JPG and loss of quality

Started Nov 12, 2012 | Discussions thread
voider Senior Member • Posts: 2,953
Re: JPG and loss of quality

lpGrumpy wrote:

I'll go out on a limb here and say that: If you save an image that was captured in RAW to a JPG set at its highest quality setting you will not be able to see any degradation in the image!

Now before all sorts of flames start, let's look at a few facts.

What you have done is to 1) compress the file size of the image and 2) thrown away some of the data that was captured. JPEG has evolved to a pretty good algorithm and only throws away data that it thinks we do not need for a good representation of the original image (i.e. the way the sensor captured it). An example: A JPG file only contains 256 levels of color for each of the primary colors. This is one byte each for red, green, and blue resulting in 16 million colors! Compare that to a RAW file from a 12 bit sensor (there are also 14 and 16 bit sensors) has 4096 levels of each color! So we go from 4096 levels to 256! That is why we enjoy creating images from RAW files and only convert them to JPG files for printing, emailing, or viewing on a monitor or screen. Each time I want to modify, re-size, crop, or otherwise process an image, I go back to the original file (yes, with my current Lightroom settings) and reprocess. Then I create another JPG file.

My point is that all JPGs from your camera or PostProcessing software are first generation files. Problem arise when you open a JPG file, make a change (any change) and save it again. You do not have the original data so you have to work with what you have. And when you save it again, there is another lossy compression applied to the image. This is just like recording tape of years ago. The first original recording could never be as good as the live performance but a good recorder did a pretty good job of playing the recording back. It's when there were subsequent dubs that brought the degradation. And it wasn't long before it really started to sound really bad.

So, shoot RAW and use that file as your master. I even limit round trips from Photoshop to a reasonable number (Photoshop returns TIF files in my setup.) And try to not make too many 2nd and 3rd generation JPGs.

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Thank you very much for this great explanation!

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