If you are requiring extra fine engraving such as fonts below 8pts; we recommend the 1.5" lens. A short focus lens has a smaller minimum spot size but spreads out more quickly after reaching the focus making them better for fine detail but not good for thick cutting. A long focus lens keeps the focus longer allowing for straighter edges but does not focus down to as small a minimum spot.
If you cutting constantly at thicker materials; a longer focus lens will give you straighter cut. A 5" lens is useful when engraving into objects where you need more working distance like engraving bowls or boxes with high sides.