The tiny black spots or thread that appear in your field of vision are called floaters. They are usually most noticeable when you look at a bright background such as a sky, and they seem to move as you move your eyes from one direction to another. Floaters can appear as spots or straight or curvy lines or strings, or even as small blobs. You may see just one floater, or many.
Floaters are caused when collagen fibers break away from the vitreous part of the eye and accumulate, then float in your line of vision.
Most floaters are annoying, but benign and many will improve or disappear over time.
In some cases, floaters can be a sign of a serious eye condition such as retinal detachment, retinal tear, eye tumors, or bleeding in the eye.
See an eye doctor/specialist if you have eye pain, floaters that worsen over time or change suddenly, if you experience any other disturbances in your vision such as flashes of light, or if your floaters develop after surgery or trauma to the eye.