Why does a7m4 camera have blue dots?

This is a small blue dot, and we call it "glare"

Glare is often considered as a reflection of poor lens quality in professional photography, and general lenses will use coating to suppress glare; However, in the film industry, glare is also used to create a unique lens sense of the film, so the existence of glare is a matter of opinion. In order to reduce glare, almost all modern lenses are coated with a very thin chemical film on each surface of each unit to reduce the reflectivity of these surfaces. Although coating can weaken glare, it can't completely eliminate glare; This is especially true when the lens is aimed directly at a very bright light source such as the sun or floodlight. Now that we know that the front and back surfaces of the camera lens may be coated, we should be extra careful when cleaning either end of the lens. Rough wiping will remove the coating.