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Cooke Triplet Lens
Nineteenth-century lens makers flattened the image with a stop placed in front of or behind the lens, at the cost of astigmatism at the edges. Harold Dennis Taylor, optical manager at T. Cooke & Sons of York, dropped that trick entirely. He showed that a negative lens images a flat object onto a surface curved exactly as much as a positive lens of equal power, but the other way - so one can cancel the other.
**US 540,122**, "Lens", Taylor, filed 5. February 1894, granted 28. May 1895, puts "a negative lens … between two positive lenses, the focal power of the negative lens being approximately equal to the combined focal powers of the two positive lenses". Three separated elements: the Cooke triplet, still the simplest well-corrected camera lens.
**You build one from stock lenses**, and compare how flat its image is with a single lens.
Ophakathi
2 hours
Imiyalelo
1
1
Choose three lenses
Choose three lenses
Two convex lenses of about 100 mm focal length (check each on a distant window) and one concave of about -50 mm. Powers: +10 and +10 dioptres against -20.
Izinto zokwakha zalesi sinyathelo:
Isethi yamalensi aqoqayo1 isethi
Ilensi egobile ngaphakathi1 ucezuAmathuluzi adingekayo:
Irula Lensimbi2
2
Mount the triplet
Mount the triplet
On the bench: convex, 20 mm gap, concave, 20 mm gap, convex. Centre all three on the same height.
Amathuluzi adingekayo:
Ibhentshi lokukhanya nemizila
Isethi yezibambi zamalensi3
3
Light a grid
Light a grid
Tape graph paper to a window or light it from behind with a torch, 1.5 m from the lens. Put a tracing-paper screen behind the triplet and focus the centre of the grid sharply.
Izinto zokwakha zalesi sinyathelo:
Iphepha Lamagridi1 ishidi
Umqulu wephepha lokukopisha amaphethini1 ishidi
Itheyiphu lokumboza1 umquluAmathuluzi adingekayo:
Ithoshi le-LED lokuhlola
Ithephu Lokulinganisa4
4
Measure the edge refocus
Measure the edge refocus
Without touching the lenses, move the screen until the grid 15 mm from the image centre is sharpest. Note how far it moved. Replace the triplet with one convex lens, refocus the centre, and measure the refocus again at the same 15 mm image height.
Amathuluzi adingekayo:
Irula Lensimbi5
5
Petzval sum and design
Petzval sum and design
Ilayisha incwadi ye-Jupyter…
6
6
History & Context
History & Context
**What the patent specifies:** a negative lens between two positive lenses, its power about equal to theirs combined, each element free of diaphragm corrections, flattening the field and removing marginal astigmatism. **Modern build (derived):** three stock singlets on a bench.
Taylor's employer declined to make it; Taylor, Taylor & Hobson did, as the Cooke lens. Tessar and many later designs grew from it. Its limits: with only three elements the aperture and angle of view are modest before aberrations return.
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