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The Digital Domain: More About File Formats
Exif
Exif stands for Exchangeable Image File Format (and yes it is written as Exif and not EXIF), and was developed in 1998 by a consortium of the Japanese Electronics Industry called JEIDA. Although we’ve just told you that all cameras use JPEG as the standard image format – the correct name for the format is EXIF (although the two are commonly interchanged, and the JPG extension on the files usually means people call them JPEG files). An EXIF file can be loaded into any application that supports JPEG files.
The Exif specification uses the existing JPEG file format and compression system with the addition of specific tags that describe a wide range of information about the image including:
- Make and model of camera.
- Date and time the picture was taken.
- Camera settings such as aperture, shutter speed, focal length, ISO setting, whether the flash fired or not etc.
- A thumbnail image for previewing the picture on the cameras LCD or image browsing software (although most imaging software creates it’s own brower thumbnails).
- Descriptions, copyright information and the photographers name.
- Geolocation – the location the image was taken. We currently only know of one camera with the built in GPS system needed to derive this information (the Ricoh 500SE).
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The date and time the picture was taken is very useful as when you modify an image – the ‘data modified’ information on the file is updated, however the Exif date and time remain unchanged.
To view the Exif information for an image in Microsoft Windows, right click on the file in Explorer, select properties, select the summary tab and then click the ‘Advanced’ button. You will get a windows similar to the one shown on the right.
A word of warning about Exif information – if you edit or manipulate an image in any way (using image editing software, or even tools included with Windows) make sure that the newly saved file retains all the Exif information. Early versions of Windows XP were know to corrupt the Exif data if you rotated the image or modified the image description data (available in the properties ‘simple’ mode). We suggest you make a copy of an image and try out your software before saving over any original image files. |
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JPEG Chroma Subsampling
The human eye is more sensitive to variations in luminance (the intensity or brightness of light) than it is to variations in the chroma (colour) information. Because of this, it is possible to reduce the amount of information used to store the colour information without any visual difference being perceived by the viewer.
The JPEG compression algorithms take account of this by reducing the resolution of the colour information in a picture but maintaining the brightness resolution. How individual cameras do this is down to the manufacturer and not usually published. However it is useful to understand that this is happening as some software packages give you the option of selecting the chroma subsampling when you save a JPEG file.
Paint Shop Pro has a list of options which works independently of the compression slider. For maximum quality, turn the subsampling off although this does increase file size considerably.
Photoshop does not give the user the option – it selects an appropriate chroma subsampling and compression amount based on the position of the quality slider.
Other File Formats
If you use a PC you may have come across other file formats - here's a brief summary of some of the other popular ones.
BMP Windows bitmap image - high quality but very large file size as so compression is used.
TIFF Tagged Image File Format (the file extension is usually .TIF). Widely used due to its extreme flexibility, although for photographs its main advantage is as a lossless compression format (although the amount of compression will be minimal). If working with images for commercial print, TIF supports CMYK (Cyan Magenta Yellow Black) as well as RGB (Red Green Blue).
GIF Commonly found on web pages - lossless and high compression although only suitable for computer generated graphics as all images are limited to 256 colours.
PNG Portable Network Graphics format - Designed to replace GIF although a lot more flexible. It can support true colour images (such as photographs) although we would not suggest using this generating images for web pages.
DNG Digital Negative Format – see the next section on "Shooting with RAW".
JPEG-2000 was developed to replace the ageing JPEG standard that was developed in 1991. It has many advantages over JPEG such as higher compression for the same quality, less visible artefacts, a lossless mode and error resilience. However despite this, the format has never really taken off and we know of no cameras that use this format. We recommend avoiding this format.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"Digital Photography allows us just not to capture memories but it also allows us to create them."
- James Wayner
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