A pixel is the smallest element in a bitmap image such as a scan. Pixel is short for ‘picture element’. Zoom in on an image in Photoshop and you will start to see the individual pixels- the fundamental building blocks – that make up the image. When working in Photoshop, you are editing pixels, changing their colour, shade and brightness.
A key factor when working on bitmap images is resolution. This is measured in pixels per inch (ppi).
Pixels can vary in size. If you have and image with a resolution of 100 ppi, each pixel would be 1/100th of an inch square. In an image with a resolution of 300 ppi, each pixel would be 1/300th of an inch square – giving a much finer, less blocky result.
When working on images that will eventually be printed on a printing press, you need to work on high-resolution images. These are scanned images whose resolution is twice the halftone screen frequency (measured in lines per inch – lpi) that will be used for final output – that is when you output to bromide or film.
For example, for a final output screen frequency of 150 lpi – a typi8cal screen frequency used for glossy magazines – you need to scan you image at a resolution of 300 ppi.
Resolutions of double the screen frequency are important for images with fine lines, repeating patterns or textures. You can achieve acceptable results, especially when printing at screen frequencies greater than 133 lpi, using resolutions of 11/2 times the final screen frequency.
To work with images for positional purposes only, as long as you can get accurate enough on-screen results and laser proofs, you can work with much lower resolutions.