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 6.4x4.8 mm sensor has a diagonal of 8.0 mm and therefore an optical format of 8.0*3/2 = 12 mm, which is expressed as 1⁄2 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:

${\displaystyle OF={\frac {p{\sqrt {w^{2}+h^{2}}}}{16000}}}$

with:

• w = width of array (in pixels)
• h = height of array (in pixels)
• p = pixel size (micrometers)