A bit like that colleague that you’ve been saying “hi” to for months, but who’s name you forgot within seconds of being introduced, most people deal with picture files every day but feel too embarrassed to admit to not knowing what the differences are between each, or why they’re important. So relax. We can’t help you with the colleague’s name, but here’s a quick guide to the picture formats you’ve most likely have been saying “hi” to for ages, and how you should use each.
THE BASICS
Three factors affect the suitability of an image for print or web: file type, resolution and colour mode.
MOST COMMON FILE TYPES
JPEG (jay-peg): widely used in both print and web media JPEG files are your best option for storing digitised photographs in compressed format. Pictures from a digital camera for example. Confusingly the common suffix (end bit) for JPEG files is ’.jpg’
GIF (pronounced as in ’gift’): GIFs only contain up to 256 colours — far less than JPEGs — but as a result are much quicker to decode, which is why they’re more commonly associated with websites and emails where speed is important. Banners, simple images, and computer graphics often contain fewer than 256 colours, so this isn’t an issue and their greater speed gives them the advantage. Suffix ’.gif’.
PDF (pee-dee-eff): A really versatile file format able to combine both documents and images into a single file, which can then easily be viewed and navigated using Adobe Reader — free software available for both Mac and PC. Beware though. With PDFs a little knowledge is a dangerous thing. A high-res image saved into a low-res PDF becomes a low-res image. But a low-res image saved into a hi-res PDF… remains a low-res image, and unsuitable for printing. Suffix ’.pdf’.
TIFF (tiff): A graphics format ideal for high resolution printing. TIFF files are enormously flexible because they support all manner of geeky flimflam useful to designers, and because they’re both Mac and PC compatible. The trade-off is that they tend to come in large file sizes as a result, so not great for emailing to others. Suffix ’.tiff’
Bitmap or BMP (bitmap, bee-em-pee): Chances are you’ll have only ever come across bitmaps if you use the Paint program on your PC. It’s default ’Save As’ setting is the bitmap file. Bitmap files are perfectly suitable for professional or desktop printing (although the above formats are more common), but — as with JPEGs and GIFs — rescaling a bitmap to a larger size will result in loss of definition. Suffix ’.bmp’ or ’.dib’.
EPS (ee-pee-es): Chances are you’ll only encounter EPS files if you have dealings with a design agency. That’s because it’s a PostScript file format used to transfer graphic images between applications and platforms. If you receive one you’ll need a program such as Adobe PhotoShop Elements to open it. You’ll then immediately be prompted to save it to a different format. Choose one of the options listed above. Suffix ’.eps’)
RESOLUTION
When image files are referred to as being either ’high-’ or ’low-res’ it refers to their quality on-screen or in print. The higher the resolution, the finer (better) the image quality. Screen (web) resolution is measured in PPI (pixels per inch), and print in DPI (dots per inch). The significance of all this is that if you include low-res images in a printed brochure, for example, they’ll appear pixillated. It’s relatively easy to convert a low-res image to a high-res one (and vice versa, see below) but bear in mind that doing so will also reduce the image size. An A4 size low-res image at 72dpi becomes only a quarter page image at 300dpi.
Use a program such as Adobe PhotoShop (Elements) to check the resolution for each image file. Click on Image › Resize › Image Size. When adjusting the resolution make sure the ‘Resample Image’ box is left unchecked. As a crude rule of thumb: 300dpi is suitable for professional printing, 150dpi for desktop printing, and 72dpi for web/email use.
COLOUR MODE
There are several colour modes but the two most common are RGB and CMYK.
RGB (short for Red, Green, Blue) is the primary colour model used by computer monitors. Images used on websites or in emails should therefore always be saved in RGB mode. Colour images that are to be printed professionally should be CMYK (Cyan, Magenta, Yellow and Key — black). The difference is down to how the human eye perceives colour, and how the printing process tries to replicate that perception. In day-to-day terms however, this is only really important if you’re using a printing press, or where colour quality is really important. Use the Image › Mode menu in Adobe PhotoShop (Elements) to change from one to the other.
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