I took some photos on a nice sunny day with my old Pentax K-r. I used fixed aperture mode because I like depth of field in photos. Settings for this particular photo are f/5.6, 1/100s, 55mm and ISO200 according to the exif data.
As you can see in this about 2-3 times zoomed in version of my photo, the yellow surface that is lit up by the sun radiates into the shadowy part behind it.
How is this effect called? How can I prevent it?
You can see bloom with your bare eyes. Ever tried to look inside an open window in a sunlit white wall? The inside is orders of magnitude darker and will be drowned from the bloom, not to mention your iris (aperture) will shrink. You have to make a "toy telescope" (rolled-up cardboard, fists or similar) and block the light parts to make the bloom disappear and see inside the room. So if we're judging cameras by how well they imitate eyesight, this is not a flaw.