by Colin Johanson (1998)

All those different types of computer files used for graphics can be confusing. They all have their strengths and weaknesses and particular roles to play. We won't bother with what their full names are, just accept that they are referred to by their file extensions.

TIF, BMP, PICT, GIF & JPG files are all raster or bitmap format files, that is they describe a picture dot-by-dot. TIFs are used by both PC and Mac and are how most scans and pictures are saved. Large size is a problem but can be saved in RGB (Red Green Blue, for on screen or colour laser printing) or CMYK (Cyan, Magenta, Yellow & Keyline, for process colour printing) form. BMPs are similar but are PC specific - used for screen backgrounds. PICTs are a Mac format. Both of these last two have their place but may be better saved as TIFs for cross-platform compatibility.

GIF is a very useful file for Internet use as it only holds information in up to 256 colours, with less colours and solid colours making smaller file sizes. Can even hold a sequence of pictures called an 'Animated GIF'. Another feature of this format is its ability to have transparent gackgrounds - very useful for Web graphics.

JPG files are used both for printing and the Net due to their compression into a small final size. 10% of a TIF size file or less is often the case. Not limited to 256 colours but can cover 16.7 million colours so are used for photos where better colour detail is required but in a small file size. The more compressed, the more quality will be lost but most people wouldn't notice the differences at moderate compression.
Raster or 'bit-mapped' graphics need to be scanned or created at sufficient resolution to avoid the 'jaggies' - stepping visible on angles or curves, that make a print look rough. A guide is 50 to 100 pixels (dots) per centimetre (130-260 dpi) of final size graphic for standard to quality work in colour. 240+ pixels per centimetre (600+ dpi) for line art (diagrams or cartoons) in limited colours.

Saving a file in RGB is used for on-screen (Internet, PowerPoint) and some direct colour printing methods. CMYK is the colour format required to create separations for offset printing and for most high end colour printers. Save your files as required for each job or ask your bureau to convert to CMYK if you don't have the required software.

EPS, WMF, CDR & Illustrator files are what's called vector graphic formats in that they 'describe' a shape such as a circle by diameter, position, x and y stretch, filled colour and edge thickness and colour. This usually gives a small file size with the capability to be scaled up without jagged edges. Note: These file types can also contain bitmap graphics that will become jagged if over enlarged.
EPSs are the standard for high end printing and can be made from a TIF to create a transparent background - very useful. You will need to print any file containing EPS graphics via a Postscript printer and driver.

WMFs are PC specific and only contain 256 RGB colours so won't separate to process colour. Great for local printing to lasers due to their size. Will often cause printer conflicts if printed via postscript, even to Acrobat format files.

CDRs are CorelDRAW files that are better saved as EPS files in most cases, as are files created in Illustrator.

The complexities get worse the more you aim at high end quality but these guidelines will give you a start. Software determines what you can and can't do and will often create strange file types. Try to keep to those described above. Your bureau or printer will be happier and end results should be closer to what you expect.

Remember, talk to your printer and your output service bureau for advice before you set your job incorrectly. Ask about line screen resolution, process or spot colour options and whether you need to have everything in CMYK. Lastly always send ALL your graphic files with any job. That way, if a problem arises with a graphic, the bureau can usually rectify it.