Right - so 2560 x 1440 is the default for a 5K studio display or old 5K iMac. 1920 x 1080 is the default in MacOS for a physical 4K monitor. Note that these n

Right - so 2560 x 1440 is the default for a 5K studio display or old 5K iMac. 1920 x 1080 is the default in MacOS for a physical 4K monitor. Note that these numbers are half the actual pixel count in each dimension because MacOS defaults to assuming the display is a 2x Retina Display. So, if you choose the 1920x1080 option you get what I see using my actual 4K display. It also avoids the downsampling, because the stream itself is 4K and so is sharper. I don't know why Apple set the default for 5K downsampled.