Important File formats – JPG

Quick and easy – not very flexible

Digital images come in a range of standard file types which computers, cameras, mobiles and scanners are able to understand. However, they are not able to interpret all image file types. Only those they are programmed to use. It is important that you know about the file types you use because they can seriously affect your photography. I am going to introduce you to .jpg files. They are the most common file types used in digital cameras.

Which file type you have is labeled by the file extension. That is the three characters after the last dot in the file name. If you cannot see this file extension refer to your computer help files to find out how to display extensions.

A file with a .jpg extension, for example ‘myimage.jpg’, is what we call a JPEG file (pronounced – ‘j’- peg). That stands for the “Joint Photographic Experts Group” who formulated the .jpg image standard published in 1992. In digital terms that is a long time ago. However, the JPEG standard has been highly successful and has proved robust. It is not perfect, neither does it do everything.

First and most importantly .jpg files are what is called a ‘lossy compression‘ format. Media files contain a lot of data – very large quantities in fact. Most of that data is used to create colour variations and tones that are very subtle. In a photograph a lot of tones and hues are not noticed by the eye. To save storage space and speed things up your camera dumps a lot of that unseen data when it creates .jpg files. That’s fine as long as the lost data is stuff the eye cannot see in the photograph. The benefit is that the lost data makes files smaller, as well as quicker and easier to move, send and store.

There is a down-side to .jpg files. Every time you open, edit and save a .jpg file it goes through the compression routine – dumping more data. For files you are going to edit on a regular basis this is bad news. Each save will lose some of the data that creates the image. As a consequence the quality of the image will be damaged over a series of edits.

Lossy compression in .jpg files damages the file each edit/save cycle

Lossy compression in .jpg files progressively damages the file each edit/save cycle. Eventually, the loss of data becomes visible. The image is a composite of four versions showing increasing compression left-to-right. Left hand side – the image shows low compression with 60% of the original data still in the file. Right hand side – only 2% of the original data is left. Over-compression, various edits and resizing all have an effect on final quality of the image.
Best viewed large. Click image for full size.

As you can see from the images the damage ruins the picture. However, a low compression is a good optimum. The file is smaller and still of acceptable quality. The small size allows easy use for things like posting on the Internet.

There are other implications of a lossy format. The lost data seriously reduces your options for editing the image. Other file formats keep that data. Most notably these are the ‘RAW’ formats from your camera manufacturer. RAW files are created by the camera directly from each of the sensor points in the digital image sensor in your camera. A RAW file stores all the data. In doing so, the file is preserving the data for you to make significant changes to the image later. These changes are often not possible with a .jpg file. While some small colour changes, brightness and other aspects of the file can be changed in a .jpg file, the degree of change is limited. When editing a RAW file you have a considerable amount of potential to recover over exposure, underexposure, colour casts, hues, tones and other attributes of your photograph.

The JPEG standard has produced a robust file standard that is widely used. The compressed size means a lot of data is dumped. This reduces the editing flexibility for the photographer. On the other hand the file is small and easy to store. It produces a good image as long as it is not over-compressed. However, to guard against loss of quality always retain an original copy of your file. Only edit your file copies so that you can go back to the original .jpg file if things go wrong.

By Damon Guy (author and Photokonnexion editor)

Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photogs.
See also: Editors ‘Bio’.
By Damon Guy see his profile on Google+.

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