回复：what makes a 300mm lens a 450mm lens on aps-c body?

The aps-c sensors are (1.5x or 1.6x) smaller than a full frame sensor, so it only capture the area equivalent to the middle portion of a full frame sensor. The effect on the resulting picture is a 1.5x or 1.6x zoom of what you get from a full frame camera, as explained here:
http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_Multiplier_01.htm
The absolute size of the subject taken from a full frame or aps-c sensor will be the same at the SAME FOCAL LENGTH. However, the full frame picture will have a bigger viewing field but a less resolution on the particular subject, if both the full frame and the aps-c sensor have the same pixel count.