The present invention is to provide a sheet conveying apparatus capable of preventing detection error without causing damage on a sheet. The present post-processing apparatus includes a sheet conveying path to convey a sheet as being formed between an upper guide member and a lower guide member for guiding a sheet, and a detection portion including a second sensor as an electrostatic capacitance sensor to detect a sheet being conveyed on the sheet conveying path with output of the second sensor. The second sensor is fixed on a face of an upper guide member on an opposite side to the conveying path and the second sensor and the sheet conveying path are separated by the upper guide member.

A method (600) for detecting and tracking a feature of a web of packaging material (102), using a first capacitor (106) placed at a distance (d) from the web of packaging material (102) is provided. The method (600) comprises; feeding (602) a section of the web of packaging material (102) to a first position (p), so that the section influences a dielectric property of the first capacitor (106), measuring (604), at a point of time (t), a capacitance of the first capacitor (106), and determining (612) the feature of the section passing the first capacitor (106) at the point of time (t), based on the capacitance.