JPEG File Format Myths and Facts

JPEGs are one of the most-used image formats on the web

With the explosion of scanners, digital cameras, and the World Wide Web, the JPEG image format has quickly become the most widely used digital image format. It's also the most misunderstood. Here are some common misconceptions and facts.

JPEG Is the Proper Spelling: True 

Although the files often end in the three-letter extension JPG or JP2 for JPEG 2000, the file format is spelled JPEG. It's an acronym for Joint Photographic Experts Group, the organization that developed the format.

JPEGs Lose Quality Every Time They're Opened: False 

Simply opening or displaying a JPEG image doesn't harm it in any way. Saving an image repeatedly during the same editing session without ever closing the image will not accumulate a loss in quality. Copying and renaming a JPEG will not introduce any loss, but some image editors do recompress JPEGs when the "Save as" command is used. Duplicate and rename JPEGs in a file manager rather than using "Save as JPEG" in an editing program to avoid more loss. 

JPEGs Lose Quality Every Time They're Opened, Edited, and Saved: True 

When a JPEG image is opened, edited, and saved again, it results in additional image degradation. Minimize the number of editing sessions between the initial and final version of a JPEG image. If you must perform editing functions in several sessions or in several different programs, you should use an image format that is not lossy, such as TIFF, BMP, or PNG, for the intermediate editing sessions before saving the final version. Repeated saving within the same editing session won't introduce additional damage. It only happens when the image is closed, re-opened, edited, and saved again.

JPEGs Lose Quality Every Time They're Used in a Page Layout Program: False 

Using a JPEG image in a page layout program does not edit the source image so no quality is lost. However, you may find that your layout documents are considerably larger than the sum of the embedded JPEG files because each page layout software program uses different types of compression on their own document files, 

If I Compress a JPEG at 70 Percent and Later Reopen It and Compress It at 90 Percent, the Final Image Will Be Restored to a Quality Setting of 90 Percent: False

The initial save at 70 percent introduces a permanent loss in quality that can't be restored. Saving again at 90 percent only introduces additional degradation to an image that has already had a considerable loss in quality. If you must decompress and recompress a JPEG image, using the same quality setting each time seems to introduce little or no degradation to the unedited areas of the image.

The same setting rule just explained doesn't apply when cropping a JPEG, however. Compression is applied in small blocks, typically 8- or 16-pixel increments. When you crop a JPEG, the entire image is shifted so that the blocks are not aligned in the same places. Some software offers a lossless cropping feature for JPEGs, such as the freeware JPEGCrops.

Choosing the Same Numeric Quality Setting for JPEGs Saved in One Program Will Give the Same Results as the Same Numeric Quality Setting in Another Program: False

Quality settings are not standard across graphics software programs. A quality setting of 75 in one program may result in a much poorer image than the same original image saved with a quality setting of 75 in another program.

Some programs have a numeric scale with quality at the top of the scale so that a rating of 100 is the highest quality with little compression. Other programs base the scale on compression where a setting of 100 is the lowest quality and the highest compression. Some software and digital cameras use terminology like low, medium and high for quality settings.

A Quality Setting of 100 Does Not Degrade an Image at All: False 

Saving an image to JPEG format always introduces some loss in quality, although the loss at a quality setting of 100 is barely detectable by the average eye. In addition, using a quality setting of 100 compared to a quality setting of 90 to 95 or so will result in a considerably higher file size relative to the degree of image loss. If your software doesn't provide a preview, try saving several copies of an image at 90, 95, and 100 quality and compare file size with image quality. Chances are there will be no distinguishable difference between the 90 and 100 images, but the difference in size could be significant. Subtle color shifting is one effect of JPEG compression—even at high-quality settings—so JPEG should be avoided in situations where precise color matching is important.

Progressive JPEGs Download Faster Than Ordinary JPEGs: False 

Progressive JPEGs display gradually as they download so they'll initially appear at very low quality and slowly become clearer until the image is fully downloaded. A progressive JPEG is larger in file size and requires more processing power to decode and display. Also, some software is incapable of displaying progressive JPEGs—most notably the free imaging program bundled with older versions of Windows.

JPEGs Require More Processing Power to Display: True 

JPEGs must not only be downloaded but decoded as well. If you were to compare display time for a GIF and a JPEG with the same file size, the GIF would display marginally faster than the JPEG because its compression scheme does not require as much processing power to decode. This slight delay is barely noticeable except perhaps on extremely slow systems.

JPEG Is an All-Purpose Format Suitable for Just About Any Image: False 

JPEG is best suited for large photographic images where file size is the most important consideration, such as images that will be posted on the web or transmitted by email and FTP. JPEG is not suitable for most small images under a few hundred pixels in dimension, and it is not suitable for screenshots, images with text, images with sharp lines and large blocks of color, or images that will be edited repeatedly.

JPEG Is Ideal For Long-Term Image Archival: False 

JPEG should only be used for archival when disk space is the primary consideration. Because JPEG images lose quality each time they're opened, edited and saved, it should be avoided for archival situations when the images require further processing. Always keep a lossless primary copy of any image you expect to edit again in the future.

JPEG Images Don't Support Transparency: True 

You may think you've seen JPEGs with transparency on the web, but the image was in fact created with the intended background incorporated into the image in such a way that it appears seamless on a Web page with the same background. This mimicry works best when the background is a subtle texture where seams are indistinguishable. Because JPEGs are subject to some color shifting, however, the overlay may not appear totally seamless in some cases. 

I Can Save Disk Space by Converting My GIF Images to JPEGs: False 

GIF images have already been reduced to 256 colors or less. JPEG images are ideal for large photographic images with millions of colors. GIFs are ideal for images with sharp lines and large areas of a single color. Converting a typical GIF image to JPEG will result in color shifting, blurring, and loss in quality. The resulting file will often be larger. It's generally not of any benefit to convert GIF to JPEG if the original GIF image is more than 100 Kb. PNG is a better choice.

All JPEG Images Are High Resolution, Print-Quality Photos: False 

Print quality is determined by the pixel dimensions of the image. An image must have at least 480 x 720 pixels for an average quality print of a 4" x 6" photo. It must have 960 x 1440 pixels or even more for a medium- to high-quality print. JPEG is often used for images to be transmitted and displayed through the web, so these images are typically reduced to screen resolution and do not contain enough pixel data to get a high-quality print. You may wish to use your camera's higher quality compression setting when saving JPEGs from your digital camera to reduce the damage caused by compression.

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