Understanding Plano-Convex Lens Orientation and Focal Lengths

Here are some pics showing the proper orientation of a plano-convex lens (curved side towards the laser) and the effect of different focal lengths.
K40 machines typically use Plano-convex lenses with a 50.8 mm (2") focal length as it offers a good balance with a small spot size for engraving and a reasonable depth of field (DOF) for cutting.