Stumbled across some information regarding what Facebook does to compress pictures. I'll post 3 pictures below of a Master Pattern that is at the Cast Iron Museum. I was discussing it in a FB group and we noticed that detail on the lettering was not as clear as it was on my original. Did a little looking at the properties and found that they make their largest changes in the DPI for the picture. That reduced the detail, but in most cases is still reasonable on the screen.
Here's some detail. My original picture for this example is 426KB, 1280x1587 pixels, 180 DPI. (number 1 here).
Pic 2 is what FB does presents after I uploaded it. 113KB, 774x960 pixels, 96 DPI (lower resolution).
Pic 3 is what Messenger does. 392KB, 1280x1587 pixels, 96 DPI.
Messenger does less alteration, but still reduces the resolution.
So if you want to share pictures that you can look at lots of close detail, share them with an individual via email or through a cloud service.
The DPI reduction will really show when you print an image.
Just FWIW.
Tom
