most sealed beam lasers do not require a 1.5" lens. It depends on how far the laser is from the lens, but in most cases if the laser is less than 6 feet from the focal lens you should be able to use a .75"D or 1"D lens. A 5" focal length is considered a long FL, but is better when cutting thick items. Most cutting lasers can use a 2 to 3 in lens, but it depends on the thickness of the material being cut