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Planar (Double-Gauss) Lens
Carl Friedrich Gauss had designed a telescope objective of two air-spaced lenses - a crown meniscus and a flint lens with differently curved facing surfaces - that corrected spherical aberration for two colours at once. Paul Rudolph at Zeiss turned it into a camera lens: two such groups placed symmetrically either side of the stop, each group achromatised with a cemented pair of glasses of nearly the same refractive index but different dispersion.
**US 583,336**, "Object-glass", Rudolph, assigned to Carl Zeiss, filed 1. February 1897, granted 25. May 1897. Descendants of this double-Gauss layout became the standard fast normal lens for most of the twentieth century.
**You test what the symmetry buys**: a grid imaged through a single lens with the stop in front, with it behind, and through a symmetric pair with the stop in the middle.
Ophakathi
2 hours
Imiyalelo
1
1
Make a stop
Make a stop
Cut a 60 mm disc of black card with a 6 mm round hole in its centre. Mount it in a lens holder.
Izinto zokwakha zalesi sinyathelo:
Ikhadi elimnyama1 ishidiAmathuluzi adingekayo:
Ummese Womsebenzi Wezandla
Isethi yezibambi zamalensi2
2
Light a grid
Light a grid
Tape graph paper over a torch or a bright window 1 m from the bench. Tracing paper on a holder is the screen.
Izinto zokwakha zalesi sinyathelo:
Iphepha Lamagridi1 ishidi
Umqulu wephepha lokukopisha amaphethini1 ishidi
Itheyiphu lokumboza1 umquluAmathuluzi adingekayo:
Ithoshi le-LED lokuhlola
Ibhentshi lokukhanya nemizila3
3
Stop in front, then behind
Stop in front, then behind
One 100 mm convex lens with the stop 25 mm in front of it: focus the grid and measure the width of the middle row and the top row across the same squares. Move the stop 25 mm behind the lens and measure again.
Izinto zokwakha zalesi sinyathelo:
Isethi yamalensi aqoqayo1 isethiAmathuluzi adingekayo:
Irula Lensimbi4
4
Make it symmetric
Make it symmetric
Add a second identical lens so the stop sits exactly midway between them, 25 mm each side. Refocus and measure the same two widths.
Amathuluzi adingekayo:
Irula Lensimbi
Ithephu Lokulinganisa5
5
Distortion from your widths
Distortion from your widths
Ilayisha incwadi ye-Jupyter…
6
6
History & Context
History & Context
**What the patent specifies:** Gauss-type objectives made anastigmatic and achromatised by cemented components of nearly equal refractive index but different dispersion, used as a doublet about a central stop. **Modern build (derived):** two identical stock lenses about a card stop.
The Planar was too prone to flare with uncoated glass; once anti-reflection coatings arrived, double-Gauss lenses became the classic 50 mm f/2 and faster. Its limits: many air-glass surfaces, and a design that needs careful glass choice to go wide.
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