The applet is very nice. Thank you. It shows the construction rays of imaging (i.e. one ray through the focal point, one ray through the prinicpal point). However in optical engineering the stop position (and the resulting pupils) is essential for imaging. Once it its defined the chief ray and the marginal ray can be drawn. These rays are much more important than the construction rays because they define for example the perspective and the resolution that can be obtained. For expample most microscopes are telecentric, i.e. the stop is located at the focal point (image side), therefore the chief ray on the object side is parallel to the optical axis. This avoids a 'breathing' of the image when focusing. Best regards Peter Ott