What changes (specifically improvements) occur to image quality when you reduce the resolution in post?

Question

If you shoot a lot of megapixels, let's say 21MP, what image improvements/changes will occur when you downsample the image to a resolution around 1920x1080 for display on a normal 1080p HD television? (assuming you are shooting uncompressed raw)

What effect will this have on DOF, perceived sharpness, and noise?

From Does reducing JPG resolution decrease, increase, or have no effect on image quality?

When you downsample a 4000×3000 image to 400×300, you are "discarding" 11.9 million of the 12 million pixels. This clearly reduces "image quality", depending on what exactly you mean by that term.

If you go from 1000×750 to 400×300, you're reducing the area by about 6 times. Again, data is discarded, but not as much.

Now, if your original image had a lot of defects — noise, aberrations, and focus error — the resolution may exceed the "effective image quality", and the extra detail is contributing nothing. In that case, downsampling to some degree doesn't "really" reduce image quality.

What exactly happens when your image has "a lot of defects — noise, aberrations, and focus errors" and you downsample? Can this improve?

Answer 1

It depends on the exact algorithm used to down-sample but generally:

1. Blur become less noticeable

   Motion blur, camera shake and out of focus areas all become less blurry (that last one will increase the DOF) but they will never look like they were sharp to begin with.

2. Noise becomes less noticeable

   The "dots" caused by noise area averaged out and this reduces noise.

3. Small details disappear or become blurred

   There's no longer enough pixels to show small details, thing that are recognizable at high resolution will become blobs of color or disappear completely.

Answer 2

The reality is that you are not discarding anything. If you did, image quality would get worse. This actually happens when cameras have to skip pixels in order to produce video because most cannot read the entire sensor and process its output at 24+ FPS.

When you downsample, you are using many pixels to generate fewer. There are techniques and research papers on the topic which achieve different compromises. At the very least, noise gets reduced because it gets averaged out. With too simple techniques, you can lose sharpness but better algorithms can produce and image that looks sharper and has more color-depth.

Anything that makes the image softer will have a reduced effect at lower resolutions. So if you are a bit off-focus it can show less. This also means that downsampling can have the apparent effect of increasing depth-of-field.