The Canon FDn 100mm f/4 Macro can be used for general photography but is also capable of focusing directly to a 1:2 reproduction ratio (.45m) or to 1:1 (.40m) with the supplied FD-50 extension tube. Many photographers prefer this focal length for photomacrography, since it gives them twice the working distance as a 50mm lens for the same reproduction ratio. This allows more room for lighting equipment and more distance from sensitive subjects. A special friction damping device has been incorporated into the helical focusing mechanism to prevent unwanted extension of the lens when used in vertical applications, such as copying.
The FD-series lenses can be used for full-aperture metering when mounted on any of Canon SLR models including and after the original F-1, EF and FTb models.