The photo quality is my iPhone 7 is very low. I found out that this may be because I switched on “iCloud Photo Library” a few months earlier, and “Optimize IPhone Store” was selected. Does doing so move all PREVIOUS photos to cloud, and replace them with low-res photos?
The answer is in the Optimize iPhone Storage setting.
When enabled, only thumbnails of photos are stored in local storage. By viewing a photo, you are downloading a compressed version from iCloud. There is no quality loss, since the original photos are still accessible by loading them one by one or turning this optimization off.
I believe that iCloud Photo Library uses some kind of lossy compression.
Here's a fix that worked for me.
Keep in mind: Although the screenshots are in Danish, the instructions below are in English, and the placement of the marked settings are the same in both English and Danish running iOS 12.2.
Go to "Settings" in your iPhone. Scroll down until you find "Photos". Tap it (Marked with a red circle in the following picture)
Now, you should see the following screen. Tap on "Download and Keep Originals" (Marked with a red circle in the following picture)
Scroll down until you find a setting called "Keep Originals". Tap it. (Marked with a red circle in the following picture)
Go back to "Settings" by tapping on the arrow in the upper left corner. (Marked with a green circle in the following picture)
Now you should see the following screen. Right under the "Photos" setting, you'll find a setting called "Camera". Tap it. (Marked with a red circle in the following picture)
Now you should see the following screen. Tap on "Formats" (Marked with a red circle in the following picture)
Now you should see the following screen. Tap on "Most Compatible" (Marked with a red circle in picture 6)
This should fix the issue :-)
I’ve seen no degradataion whatsoever by optimize and yes, the compression used is lossy, but I believe Apple keeps the actual screen resolution / print resolution in mind and does very light handed compression (as opposed to severe compression at high ratios that causes bad image quality.)
What do you see for the size of an exported image? You should be able to save off the images to Files / iCloud and then download them from the computer if you don’t use an iOS tool like ViewEXIF to see the resolution (pixels) and size (KB/MB) of the images.
For me, when I need to do an operation like edit the photo and I have a reduced resolution size image (if it were added recently and I only have a placeholder), iOS shows me that it’s downloading the full resolution image from the cloud so I can then share / export a good quality image.