With flatbed scanners now priced within most people's reach, photoshoppers are expected to be able to process and prepare a scan at the drop of a hat.

Before we get into the heavy stuff we'd better make sure we are all talking the same language!
 

Resolution Basics

'Image Resolution' refers to the pixels in an image and is best described in pixels per inch (ppi). A one inch square file for example with a resolution of 72ppi would contain 72x72 (5184!) pixels and would have a file size of around 6k.

The higher the resolution, the smaller the pixels, the more detail and clarity an image will have, the larger the file size!

A bad habit that a lot of people ( and scanning utilities ) have is to describe image resolution in dots per inch (dpi). This leads to confusion as dpi is also used to describe output resolution and monitor resolution. We are describing pixels, so we should use pixels per inch. PPI makes sense and leaves no chance of confusion!

'Monitor Resolution' should not be confused with image resolution or the physical size an image will print out at. PC monitors generally run at around 96dpi, MAC's 72dpi. Modern monitors can be run at a range of screen sizes, typically 640x480, 800x600, 1024x768 and 1600x1200 pixels. The size is set by the user but is limited by the capabilities of the video card.

When an image is opened in photoshop if it can be displayed at one image pixel to one screen pixel ('actual pixels') it will be, otherwise photoshop will display it in a 'fit to screen' mode. The size of the displayed image is irrelevant for printing purposes. It serves only as a way to see and work on an image.

'Output Resolution' refers to the number of dots per inch (dpi) that an output device produces.

Coupled with the dpi of an output device is it's 'screen frequency' or 'line screen ruling'. This refers to the number of halftone cells per inch in a colour separation or grayscale image and is measured in lines per inch ( lpi ). Luckily for us web guys we can snooze through lpi!