Step 3 of 8: Find the Vertex for each surface

Identify the vertex of each surface by finding where it crosses the optical axis. The vertical lines are just for finding distances and identifying surfaces.

Now label the surfaces. Start at the left with the object surface "OBJ". The next surface to the right is "1", followed surprisingly by surface "2". The last surface is the image surface "IMS".

Unfortunately, OSLO's programmers did not like this numbering scheme. Instead, they observed that in any optical system, one surface will always be the aperture stop "AST". Therefore one surface must be labelled "AST". Whether or not this is a good and interesting thing is not something that you are allowed to have an opinion about. For our optical system, it makes no difference, so we will just label the first surface "AST" instead of "1".

So far, you're killing it. Am I right?