A lens’s focal length (like 200mm) doesn’t tell you how long the lens physically is – it’s the distance between the lens’s optical center and your camera’s sensor. More specifically it’s the effective focal length when focussed on infinity.
So why are some 200mm lenses much longer than others?
-
Optical design – Telephoto designs use special lens groups to compress the light path, allowing a long focal length to be built into a shorter, lighter lens body.
-
Zoom vs. Prime – Zoom lenses need more glass and mechanics, so they’re bulkier.
-
Aperture size – Wider apertures (like f/2.8) require bigger front elements, adding length.
-
Extra features – Image stabilization, weather sealing, and pro build all add size.
💡 Bonus fact: Some 200mm mirror lenses are very small because they bounce light with mirrors, folding the optical path into a compact package! Unfortunately, these mirror lenses generally suffer from a loss in image quality.
Imported from legacy forum.