The Image Sensors are rectangular, usually with a 4:3 aspect ratio. The size of the sensors are marked as 1/3, 1/2, 2/3, and 1 inch, or two dimessional values like 22.5mmx15.0mm. These sizes are not the diagonal length of the sensors but the standards based on the "old video tube" sensors that the new image sensors replaced.
The Focal Length is defined as the distance between the optical center of a lens and the plane where parallel light rays from an object at infinity come to a sharp focus.
The field of view (FOV) is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically.
Typical focal lengths and their 35mm format designations |
|
< 20mm |
Super Wide Angle |
24mm - 35mm |
Wide Angle |
50mm |
Normal Lens |
80mm - 300mm |
Tele |
> 300mm |
Super Tele |
Since the aspect ratio of lenses is not 1:1, the optimum Focal Length to completely fill the sensor Horizontal or Vertical directions is different. So you may use either one to calculate the final size. The following is a short table to help to the actual size of some popular sensors.
| Sensor Type | 1/3" | 1/2" | 2/3" | 1" | Canon EOS 20D |
| Horizontal Size (mm) | 4.8 | 6.4 | 8.8 | 12.7 | 22.5 |
| Vertical Size (mm) | 3.6 | 4.8 | 6.6 | 9.5 | 15.0 |
