Image Formats
There are four main file formats for images: TIFF, JPEG, GIF and PNG. The format of a particular file is identified by the three letter file extension at the end of a filename (.tif, .jpg, .png, and .gif respectively), and each format has unique characteristics that make it suitable for particular uses. A brief explanation of each format is given below, together with some sources of further information.
Note: The image formats covered in this section are all 'raster' formats (images composed of pixels). Vector images (images that are composed of lines and co-ordinates) are not covered by these guidelines, but you can follow this link for further information: JISC Digital Media - Vector file formats.
TIFF (Tagged Image File Format)
Description: TIFF images are usually used for master image files. They contain image information in a lossless format (ie no image information is lost when images are saved) and so tend to be fairly large in size. They are therefore a good format for archiving images, but the large file size makes it an unsuitable format for use in web delivery or in presentation software, such as PowerPoint.
Good for: master copies of images as all image information is retained when files are saved (lossless format).
But: file sizes tend to be large due to lossless format, so TIFF files are not suitable for web delivery or inclusion in PowerPoint presentations.
JPEG (Joint Photographic Experts Group)
Description: This is the main format that is used for photographic-type images on the web. It is a 'lossy' format: images are compressed when saved and so image information is lost each
time the image is edited and saved. The benefit of compression is a reduction in file size, but the downside is that if too much compression is applied, visible artefacts such as highlighting around areas of high contrast may occur, as shown on this photograph - notice the blurring around the edges of the statue.
However, by choosing the level of compression that is used and you can control the file size and quality of the resulting image: as a general rule, you can save at 70-80% quality without noticing much degradation in image quality.
Good for: web delivery of photographic images due to ability to compress images without too much loss of quality, therefore giving smaller file sizes than TIFF formats.
But: too much compression can lead to a loss of quality so care needs to be taken with the quality setting used when saving images.
GIF (Graphical Interchange Format)
Description: Another format encountered on the Internet, the GIF format is usually used for icons or graphics that contain a limited range of flat colours. It is a lossless format (no information is lost when saving), but has limited colour capabilities and so is not suitable for displaying photographs.
Good for: web delivery of icons and graphics due to small file size and lossless format.
But: supports limited range of colours, so is only suitable for certain types of images.
PNG (Portable network graphics)
Description: PNG is a relatively new web graphics format, designed primarily to replace the GIF format for use on the Internet, and potentially rival TIFF in the long term as an archival format due to its better compression performance. Its main advantages over GIF are an improved lossless compression method and support for 'true colour'. Although software support for the PNG format has been slow in developing, this is now beginning to change and it may become a more common format in the future.
Good for: web delivery due to lossless compression technique resulting in files of small size but high quality.
But: JPEG format gives better results for photographic images, and older web browsers and programs may not support the PNG format.
Further information
For more information on file formats, have a look at the following web resources:
- MS Office picture formats - details of formats suitable to use in MS Office documents
- JISC Digtial Media - Choosing a file format
- Scantips.com - Which image file format?