Help and How To - JPEG Images

September 13, 2006

Introduction

This article will help you become familiar with the JPEG image format.  We'll show you the various features of the JPEG format, and help you understand how to use JPEG image effectively with your digital camera.

Background

JPEG (pronounced "jay-peg") is a digital image format commonly used to compress photographic images.  It's an acronym which stands for Joint Photographic Experts Group, which refers to the original committee that designed and created the standard.

The JPEG image format is commonly used in many areas of the computer industry that require the storage of photographic images.  As a result, virtually all digital cameras use JPEG for storing their digital images.  Some digital cameras also support other image formats, but that is beyond the scope of this article.

JPEG Image Compression

Why is the JPEG format so common?  The main reason is because JPEG images achieve very high levels of compression.  This is possible because the JPEG format is a lossy image compression format.  What does this mean?  It's means that when you save a digital image as a JPEG file, some subtle details in the image are discarded.  This is done to significantly reduce the storage requirements for a photo.

When you save a JPEG image, you can usually choose a compression level.  This compression level determines how much information is discarded when the image is saved.  If you set the compression level low, more details are discarded and the resulting file is smaller.  If you set the compression level to a higher setting, the image is more accurately preserved, but the resulting file is larger.  You'll find that some digital cameras offer an option to change the compression level.

Take a look at the sample image below.  Here, you can see how the various compression levels affect the quality of a photo.  (Images were created using irFanView). Your actual results will vary depending on the program you use to save your images.  (Please note that you will need javascript enabled for this page to work properly).

Sample image Orange car Solid text Choose a quality level 100 90 75 50 30 15 0

This is what the original image looks like, being compressed at quality level of 100.  Please note that even when a JPEG is saved at maximum quality, some information is still lost. Image compressed at a quality level of 90. You'll notice that image has not visibly degraded. Image compressed at a quality level of 75. Image still looks quite good. Image compressed at a quality level of 50. Image still looks ok but has degraded slightly. Image compressed at a quality level of 30. Image has begun to degrade noticeably with a lot of JPEG artifacts being present. Image compressed at a quality level of 15. Image has lost a lot of detail. Image compressed at a quality level of 0. Image is not usable.

Here's a table with file sizes for a sample JPEG compressed image.  Actual compression levels will vary based on the image you are compressing and the software you use to save your image.  JPEG images file sizes will also vary based on the image you are saving.

As you can see, JPEG achieves excellent compression on photographic images.  This translates into a significant advantage, since it allows you to store a much large number of photos on your digital camera card.  This saves you time and reduces storage requirements significantly.

Here are some important points we covered in this article.

• JPEG is a lossy compressed image format (it discards subtle details).
• JPEG can achieve very high compression.
• JPEGs are suited for photographs, not images that contains a lot solid colors with sharp edges.

Tips

• If you are editing or retouching your photo, save your file as a TIFF or Photoshop document during your editing phase to avoid gradually degrading your photo.  Remember that when you save to JPEG format, you lose a little bit of detail in the image each time.
• Avoid saving your JPEG images at too low of a quality setting if you plan on archiving your photos.
• Avoid the temptation to save your photos at 100% quality all the time.  Even when you save your photos at this quality level, you still will lose a very small amount of detail in your photos.  A quality level of 90% to 95% is more sensible for archiving photos.
• It's ok to save at a lower quality setting if you're sending your JPEG files by email.  Most people only want to view the photo.  It's also a good idea to resize the photo so that the photo doesn't appear too big on screen.
• To see JPEG artifacts in a JPEG compressed image, increase the colour saturation to 100%, or try doing a  heavy sharpen twice on your photo. The artifacts become readily apparent as the image degrades at every 8 pixel interval.
• Since the human eye is much more sensitive to changes in brightness than colour, a JPEG image discards a lot of colour detail, but tends to more accurately preserve details in the brightness of an image.