Simpson Optical Chicago Focal Length Modifier 16mm 2.5 2-1/2 Inch Lens

Simpson Optical Chicago Focal Length Modifier 16mm 2.5 2-1/2 Inch Lens Attachment

This is a vintage Simpson Optical focal length modifier made by Simpson Optical of Chicago, Illinois. Engraved on the barrel are the markings "16mm Focal Length Modifier" and "2-1/2 Inch," identifying its designed focal length and physical diameter. The unit features a multi-ring knurled aluminum barrel construction and contains glass optical elements. It disassembles into two sections as visible in the photos.

Specifications & What's Included

• Brand: Simpson Optical, Chicago, IL
• Type: Focal Length Modifier / Lens Attachment
• Marked Focal Length: 16mm
• Marked Diameter: 2-1/2 Inch (2.5 Inch)
• Barrel Material: Aluminum with knurled adjustment rings
• Lens Elements: Present and visible in photos; optical clarity not tested
• Construction: Multi-section threaded barrel; disassembles into two pieces
• Included: Focal length modifier unit only (two sections as shown)
• Sold as-is — no functionality, optical quality, or compatibility guaranteed

Applications

Simpson Optical was a Chicago-based optical manufacturer that produced a range of lenses, modifiers, and optical accessories primarily for the 16mm film, industrial, and photographic markets during the mid-twentieth century. Focal length modifiers of this type were used to alter the effective focal length of a primary lens — typically mounted between a camera or projector body and an existing lens — allowing a single base lens to cover a different angle of view or projection distance without replacing the primary optic. They were commonly used in 16mm film projection, industrial machine vision, and documentary or educational film production. Today, items like this are of interest to collectors of vintage optical equipment, technicians working on period projection or camera systems, and experimenters exploring vintage optics for custom builds or artistic use.