What’s the difference between jpg, eps and tif files?
JPEG is actually a compression system invented by the Joint Photographic Experts Group (hence the name). When using this system in a programme such as Photoshop, the amount of compression put onto an original photograph can be controlled, but to a greater or lesser degree the file is altered and some data will be lost*. The theory is that the system will throw away non-essential information to reduce the file size, and then replace with similar (though not identical) information when decompressing. With a high quality setting for the compression, the loss may not be visible to the human eye – especially on screen – but with a low quality setting and especially when the image is printed on paper, it may appear substantially altered. Once compressed through a “lossy” system such as .jpg, there is no way of retrieving the information, and it can be deduced that repeatedly compressing a file will result in more and more information being discarded.**
.eps and .tif bitmap files, on the other hand, are essentially noncompressed, and so retain all the original information from the scan or photograph – except for those pixels which have been lost through restricting the colour range to the final output device. For example, when preparing an image for print we have to turn RGB images into CMYK, which means that the software will remove all those colours which cannot be achieved via ink on paper. This in itself can vastly reduce the colour range, which is why we are wary of throwing away any more info through the .jpg process, and prefer to receive images directly from the photographer or scanner as .eps. ,tif or RAW files.
Having said that, photographers using .jpg will generally set the compression to a very high quality so it’s not necessarily a disaster – but the blanket use of low quality .jpgs is something we need to be very aware of.
* there is a “loss-less” JPEG system but this is not normal so we assume that all .jpg files have undergone “lossy” compression.
**If you are interested in exactly how the compression algorithms work, the best explanation we’ve seen is on Wikipedia.
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