'JPEG' File Format

Save Options

Figure 1  The first set of saving options

When a JPEG file is saved, two panels are displayed.  The first asks for the file name, where to save the file, the file format, and at the bottom of the panel are some options (shown above).  Most of the "Save" options will be greyed-out as the JPEG format can not hold extra features, such as Layers.  The important part here though, is in the "Colour" section.  The "ICC Profile" option will be ticked by default, but do check that this is the case.  It is very important that the image Colour Profile is always saved with the file, not just for your own work, but also for those images supplied to clients.

'JPEG' Compression

Figure 2  The JPEG options panel

The most important JPEG setting is in the Image Options section which allows three ways to set image quality from '0' to '12'. 
• Type the value within the Quality field.
• Use the drop-down list to choose Low, Medium, High or Maximum.
• Use the slider.
Which method is used is unimportant as they all have the same result. 

NOTE:  Normally the word maximum means what is says, the highest value.  But in Photoshop, setting the JPEG quality level to "Maximum" can mean values of '10', '11' or '12'.  This comes from a time when the quality levels ranged from '0' to '10' and "Maximum" did mean '10'.  Some other programs still use a '0' to '10' levels range, and yet others use percentages of 0% to 100% instead.

Figure 3  The biggest gains in compression are between Quality '12' to Quality '8'.

With a quality setting of 12 the reduction in saved file size is quite noticable, usually around a quarter to a third of the original. As quality is reduced the file size decreases, but at a diminishing rate. From 12-8 (Maximum & High) there is a marked decrease in file size, but from 7-0 (Medium & Low) there is only a gradual change.

NOTE:  If you are working with older graphic programmes then note that some use a JPEG scale representing Compression rather than Quality. In this case 0 means least compression and therefore maximum quality, the reverse of what is used now.

'JPEG' Damage

Figure 4  The more the image is compressed, the more the damage becomes visable

The increase in JPEG compression does not come for free, though, as to achieve smaller sizes the JPEG file format removes increasing amounts of colour information from the pixels. The result is a distinctive pattern, known as JPEG artifacts, that increase in visability as the image quality settings decrease. As some information is always lost the JPEG is known as a lossey file format. In contrast the TIFF file format is known as lossless because it can compress with no loss (usually) of quality.

At the highest quality setting of 12 (the least compression) the loss of image detail can be virtually undetectable, but as the quality slides down the scale this image detail loss soon becomes noticeable. An image with a JPEG compression quality of 6 will begin to show a sub pattern of blocks of pixels. An image with a JPEG quality 3 will have a distinct blocking of pixels. When the JPEG quality gets as low as 0 the pixels will start to resemble a parque floor pattern.

A Rule of Thumb:
• To hold good quality do not compress below level 8 (High Quality setting)
• If quality is not so important, for quick reference photos for example, then do not compress below level 3

• Many picture libraries will require a quality level of 10 or higher, in which case always follow their standards.

NOTE:  When the JPEG file format removes colour information from the pixels, to allow for better compression, it is the colour values in each pixel that are altered.  The  pixels themselves are not deleted and so the open working file size remains the same, no matter what the compression was set to.

Figure 5  The JPEG blocks of 8x8 pixels

The image used here has been compressed with the JPEG quality level set to 0.  The resulting parque floor pattern is caused by the JPEG compression format dividing the image up into blocks of 8x8 pixels as seen above. The more the image is compressed, the more obvious this pattern of JPEG artifacts become. In the enhanced colour detail of the image notice that the centre block of 8x8 pixels have now become a uniform colour indicating that subtle differences in the image have been removed thus improving the compression. Although a high JPEG quality setting of 10 to 12 will show little or no damage, this parque floor pattern is worth looking out for. It is a sure sign that JPEG compression has been used and that the quality level has been set too low. This pattern can not be removed later, so even saving the image as a TIFF file will not hide the fact that JPEG compression has been used at some stage.

NOTE:  Even if a JPEG image has used a high quality setting, repeatedly saving the file later will only increase the artifacts as the JPEG compression effects are cumulative.

NOTE:  Since Photoshop 6 it has been possible to save TIFF files with JPEG compression. So if a TIFF image, that has never previously been saved as a JPEG file, can still show the same artifacts if JPEG was selected for the TIFF compression.

Figure 6  JPEG damage ia a gradient

Because JPEG compression simplifies the tonal differences in groups of pixels it can result in problems with gradients.

Figure 7  JPEG damage in a graphic

With graphics that are not photographic, ones which have large blocks of solid colour, other artifact problems can occur with JPEG compression. The illustration here has been compressed with the JPEG quality level set to 0.  Areas around fine detail that should be a solid colour have now taken on a feathery look and gradients have started to posterise. This effect is not exclusive to graphics as photographic images with adjacent blocks of smooth detail can show the same problems. The image is still being divided up into 8x8 pixels but the details and edges are the most noticeable. Higher quality settings of JPEG will reduce the damage, but to avoid the problem altogether it is best to use the GIF and PNG formats instead when compressing graphics.

NOTE:  Over sharpening can often increase this feathery pattern, especially where black & white lines are added to blocks of detail.

Compressed File Size

Figure 8  The image on the left has "busy" detail, whereas the image on the right has "smooth" detail, which means that the two images will have very different compressed file sizes when saved on the disk

When saving a JPEG image the options panel will estimate the resulting saved file size, but this will not be exact.  Predetermining what the actual compressed file size will become is not possible without actually doing the compression and viewing the results. Both images shown here are identical in proportions, at 560 pixels wide by 833 pixels high, creating a working file size of 1.34Mb. Both were compressed at a quality level of '3'. Yet because the first image has lots of busy detail it has ended up with a saved file size of 145 Kb. This is nearly 3 times the saved file size of the second image, at 54 Kb, as it has large areas of 'smooth' detail instead.

'JPEG' Format Options

As a general rule always use the 'Baseline ("Standard")' setting when supplying clients, although no damage will be done if you do use one of the other options.  The client's standards will state which option they prefer.

If you are supplying images for web use only then the normal setting is to use the 'Progressive' option instead.  Seek advice from the website manager.

Quick Summary

Always save the images with the Colour Profile attached, by ticking the "ICC Profile" option.

Using JPEG quality settings of "10" or higher will result in very little damage, and you probably won't see any difference.  But again, do follow the client's standards when supplying images.

NOTE:  Traditionally Macs used the four letter file extension of 'JPEG'', whereas PCs used the three letter 'JPG' instead.  It has now become standard to always use the three letter 'JPG' file extension with images.  Macs don't have to have file extensions, but when supplying clients, then always, always, but ALWAYS supply images with file extensions.  This can not be stressed enough.

WARNING:  Repeat saving of a JPEG file will damage the image every time the file is saved.  The JPEG damage is cumlitive, so the damage will gradually get worse.  However, opening JPEG files to view them, and then closing them without saving, is not a problem.  If you receive JPEG files that need editing, and you want to prevent any further danmage, then the first thing to do is open them and save as TIFF files with no compression.  You won't undo any damage that may already be in the files, but you will now stop any extra damage from being added.