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I studied and think I got a good grasp on the relationship between image resolution, DPI\LPI\PPI, and the difference between re-sampling or not. The only thing I can't understand is what I read (including on the Adobe website) about how in PS (and in general), you don't have the option to choose the file-size. This will be determined based on the other settings you chose (all the above, plus output format and other settings).

So my question is about 3-rd party software, specifically the Microsoft Power Tool "picture resizer" and I'll clarify...

I took an original image. File size is 16MB. Here are the specs:

enter image description here

I used the power tool to convert it to the same dimensions:

enter image description here

The result SEEMS to be the same quality, both to the eye AND in "image size" settings in PS (see comparison screenshots bellow) but the new file-size is now 2.42MB! (Both are jpg).

This is not possible in PS to export the same dimensions, with the same DPI, to the same format and to get an %84.88 decrease in file-size reduction.

So yes, there is definitely some algorithm in play here but obviously, the reduction in file size must mean that some file information was deleted, which I assume would be pixel information (compression), so how is it that the quality seems to stay the same? Is this just an illusion? If both files were now printed in the same size (30x80 cm), would they print in the same quality?

Comparison screenshots in PS:

enter image description here

enter image description here

enter image description here

enter image description here

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It's all about the degree of compression. The smaller file has less ultrafine original details left.

JPG compression is compression because it shaves off those frequency components that are cosidered to be too weak to be noticed by eyes. Thus less storage is needed. In practice noiselike grainy details seem to be diminished and sharp edges seem to be more fuzzy. Term "frequency component" comes from the same mathematical basis (=Fourier spectral analysis) that is used in MP3 and many other compressed audio file formats.

In Photoshop "Save as JPG" lets you to choose from 13 different compression levels and all them in three different basic algorihms. It also shows the filesize that will be resulted. The numerical index for compression levels is named "Quality". So, the smaller is the Quality, the higher is the compression and the more details are lost.

JPG compression makes more than shaves off details and sueezes the file. It also introduces "JPG artifacts". They are all kind unwanted crap near color and luminosity borders. The sharper border or the higher the compression, the more artifacts.

Save a sharp edged coloured shape on a white background as JPG with low quality. See, how a lot of complex small details have stepped in, when you close that image and reopen it. Often artifact crap is nearly invisible but it's there and affects, for example, on how the backgrounds can be removed easily. The Magic wand is often useless just beause JPG arifacts.

The following is an extreme example: only sharp edges and very high compression.

enter image description here

You asked, if JPG compression affects to printing quality of your photos? Yes, if the difference is visible when you zoom in onscreen so much that the screen resolution is same than the printing resolution.

Comparing your uploaded images here is not the same as comparing your files because the website can add compression to save space. Assuming this to be insignificant, the original seems to have a little higher quality. It maybe is noticeable after printing both images in the same printhouse in high resolution onto best glossy white paper. But you have to look both images using a magnifying glass or to be young enough to see them well at the distance less than 20 cm.

  • So what you're saying is although to me it looks the same, there is no way that a 16MB jpg file can be the same quality when compressed via a 3rd part tool to 2.42MB? – Zvi Twersky Jan 8 '17 at 7:36
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    Check the enlargened pic of the eye. The original has a little more details in the skin around the eye. See the face portrait. In the original the lines between the teeth seem to be sharper. The latter may be noticeable when one looks at high quality prints through a magnifier glass. Nothing visible exists for ordinary watcher who are not trying to find a beforeknown error. But in math, the difference exists.. I have assumed the uploaded samples not to be spoiled by this website. For example Facebook renders everything to somehow low-detail and bleak . – user287001 Jan 8 '17 at 9:27

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