What the *Fresnel* is that?

A prism correction is needed in many different cases, but most commonly used for things like double vision, convergence correction, or positional correction. The addition of a prism to a person’s prescription helps to refract the light to whatever direction is needed for correction (up, down, left, right). In general when high powered prisms are involved in someone’s prescription, no matter what the material, it can be a very thick lens. Fresnel prisms (also known as Press-On Prisms) are made of little rows of baby prisms in a flexible vinyl material that is amazingly thin and can be cut and to fit all types of ophthalmic lenses. 
Besides the thinness, another advantage to a Fresnel is that it can be applied or removed at any time… making it a great solution for ophthalmologist/optometrists to “test” the correct level of prism before finalizing a prescription. The downside? It’s only a temporary solution because even though it may be aesthetically pleasing (in thinness – obviously the lines are visible when wearing, so it’s not perfect), wearing a Fresnel prism can reduce contrast and objects aren’t as clear as they would be if the prism were built into an ophthalmic lens (again the fault of those pesky lines).
I’ve been an Optician for just over a year and a half (ABO certified for only 10 months) and am surprised at how many optical professionals are clueless when it comes to the “prism patient”. Local ophthalmologists and optometrists send their patients who need prism corrections to our office so we keep an entire drawer dedicated to Fresnel Press-On Prisms so that we can accommodate… even in extreme cases we have up to 40 diopters of prism (which have a weird surplus of). Perhaps I’m spoiled in the amount of interaction I’ve had with different prism scenarios in such a short time, but I feel extremely lucky to have had the opportunities. 
Shout out to Hillary Clinton for shamelessly rocking a Fresnel prism.