Sunday, January 27, 2008

An Overview of JPEG

A previous post introduced the basic ideas of raw and JPEG image files. This post is about JPEG. I’m no expert on JPEG so my understanding and the following explanation is probably an oversimplification but, even so, may be a useful starting point for many new photographers.

Most of the pictures that we see every day were made from JPEG files. Almost every picture on the Internet is a JPEG. The photo sharing websites require or prefer JPEGs. The local discount digital photo printshop requires JPEGs. Many photo contests and online galleries require JPEG files. Your digital camera – even if it is an expensive DSLR – shoots JPEGs and probably defaults to JPEGs; in fact, your digicam may shoot only in JPEG mode. Photojournalists are likely to be required to shoot in JPEG mode. The micro-stock agencies post JPEG images for downloading. So, what is a JPEG?

Instead of going into the full definition of JPEG with all the mathematical details and possible variations, for our purposes there are three important features of a JPEG file: 1) JPEG is a data file for storing digital images, 2) JPEG is an 8 bit data file and 3) JPEG is a lossy data file.

In computerspeak, current digital cameras can capture 10, 12 or even 14 bits of information from every pixel. When this captured information is to be stored in an 8 bit data file, some of it must be discarded. Even without going into all the details of “bits” the intuitive conclusion is correct: A JPEG data file does not contain all the information originally captured by the camera. This is troublesome to some people; however, monitors and printers are actually 8 bit devices.

Instead of storing all the 8 bit data, the size (and quality) of the JPEG data file is further reduced by discarding data that might not be noticable to the human eye or interpretation. This is called “lossy” compression and is an ingenious method of reducing file sizes. Before the image is saved to the JPEG data file, a “quality” is specified. At the highest quality, very little data is discarded. In Photoshop, JPEG quality is specified from 1 (lowest quality) to 12 (highest quality). The first image shown in this posting was saved at a Photoshop quality of 1; its file size is 34 KB. This image was saved at 6; its file size is 70 KB (click for a larger image). These images are 100% crops based on the in-camera JPEG taken at the same time as this previously shown raw capture from this photo.

Obviously, a JPEG image file is extremely useful. We all benefit from JPEG. In the next posting, I’ll give an overview of the JPEG image files and options for the Canon G9 .


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