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Retrofocus Wide-Angle Lens
Reflex cameras put a mirror between the lens and the film, and a conventional wide-angle lens needs to sit closer to the film than the mirror allows. Pierre Angénieux in Paris reversed the telephoto idea: a diverging group in front, a converging group well behind it, so the lens's focus lies further back than its focal length.
**US 2,649,022**, "Wide-angle photographic objective lens assembly", Angénieux, filed 29. July 1950, granted 18. August 1953, describes an objective whose "distance along the axis between the back surface of the last lens and the focal plane … is relatively very large with respect to the focal length". (His US 2,649,021 of the same day is a different lens - a large-aperture projection objective.)
**You build one on the bench**, measure its back focus against its focal length, and fit a mirror in the room it leaves.
初学者
90 minutes
说明
1
1
Two lenses, reversed
Two lenses, reversed
Front: concave about -50 mm, facing the scene. Rear: convex about 50 mm, 60 mm behind it. Aim out of a window.
此步骤所需材料：
凹透镜1 个
凸透镜套装1 套所需工具：
光学导轨与滑轨
透镜支架套装2
2
Find the focus
Find the focus
Move a tracing-paper screen until the distant scene is sharp. Measure rear lens to screen: the back focus. Measure the height of one far feature on the screen.
此步骤所需材料：
裁剪拓样纸卷1 张
美纹纸胶带1 卷所需工具：
钢直尺3
3
Compare one lens
Compare one lens
Remove the concave lens. The +50 mm lens alone focuses about 50 mm behind itself; measure the same feature's height.
所需工具：
钢直尺4
4
Fit the mirror
Fit the mirror
Put the concave lens back. Stand a small mirror at 45 degrees between the rear lens and the screen position and turn the screen to catch the image from the side - a reflex viewfinder. With the single lens there is no room.
此步骤所需材料：
平面镜1 个5
5
Back focus vs focal length
Back focus vs focal length
正在加载 Jupyter 笔记本…
6
6
History & Context
History & Context
**What the patent specifies:** a diverging meniscus front element and a converging group far behind it, giving a back focal distance large compared with the focal length. **Modern build (derived):** two stock lenses and a mirror.
Angénieux sold it as the Retrofocus; nearly every SLR wide-angle since uses the layout. Its limits: the asymmetry brings barrel distortion, and the big front element makes the lens larger and heavier than a symmetric design.
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