Dear All,
 
since threads about FilmBack sizes are comming up regulary on this forum, I propose a specific topic on this issue.
 
Historicly speeking, the "FilmBackSize" (FBS) is the size of the apeture of the film gate (expressed in mm or inch) which determins the size of the exposed negativ. For a 35mm film camera with a so-called "full apeture" it is approximatly 24x18mm (I will cover film-camera's FBS in a later topic), for a photo-(still)-camera 36x24mm (in "landscape"-mode).
 
With video cameras it is a little bit different:
The size of the video capture device (most commonly a CCD ("charged couple device" ) or CMOS) is expressed in the diagonal length in inch (1 inch = 25.4mm).
Professional broadcast camers usually have 2/3 CCD, consumer-camcorders 1/3inch.
A 2/3 CCD has a "theoretic" value of 16.93mm diagonaly (in fact, the used area to capture the image is smaller than the CCD surface).
Depending on the "AspectRatio" (or just "Ratio" 4/3 or 16/9) this value determins the "FilmBackWidth" (FBW) and -Heigth (FBH)
A typical 2/3-16/9 FBW = 14.76x8.3mm (0.581x0.327inch), a 1/3-16/9 = 7.38x4.15mm (0.291x0.163inch)
 
A full resolution HD image is 1920x1080 pixels (or 1280x720) with a square "PixelAspectRatio" (PAR).
Some HDV-cameras record 1440x1080 with a PAR of 1:1.333 (the actual pixels of the capture device are square, but spaced by 1/3rd of a pixel-width horizontally :-) standart PAL-resolution is 720x576 with a non-square PAR of 1.0667 in 4/3 and 1.422 in 16/9)
 
The "Focal Length" is usually expressed in mm.
A film-camera lens (as well as still-camera lenses) with a variable focal length (zoom-lens) is usually given with its shortest (widest) and longest (most tele) values in mm (ex: 10-80)
Zoom-lenses on video-cameras though are often given with its shortest value and the multiplication-factor for its longest focal length (ex: 10x8)
 
More to come... :-)
 
Tim