How can I find circles that are in certain distance?

I want to find on image that circles wich are for example 50px from each other.

[Sven]

1. Find your circles.
2. Find those that are 50px away from another circle.

If you don't show what you have done, we can only give answers like this.

Answer 1

[Image Analyst]

First find the circles, for example with imfindcircle(). Then find the distance of every circle to every other circle with the Pythagorean theorem or the hypot() function.