Paper: DSDN144 Digital Photographcis Info Sheet: Image Resolution Image Resolution:

• refers to the number of pixels per unit of measure in the digital image

• expressed in pixels per inch (ppi), should not be confused with dots per inch (dpi) - which is a measurement of output resolution on a printer

If you keep increasing the size a digital photo the pixel size will continue to increase until you can see each separate pixel, resulting in an image that looks jagged and blocky

A 457.1 Kb digital image with a resolution of 300 pixels per inch will print at 3.34cm2. The pixels are too small to see.

A 457.1 Kb digital image with a resolution of 72 pixels per inch will print at 13.93cm2. The pixels will be visibly noticable.

A 457.1 Kb digital image with a resolution of 10 pixels per inch will print at approx. 1m2. The pixels will be extremely obvious.

Put in simple terms, smaller-sized prints will have smaller-sized pixels and more of them to the inch (a higher ppi count) while of the same photo larger-sized prints will have larger-sized pixels and less of them to the inch (lower ppi).

Using too low a resolution for a printed image results in pixellation--output with large, coarse-looking pixels.

Using too high a resolution (pixels smaller than the output device can produce) increases the file size and slows the printing of the image; furthermore, the device will be unable to reproduce the extra detail provided by the higher resolution image.

72 ppi 300ppi

Interpolation

• resamples the image by adding more pixels to it

• if you increase the dimensions in Photoshop’s Image Size dialogue box with the Resample Image box ticked your image will interpolated

• Photoshop uses a mathematical calculation to “guess” the colour of the added pixels.

• Can be done within reason but too much and your image will be noticably degraded.

• Try and avoid increasing your image more than 130% and never more than 150%.