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When i go to the camera values in the datablocks, I can change all the camera parameters:

Horizontal/Vertical FOV and the Focal Length and Sensor Size.

When I update the Horizontal or Vertical FOV, the Focal Length gets updated correctly.

When I update the sensor width or sensor height, the corresponding FOV parameter gets updated, but the Focal Length does not.

Mapping between sensor size and focal length

It seems to me I will make the mapping completely off and as soon as I change the sensor size, the focal length is no longer correct. Why is that? I need correct camera values including the sensor size and I am not sure right now in which combination I can achieve them.

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The focal length doesn't change with sensor size. A lens with a 35mm focal length is always 35mm. However, the FOV changes for the same focal length with different sensor size.

The reason for the common misconception that the focal length gets longer with a smaller sensor, is that lens and camera manufacturers often (if not always) tell us that for instance "This 50mm lens on this camera is equivalent to an 80mm lens on a 35mm sensor". What this means, is that on a Canon APS-C sensor1 a 50mm lens gives the same FOV as an 80mm lens would on a fullframe 35mm2 sensor.

Crop factor on Wikipedia explains it quite well.

1Nikon has a different crop factor, so a 50mm lens on one of their cropped sensors would be equivalent to 75mm on a fullframe.
2A fullframe 35mm sensor is actually 36mmx24mm. 35mm comes from the fact that film is 35mm wide, including the edges above and below the recorded image.

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  • $\begingroup$ If I had the focal length in pixels as calibration software often returns, then the focal length would depend on the sensor size right? Because focal length_in_pixels = focal_length_in_mm * num_pixels / sensor_size_in_mm. Am I correct with this formel? $\endgroup$
    – Hakaishin
    Commented Sep 26, 2018 at 13:30
  • $\begingroup$ "a fullframe 35mm sensor". Video and photo standards are sometimes just confuse (what is the pixel aspect ratio?), including for picture sizes: 35mm is a movie film gauge for images of various sizes (e.g. 16x22mm). It used to be packaged for photo into the 135 film cartridge (e.g. Ilford Pan F film) to record 24x36mm pictures but also 18x24mm and 24x65mm. (I know you already know.) $\endgroup$
    – mins
    Commented Jun 11, 2022 at 17:33
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A lens of a given length will create an image circle at the taking plane (focal point behind the lens) of a fixed size. The sensor fits within this image circle, first it crops to a rectangular shape, then (depending on sensor size) it can be cropped (or smaller) within the circle. Thus the effective Field Of View will change (like a zoom in) but the lens length can remain the same.

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