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Basically the scale factor is used to create your scale pyramid. More explanation can be found on this link:
https://sites.google.com/site/5kk73gpu2012/assignment/viola-jones-face-detection#TOC-Image-Pyramid
In short. Your model has a fixed size defined during training. This means that this size of face is detected in the image if occuring. However, by rescaling the input image, you can resize a larger face towards a smaller one, making it detectable for the algorithm.
Using a small step for resizing, for example 1.05 which means you reduce size by 5 %, you increase the chance of a matching size with the model for detection is found.
