Optical format

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 where the actual sensitive target is smaller than the overall size.

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 the convenient 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 more helpful equation is to convert the pixel size, and array size, directly to optical format. The equation for this is:

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)

References

↑ http://www.dpreview.com/news/2002/10/7/sensorsizes (table of sensor sizes at bottom)

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