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Varifocal Projection Objective
A lantern-slide projector threw one fixed picture size at a given distance; to fill a different screen, the lantern had to be carried closer or further. Clile C. Allen of Chicago put three lenses in one tube, fixed the first and last, and made the middle one slide on a spiral track as the tube was turned - changing the focal length, and so the picture size, from the same spot. A rack and pinion moved the whole tube to refocus.
**US 696,788**, "Optical objective", Allen, filed 25. February 1901, granted 1. April 1902: "three lens elements, the first and third of which are permanently fixed … while the second is movable and capable of being placed in any intermediate position". It is one of the earliest variable-focal-length objectives - the idea behind every zoom lens.
**You build it on a bench as a projector**, and measure picture size against the middle lens's position.
Ophakathi
2 hours
Imiyalelo
1
1
Draw a slide
Draw a slide
Draw a 24 mm square grid with a marker on transparency film. Tape it over the torch lens: this is the lantern.
Izinto zokwakha zalesi sinyathelo:
Ifilimu ecwathile ye-inkjet1 ishidi
Itheyiphu lokumboza1 umquluAmathuluzi adingekayo:
Ipeni Elingesulekiyo
Ithoshi le-LED lokuhlola
Irula Lensimbi2
2
Mount three lenses
Mount three lenses
Wall side: convex ~100 mm. Slide side: convex ~50 mm, 80 mm from the first. Between them: concave ~-50 mm on a holder that slides. Aim the bench at a wall 2 m away with the lit slide behind the 50 mm lens.
Izinto zokwakha zalesi sinyathelo:
Isethi yamalensi aqoqayo1 isethi
Ilensi egobile ngaphakathi1 ucezuAmathuluzi adingekayo:
Ibhentshi lokukhanya nemizila
Isethi yezibambi zamalensi3
3
Slide, refocus, measure
Slide, refocus, measure
Set the middle lens 10 mm from the wall-side lens. Move the slide (or the whole lens group) until the grid is sharp on the wall; measure its width. Repeat at 30 and 60 mm.
Amathuluzi adingekayo:
Ithephu Lokulinganisa4
4
Focal length vs position
Focal length vs position
Ilayisha incwadi ye-Jupyter…
5
5
History & Context
History & Context
**What the patent specifies:** a three-element objective with fixed outer elements and a movable middle element carried by a spiral track, plus a rack and pinion to move the whole tube. **Modern build (derived):** stock lenses on an optical bench.
Allen's lens must be refocused after each change - a varifocal. A true zoom keeps focus while zooming by moving two groups together, which came later with cam-driven designs. Its limits: the picture dims as it grows, and aberrations change across the range.
Izinto
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