Why does render resolution affect output quality? The two images below are sections of the same image rendered at different resolutions.


The blurry one to the left is rendered at 600x350 and then cropped to the section shown here. So there is no scaling or other processing of this image except cropping for the sake of clarity.
The sharp one to the right is rendered at exactly the same settings except for the higher resolution of 1200x700 then down-scaled and cropped to the same size as the first one.


I'm fairly new to blender and 3D-rendering in general so do not understand why the first one is so blurry when the scaled one sharp. Can this be fixed and in that case how? I thought that the imgages would be rendered as sharp as possible for the given resolution. But in this case that is obviously not the case.

Is there some switch or slider or property that should be adjusted to get it right? Or is this behaviour to be expected?

I'm running blender 2.80.42.
There are 2 reasons:


the down-scaling can have a sharpening effect on its own - depends on the algorithm used
the higher resolution image contains more traced samples per the same area = less noise. The less noise the less blurry image. Let's say single pixel in original size received 100 samples, the same area in double the resolution would receive 400 samples.


So the apples-to-apples comparison should be rendering the original resolution with 4x the sample count and down-scaling the higher resolution with a method that does not add any sharpening.