Trying to understand CMTime

A CMTime struct represents a length of time that is stored as rational number (see CMTime Reference). CMTime has a value and a timescale field, and represents the time value/timescale seconds .

CMTimeMake is a function that returns a CMTime structure, for example:

CMTime t1 = CMTimeMake(1, 10); // 1/10 second = 0.1 second
CMTime t2 = CMTimeMake(2, 1);  // 2 seconds
CMTime t3 = CMTimeMake(3, 4);  // 3/4 second = 0.75 second
CMTime t4 = CMTimeMake(6, 8);  // 6/8 second = 0.75 second

The last two time values t3 and t4 represent the same time value, therefore

CMTimeCompare(t3, t4) == 0

If you set the videoMinFrameDuration of a AVCaptureSession is does not make a difference if you set

connection.videoMinFrameDuration = CMTimeMake(1, 20); // or
connection.videoMinFrameDuration = CMTimeMake(2, 40);

In both cases the minimum time interval between frames is set to 1/20 = 0.05 seconds.