Calibrate image scale automatically

I'm working with an image bank to find the leaf area of the leaf.

For that I would get an algorithm that would find my scale and calibrate my image automatically.

Here is an example image:

In all my images I have something like this. What I want is to find that piece of the image through an algorithm and make it my scale.

Here is another image with the part of the image of interest:

Answer 1

[Benjamin Thompson]

Use the Camera Calibration app, and see the example "Measuring Planar Objects with a Calibrated Camera". Use a checkerboard image to calibrate your own camera setup if possible, then you can use pointsToWorld to calculate any distance within your plane of interest. This method has the nice advantage of removing camera distortion effects for you.