If you prescribe prism on a fairly regular basis, chances are one of your patients will raise the complaint about seeing double. That wouldn’t be surprising if it had something to do with fragile fusion, but would surprise you if the complaint occurred even when looking though one eye. It is the law of averages that this will occur, typically with observant patients (no pun intended) having prism in their prescription for the first time. You’ll be more likely to encounter it with patients who have a low Rx, and making the observation specifically when looking at a discrete light source such as a light fixture, street lamp, car headlight, or LED digital clock or timer.
Puzzled the first time I heard this, I thought I’d share the observation. A property of prism is that it displaces light, and the greater the power of the prism the more the second image will be perceived to shift from its original source. Fortunately this is something to which the patient usually adapts, and a little bit of reassurance that this is normal goes a long way. Here is the visual evidence. The first image is the reflection of a trans-illuminator light in a 0.25 prism diopter lens held base up, and there is only a single light source seen.
It takes a 0.50^ lens before the second image is barely perceptible, and when the power reaches 1^ it is definitely noticeable, as seen here with the base up orientation creating the second image below the first.
Increase the power power up to 3^, and the double image of the light source is blatant.
Patients who periodically receive increases in Rx lens power, particularly minus lenses, are accustomed to adapting to the subtle changes that occur which include monocular prismatic effects. They are unlikely to notice the type of point spread of light induced by a relatively small amount of prism ground into their Rx. They’ve already experienced the phenomenon subconsciously since optically speaking, any ophthalmic lens can be considered as a series of prisms arranged base-to-base or apex-to-apex.
This is one reason, among many, why it is prudent to make relatively small changes from the habitual Rx for patients who are visually sensitive. In the case of prism added to low power Rxs, it is another good argument for keeping the prism power as low as possible. Remember: Prescribe the least amount of prism to gain the maximum therapeutic benefit.