Optical format is a hypothetical measurement approximately 50% larger than the true diagonal size of a solid-state photo sensor. The use of the optical format means that a lens used with a particular size sensor will have approximately the same angle of view as if it were to be used with an equivalent-sized video camera tube (an "old-fashioned" TV camera). In a video camera tube, the diagonal of the actual light-sensitive target was about two-thirds the outside diameter, which was the measure used.
The optical format is approximately the diagonal length of the sensor multiplied by 3/2. The result is expressed in inches and is usually (but not always) rounded to a convenient fraction. For instance, a 6.4x4.8mm sensor has a diagonal of 8.0mm and therefore an optical format of 8.0*3/2 = 12mm, which is expressed as inch in imperial units. The reason it is expressed in inches is historical, dating back to the early days of television. [1]
Many image device sheets do not list the actual optical format but do list the size of their pixels in terms of micrometers; a helpful equation can be used to convert the pixel size and array size directly to optical format. The equation for this is:
OF=
| p\sqrt{w2+h2 | |
with: