It may not be surprising to hear, as in most subject areas, that there is a substantial amount of mathematics in Optics. Luckily for most of us (or unluckily if you like maths) this maths is ‘hidden’ by using rule of thumb systems, data tables or computer software that does all the work for us. This is also largely true of Optics, though there are some purists out there who prefer the aid of mathematical equations in deriving specific results. I am naturally one of those purists, proud of it too. However, there are some equations that are routinely used in practice, both by Dispensing Opticians, and Optometrists. Firstly let us consider something fundamental, the power of a correcting lens.