A perfect circle can always be drawn inside a perfect square, the centre of the square also being the centre of the circle. The circle in perspective can also be drawn inside of a foreshortened square, drawing the diagonals will hence forth also give the centre of the circle.

Major axis of an ellipse is just the midline between the top and bottom lines and minor axis is the vertical midline between the two side lines, so combining the above we see that the circle's diameter falls slightly behind the ellipse's major axis, minor axis is the line which is the most foreshortened line among all the vertical lines or is just the centre most line in the making.

Cylinders: bellow the cylinders are showing in terms of two point perspective from different angles so, basically you are seeing ellipse not the proper circle as mentioned earlier.

Fig.102: Drawing of a cylinder through ellipse.