Ghost images at low-minus sphere

[Phillysun]

I'm inquiring about an alleged phenomena that creates an unsusual amount of ghost-image reflections in low-power (-0.75, or so) minus-spherical lenses. The apparently internal reflections are subtle but excruciatingly persistent, although anti-reflective coating does help. I cannot find a source on this specific problem, although I vaguely remember one of my teachers addressing it. Is this just optical folklore, or does such a prescription truly lend itself to greater ghost imaging? (I've seen folks in this Rx range complain of it myself, moreso than those with stronger lens powers.) To be clear, I'm not talking about exceptional reflected glare off the external front and back lens surfaces, such as that derived during nighttime driving or from bold lighting; I mean subtle double images of even softly-lit objects perceived within the lens. I'd appreciate any info or sources avaiable. Thanks! - Phillysun

[Darryl Meister]

The ghost image you're referring to is the result of internal reflection between the back and front surfaces of the lens. Because of the "vergence" or "power" of this particular reflection, it is generally most troublesome in Plano and low Minus powers. Generally, for reflections to be bothersome to the wearer, several criteria should be met:

• If the ghost image is sufficiently bright compared to its background illumination, it will be more visible to the wearer (just as stars are only visible at night).
• If the ghost image is sufficiently in focus, or can be brought into focus by accommodation, it is more likely to be visually distracting to the wearer.
• If the ghost image is slightly displaced relative to the object of regard, it is more likely to draw attention from the wearer.

This particular ghost image meets the last two criteria for Plano and low Minus lenses; this reflection is relatively in focus, or can be brought into focus with accommodation, and this reflection is only displaced slightly from the object of regard since there is only a small amount of prism induced in these prescriptions. Consequently, in low-light conditions such as night driving, which often results in a situation that easily meets the first criterion, this particular type of reflection is more likely to illicit a complaint from emmetropes and low myopes than for hyperopes or high myopes.

[Phillysun]

Thanks for the qiuck and helpful reply!