The equivalence refers to a) angle (field) of view aperture size. As the sensor is 1,5 (approx.) smaller, the angle of view is also smaller, and aperture is also smaller. Imagine the A4 print as equivalent of full frame. Now the APS sensor is just rectangle in the middle not covering the whole A4 print (the . Which means that the APS sensor, from the same perspective and distance, "sees" less of the picture (and light gathering is also "weaker"). So in terms of length of the optical system (lens) is the same as in full frame (35 mm is always 35 mm but angle of view and light gathering capability is smaller. So using APS compared to FF gives you: a) "zoom" effect" larger depth of field (smaller aperture). kind regards, mat
