Occasionally I am asked why it is that floaters “disappeared” for the few hours after having the pupils dilated pharmacologically for the eye examination. The answer is relatively simple and is much better explained graphically than in words.
Remember, you are not seeing floaters, but rather you are aware of shadows cast onto the retina by optical densities in the vitreous space. Your awareness of these shadows depends on a multitude of optical, and psychological factors:
- Central vs. peripheral location
- Anterior vs. posterior position
- Large vs. small size
- Dense vs. translucent vs. transparent opacities
- Visually tasking vs. non-tasking activity
- Bland/plain vs. irregular visual background
- Personality type
- Bright ambient light vs. dim/dark background light
- Normally reactive vs. Pharmacologically dilated pupil.
It is the last comparison I’ll illustrate here. The following shows the same floater in the same position of the eye. Assuming all the other variable are the same as well, the only thing that is different is the size of the pupil. With the dilated pupil (upper image), the light is entering the eye from a broader light source. The trailing shadow will be shorter, and in this case, will not reach the sensory retina in the back of the eye (or will be diminished). Below that image, the pupil is constricted, and the light entering the eye is from a smaller source. The trailing shadow will be longer and in this illustration, will cast a much more distinct shadow onto the retina.