How To Determine Camera Focal Length
It is not possible to determine both lens and sensor size simultaneously.
Information technology is relatively easy however, to find the value of a lens given an arbitrary sensor size.
Stuffmatic's Blam (now Fspy) addon can lucifer the photographic camera placement based on the perspective of the image using lines drawn with the grease pencil. The improver volition modify the render size of the scene (in pixels) based on the reference image, and will also determine the lens size, based the sensor size for the electric current scene in blender.
Note: Blam is no longer mantained and has been replaced by Fspy: https://fspy.io/, just there is no importer for blender 2.79 or previous versions.
For a detailed post on how to utilise blam and Fspy read: How can I recreate geometry using a photograph?
Knowing the lens size will aid you determine the Field of view in degrees.
The field of view's value is unique for the combination of the length of the lens and the dimensions of the sensor.
For example: yous volition have the same field of view (roughly 35 degrees) with a 35mm lens on an APSc size sensor, every bit you would have on a Full Frame sensor with a 50mm lens, or with a 75mm lens on a medium format photographic camera.
Y'all tin can know the lens field of view in the camera settings for blender, and if you alter the sensor size then you can use that data to find the lens equivalent.
Why should we care for the sensor size then?
While the field of view of an image with different lenses and sensor size combinations might be the same, the depth of field will vary depending on the length of the lens.
If your goal is to have a perfect integration between CG images and a photographic image, ideally you would need to match the values with those of the photographic camera and lens used.
Afterward reading all of the above... what is the conclusion?
Sadly, that while it is piece of cake to detect the field of view of an image, at that place are no tools to determine the size of the original sensor size, other than an educated guess based on feel (like the calorie-free conditions, estimated depth of field, etc) or other external information (like the Exif data on the file, or historical records, or hearsay).
As you tin can guess, those who make special furnishings for photographs and movies make all kinds of notes on the field to know the conditions in which images are made, so that they don't have to guess any of it when integrating images with CG.
Source: https://blender.stackexchange.com/questions/67844/is-there-any-way-to-calculate-camera-focal-length-and-sensor-size-from-a-single
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