Some microscope objectives have the inscription LWD or ULWD on them. This stands for Long Working Distance or Ultra Long Working Distance. Working distance is the vertical distance that must be between the tip of the microscope objective lens and the part of the specimen you wish to view through the microscope, in order for it to be in focus. Long Working distance microscope objectives require that this distance be greater.
So why would you want more distance? Having a longer working distance is invaluable when viewing specimens through a culture vessel or Petri dish while using an inverted biological microscope. When using a metallurgical microscope, a long working distance microscope objective is helpful if viewing IC wafers in order to avoid inadvertent contact with the wafer. Inspecting solder joints is another process that often requires use of a long working distance objective. Additionally, any time more working room is required under the microscope, long working distance objectives are usually preferred.