Few types of surgery require more precision than eye surgery, for which standard steel surgical knives are insufficiently dull. Diamond knives provide micrometer precision for cataract surgery and other typical ophthalmic procedures. Knives are typically around 1-3mm in thickness, but can measure less. Not all blades and handles are shipped together. Handles should be chosen based on the size of blade the customer expects to use; blades can either be single, double or triple edged. Moreover, they are either configured as spear, tapered spear, lance or crescent shapes. Blades are extended (to about 3mm) or contracted with step wheels that move the blade in precise increments.