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2015 International Year of Light and Light-Based Technologies

Kodak  Brownie Number 2,  Models A to F (1901-1935) The original Brownie, introduced by the Eastman Kodak Co. in February 1900, was taking square pictures on a new type of roll-film  (the now defunct 117).   In 1901, the  Number 2  Brownie became the first camera to use  120 roll-film,  (now one of only two formats still widely available;  35 mm being the other).  120-film differs from its 117 predecessor only by the diameter of the flanges on its reel  (0.991'' vs. 0.875'' for the 117).  Both formats used the same core diameter  (0.468'')  and film width  (2.466'').

(2015-07-02)   Basic specifications of the No.2 Brownie
The diagonal of the image and focal length of the lens are both 100 mm.

The rectangular format for the Number-2 Brownie was specified to have a diagonal of exactly 100 mm and a width of  57 mm  (2¼'' rounded down).  By the theorem of Pythagoras, the nominal height of the picture is equal to:

Ö ( 100² - 57² )   =   Ö 6751   =   82.1644692...mm.

Accurate measurements on my own Brownie confirm this.  The advertised size of  2¼''  by  3¼''  is definitely just a rough approximation...

This image size had been inaugurated in August of 1897 with the  Folding Pocket Kodak  (on 105 roll-film, discontinued in 1949)  which Frank A. Brownell (1859-1937)  designed and manufactured for Kodak.  In 1898, George Eastman commissioned Brownell to design what would become the Brownie cameras:  Affordable easy-to-use cameras which would encourage everybody to practice photography, thereby increasing film sales for  the Eastman Kodak Company...
 
Officially at least, the Brownie camera wasn't named after Brownell but after the cartoon characters "The Brownies" created in 1879 by Palmer Cox (1840-1924).

The lens was precisely a  normal  one, according to the wisdom of that era, with a focal length exactly equal to the diagonal of the picture  (100 mm).

With such a lens focused to infinity, a camera would have a field of view of  53.13°. However, to maximize its  depth of field,  the fixed-focus of the  No.2 Brownie  was set to the  hyperfocal distance  instead,  which narrows its field of view by one or two degrees.

I measured the diameters of the three apertures on my No.2 Brownie to be approximately  3.5 mm,  5 mm  and  7 mm.  These are obviously one f-stop apart. 

Beginner's question   |   f-stop equivalents on a Kodak Brownie No.2

(2015-07-02)   Wollaston's Lansdcape Lens
Meniscus lens with an aperture stop on the concave side.

This was the key to the Brownie design.  The concave side is facing outward behind the iris and the shutter  (which protects the lens).

Wollaston's Lansdcape Lens (1804, 1812)   |   William Hyde Wollaston (1766-1828)

(2015-07-01)   No. 2  Brownie  Model F
Patents dated   1 Feb. 1916,   21 Mar. 1916   and   4 Aug. 1925.

• First all-metal Brownie model, released in February 1924.
• 100 mm "normal" lens, equal to the diagonal  (45° field-of-view).
• Two reflective finders  (portrait  & landscape).
• Two  UNC 1/4''-20  tripod mounts  (portrait  & landscape).
• Central slider gives a choice of three apertures f/16, f/22 and f/32.
• Corner slider provides a shutter lock (T-mode).

No. 2  Brownie