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JPEG Myths and Facts

Don't believe everything you hear about JPEGs. Get the facts here.


JPEG Myths and Facts

JPEG Myths and Facts

© S. Chastain

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's a collection of some common misconceptions and facts about JPEG images.

JPEG is the proper spelling

True. Although the files often end in the three-letter extension JPG (or JP2 for JPEG 2000), when referring to the file format it is spelled JPEG. JPEG is an acronym for Joint Photographic Experts Group, the organization that developed the JPEG format.

JPEGs lose quality every time they are opened and/or saved.

False. Simply opening or displaying a JPEG image does not harm the image in any way. Saving a JPEG 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. To avoid more loss you should duplicate and rename JPEGs in a file manager rather than using "Save As JPEG" in an editing program.

JPEGs lose quality every time they are opened, edited and saved.

True. If a JPEG image is opened, edited, and saved again it results in additional image degradation. It is very important to 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 (TIFF, BMP, PNG) for the intermediate editing sessions before saving the final version. Repeated saving within the same editing session won't introduce additional damage. It is only when the image is closed, re-opened, edited and saved again.

JPEGs lose quality every time they are used in a page layout program.

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

If I compress a JPEG at 70%, then later reopen it and compress it at 90%, the final image will be restored to a quality setting of 90%.

False. The initial save at 70% introduces a permanent loss in quality that can't be restored. Saving again at 90% quality only introduces additional degradation to an image that has already had 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.

However, the same setting rule just explained doesn't apply when cropping a JPEG. JPEG 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. In other words, 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. It's also important to know what your software is asking for when you set the quality. 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 the quality settings. See screen shots of JPEG save options in various image editing software programs.

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, though the loss at a quality setting of 100 is barely detectable by the average naked eye. In addition, using a quality setting of 100 compared to a quality setting of 90-95 or so will result in a considerably higher file size relative to the degree of image loss. If your software doesn't provide a JPEG 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 image, but the difference in size could be significant. Keep in mind, though, that 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.

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