This lens is part of a FULL SET OF 14 Lenses. This lens is designed largely for close-up photography, and the extended shooting distance it allows gives increased flexibility. It has a magnification of 0.5X at its minimum focusing distance of 45cm. (The same magnification is produced at a focusing distance of 23.2cm with the 50mm Macro.) Even at the maximum magnification of 1X achieved when the lens is combined.with the FD 50-U extension tube, a extended comfortable film-to-subject shooting distance of 40cm can be maintained. This lens is comprised of 5 elements, 3 groups lens construction which also facilitates optimum correction of astigmatism distortions, an all important factor in close-up photography. It also suppresses aberration fluctuations caused by varied shooting distances ranging from infinity to a magnification ratio of 1. A friction mechanism is provided on the focusing ring to prevent the lens barrel from accidentally extending when shooting with the lens pointing down. This lens has been amazingly REMOUNTED to EF, DECLICKED AND with a CUSTOM LENS CAP from Ron Sim of Simmod, in Indiana. The lens's FOCUS, and IRIS have been GEARED by the very Sean McCurry at Follow Focus Gears in Georgia... **Who, for the first time ever, learned how to ENGRAVE F-STOP MARKINGS. He did this, for the first time into every one of these lens's iris rings and matched them appropriately to their LIGHT MEASUREMENTS. Way to go above and beyond, Cap'n. The iris turns past F16, F22 or F32 on all of the lenses, in this set, except the 50mm f3.5, 55mm f1.2 and the 135mm f2.0. Be careful when turning not to turn it past that point, otherwise it won't be adjusting exposure, accordingly. Ron Sim informed me that those connecting the iris to a follow focus should be careful. This is because the iris turns past it's "most closed" point on most of these lenses. My advice is, if you do connect the iris to a follow focus, you will need to manually calibrate the follow focus to know that the end of the iris is where it is, and not where the lens turns to. That would tear at the iris ring. So the iris ring is for reference and the knowledgeably careful. Turning the whole iris turn on a follow focus is bad for the lens and it isn't good to have that part in the focus puller's calibration either. So please leave that part of the iris turn for all practical purposes. These lenses all have 77MM INNER DIAMTERS and 80MM OUTER DIAMTETERS. So you can easily attach and swap 77mm variable nd filters between these lenses and you can attach an attachable mattebox such as the Misfit Atom to all of these lenses. Focal length:100mm Aperture ratio: 1:4 Construction: 3 groups 5elements Coating: S.S.C. (super spectra coating) Angle of view: Diagonal: 24° Vertical: 14° Horizontal: 20° Distance scale: (m) 0.45 (magnification 0.5X) to7.oo (ft) 1.48 to 15.oo Focusing mechanism: Helicoid Magnification: 0.5X for lens used alone, 0.5Xó1.0X with Extension Tube FD 50-U (0.4m) Minimum aperture: f/32 Diaphragm: Automatic Filter size: 52mm Hood: BT-52 Cap: C-52 (CG2-0070)