as dpi (dots per inch).

So let’s do some math:

• My original image is 3000 pixels wide.
• If I want my image to be 10 inch (around 25 cm.) wide, the resolution of the image will be 3000/10 = 300 ppi.
• If I want the image to be 5 inches, the resolution goes up to 600 ppi.

Resolution and output

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 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 many 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.
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 logo’s 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.

Printing photos

Usually 250 dpi is considered the optimum resolution for printing high quality photos. Don’t be fooled by the fact that a 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 photolab 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 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.

Images used in billboards

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 camera’s are perfectly capable of generating such files.

Resolution too high or too low

If you are not sure what resolution images need to have for a certain project, consult your printer.

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.