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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.
Intermediate
2 hours
Instructions
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.
Materials for this step:
Inkjet Transparency Film1 sheet
Masking Tape1 rollTools needed:
Permanent Marker
LED Inspection Flashlight
Steel Ruler2
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.
Materials for this step:
Convex Lens Set (Assorted Focal Lengths)1 set
Concave Lens1 pieceTools needed:
Optical Bench & Rail
Lens Holder Set3
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.
Tools needed:
Tape Measure4
4
Focal length vs position
Focal length vs position
Loading 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.
Materials
4- 1 sheetPlaceholder
- 1 rollPlaceholder
- 1 setPlaceholder
- 1 piecePlaceholder
Tools Required
6- Placeholder
- Placeholder
- Placeholder
- Optical Bench & Rail10% commissionPlaceholder
- Lens Holder Set10% commissionPlaceholder
- 1 vendor sell this, none ship to you yetPlaceholder
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