How does Image Sensor size relate to Optical Format?

Image Circle Calculation| How to calculate an Image Circle for a Lens

Optical Format is a photonics term that is a carryover from the days when video cameras where using Vidicon tubes.  You many have heard references to lens format as 1/4, 1/3, 1/2, 2/3, 1, and 4/3-inch format.  The description is defined in inches.  The Optical Format can be approximated as 3/2 x the sensor Image Circle.   As an example, let’s take one of the most widely used lens format, 2/3 inch often referred to as a C-mount Lens.  The accepted definition of a 2/3-inch lens is one that has an image sensor that has an 11mm diagonal.  If we take 11mm x 3/2 = 16.5 mm = 0.6496 ~ 2/3 inch.  As convent as it may seem to carry forward the idea of Optical Format, todays advanced sensor technology has formats that are better described using the description of an Image Circle.   An Image Circle is the circle that an image sensor can fit within without cropping the corners or area of the sensor.  The size of an Image Circle also relates to the camera Lens Mount.  As an example, if you have an image sensor with an Image Circle of 66.7 mm, the Lens Mount that would not crop or cause keystone effect would be the M72 Lens Mount.  Listed below are some common Lens Mounts and their corresponding Image Circle.

To calculate an image sensor Image Circle, the following formula is shown below.






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