Method for determination of focal length for a zoom lens

A method for determining a focal-length setting of a lens mounted to a camera, the method comprising:

• capturing an image of a scene through the lens, which is introducing distortion to the captured image;
• identifying edges in the image, using an edge-detection algorithm;
• identifying functions describing at least a fraction of the identified edges;
• comparing the identified functions or a selection and/or combination of the identified functions with data compilation for the lens, wherein the data compilation comprises transformation data correlating a distortion-correction function with a number of focal-length settings for the lens;
• identifying, by use of the data compilation, a focal-length setting which is related to the identified functions, or the selection and/or the combination of the identified functions;
• wherein the identifying functions is based on fitting of the distortion-correction functions available in the transformation data, comprising:
• applying distortion-correction functions on the distorted image or portions thereof;
• evaluating the amount of straight lines in the image or portions thereof;
• identifying distortion-correction function resulting in the maximum amount of straight lines;
• and
• identifying the focal-length setting corresponding to the identified distortion correction function.