Smartphone cameras have a lot of post processing via the camera app. Performance, quality and image processing are typically at odds with each other. Finding the compromise between all the variables (compression type and quality, iso, exposure, f-stops, image processing amount and quality, better at low light or bright light, etc etc etc). Updating the app between iOS versions can effect the images quality fairly easily.
I dont have a problem with grain at all unless its really low light but this is common. iOS 13 provided tools for noise reduction in photos. That might help you out some.
This image will not come through for you guys (compression from the camera, export, screenshot and finally MacRumors upload). Its just the point of reference I used on my Mac directly from the originals between iOS 12 and iOS 13.
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The image is a specific forum I use at work since they are using similar distances, lighting, etc, this is as close as I can get to comparing similar images from 2 different OS's and 2 different phones. The iOS 12 version is using a bolder font (since its a year old things change) then both 13 versions. Also keep in mind this is at a macro level nitpicking here (300-400% zoom).
iOS 12 adds quite a bit of noise reduction. This is required with smartphones to an extent but too much and the image starts to look mosaic (painted effect) and overly soft, borderline blurry. The colons are supposed to be 2 squares but iOS makes them look a bit round.
I can't tell if its sharpened in anyway or not but I think it is since I can see a bit of gray in the middle of the black text if I look really close.
White has a slightly red hue. Naturally the paper in my work environment should be gray with a hint of yellow (gray drywall walls with an ugly depressing yellow cubical walls). All the colors are going to be slightly off if white is off.
I also think the white is also too bright causing the colors (not in the image) to look too dark.
iOS 13 isn't perfect by any means but I think the main difference is they just reduced the amount of image processing automatically applied by the camera app. In turn they gave us the editing tools for post processing because one size fits all with photo editing is basically one size fits none.
Again I want to mention this was at nitpicking levels. The reality is the images