Hello, this isn't a homework problem, so i'm hoping it's okay to post here. I would like to know the correct way to mathematically express the idea in my title. It is intuitively obvious that as the radius of a circle increases, it's curvature decreases. I looked it up and found that the curvature of a circle is equal to the reciprocal of it's radius. Certain assumptions are often made when looking at lenses, i.e the wave fronts reaching the lens are parallel, or have 0 curvature - In other words, the object distance is infinitely far away. But, 1/∞ ≠ 0 So how do I express it properly? In words, I think it goes something like this - As the radius tends towards infinity, the curvature of the circle tends towards zero.