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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.
Katamtaman
2 hours
Mga Tagubilin
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.
Mga materyales para sa hakbang na ito:
Transparency film para sa inkjet1 pilyego
Teyp na pantakip1 rolyoMga kailangang kasangkapan:
Pangmarkang Hindi Nabubura
Lampara ng LED na pansuri
Panukat na Asero2
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.
Mga materyales para sa hakbang na ito:
Set ng kumbeksong lente1 set
Lenteng malukong1 pirasoMga kailangang kasangkapan:
Optical bench at riles
Set ng hawakan ng lente3
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.
Mga kailangang kasangkapan:
Panukat na Tape4
4
Focal length vs position
Focal length vs position
Naglo-load ng Jupyter notebook…
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.
Mga Materyales
4- 1 pilyegoPlaceholder
- 1 rolyoPlaceholder
- Placeholder
- 1 pirasoPlaceholder
Mga Kinakailangang Kasangkapan
6- Placeholder
- Placeholder
- Placeholder
- Optical bench at riles10% komisyonPlaceholder
- Set ng hawakan ng lente10% komisyonPlaceholder
- 1 vendor sell this, none ship to you yetPlaceholder
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