Why do some lenses make your eyes look small? 

If you are short-sighted, it means you can see better up close but not as well far away: the light that enters your eye focuses too far in front of the right spot (the retina).

This lens uses its shape (thinner in the middle but thicker on the outside) to refocus the light so that it reaches the right part of your eye. But it minimises your eye in the process!

The good part is that we can use hi-index lenses to reduce this effect - you may have heard of 1.6 or 1.67 lenses.

Why do some lenses make your eyes look bigger?

If you are long-sighted, the light focusses too far behind the right part of the eye, which means you can see better further away than you can up close.

This lens causes the light to focus at the right part of the eye by its shape (thicker in the middle but thinner on the edges). This maximises your eyes.

We can thin these lenses to reduce the effect with hi-index materials.