Why are there subtle off-color arcs in my photos?

Question

Have been dabbling with photography for a few months. I took a couple of photos using a Sony A7ii mirrorless camera. It's not the best camera out there but it's fairly high quality. I'm taking the photos in RAW format.

Can someone take a look at a couple of my photos and explain to me why where are these white-colored arcs in them? I'm not quite sure what's going on. My friend thinks that JPEG is not registering the colors in the RAW format and auto-defaults to white. I'm a bit skeptical of that because the majority of the photo is a clear blue sky.

NOTES: Thanks everyone. This was very helpful. I wasn't aware of the technical term "banding" but will spend some time to learn more about it.

In case it helps others, I'm including a link to a helpful article I found online: https://www.dpmag.com/how-to/tip-of-the-week/identifying-repairing-banding/

Answer 1

This is known as "banding". This happens when you have uniform color gradients and the quantization by the camera (because JPEG is only 8bits/channel) transforms them into uniform areas. Along the edge of these areas the value "jumps" and our eyes are quite sensitive to this. This can be checked with the histogram, which assumes a hair comb shape (the spikes are the values in the areas):

Ironically, this is less visible if there is noise, so bad cameras are pretty much immune from this. If you have the raw image this can be remediated (possibly by adding some noise...). "Spread noise" (where pixels are randomly swapped) is another way to mitigate this.

Answer 2

Suppose you were given the task to condense a book. One scheme you could use would be to cast out all commonly reoccurring words like “is”, “and”, “the”, “are”, “a”, “that”, etc. If you substituted a short code for each extracted word, the size of the book would shrink. The reader, knowing the scheme of the code would be able to reinsert words and make sense of this now condensed book.

As you know, a digital image file consists of a googolplex of numerical data. This data mirrors scene brightness. It is needless to point out that enormous data files take up lots of storage space. Mathematical schemes are used to abridge this mass of data. JPEG (Joint Photo Experts Group) is a common scheme of compression.

JPEG rules state how mundane data like “blue sky” or uniform expanses of “blue water”, etc. are extracted, coded and reinserted. The undesirable patterns of reinserted pixels are due to the inaccuracies of the reinsertion.