Floaters are located in the vitreous and are described by patients as floaters, bubbles, bugs, cobwebs, or dark spots that move during eye movement. Whether the patient and/or practitioner can see them depends on the location of the floaters.
When light enters the eye, floaters cast a shadow on the retina. It is this shadow that the patient ‘sees’ rather than the floater itself. The further away from the retina, the floater is located the bigger the shadow and the more noticeable the floater is to the patient. The closer the floater is to the retina the smaller the shadow and the less noticeable the floater is to the patient.
A useful analogy is the position of the Moon. When the Moon moves to be between the Sun and the Earth, because of the Moon’s distance (around 239 000 miles) from the Earth, it casts a large shadow on the surface of the Earth, which is noticed by many thousands of people. This is known as a lunar eclipse. If the Moon where at cloud level, that is further from the Sun and closer to the Earth, the shadow would be much smaller and less noticeable.
So, small floaters in the anterior vitreous (far from the retina) which can be impossible for the practitioner to detect will cast a relatively large shadow on the retina and be very noticeable to the patient. Conversely, large floaters in the posterior vitreous (close to the retina) which can be very easy for the practitioner to detect will cast a relatively small shadow (if at all) on the retina and not be noticed by the patient.
Furthermore, patients also often report that their floaters become more noticeable and sometimes more bothersome when they are in a bright environment and/or looking at a featureless scene such as a clear blue sky, a plain white wall or the screen of a device. The extra light enhances the appearance of the shadow while the featureless scene means there is little to mask the shadow.