Hi Marzanna,

In a Spot Diagram, the IMA label indicates the position of the grid with respect to a reference defined in the settings. If you click on Settings in the top-left corner of the Spot Diagram, you should see something like so:

In that settings window, you have a drop-down menu labelled Refer To and this is what IMA will use to display the different spots. By default, it should be Chief Ray. Meaning the position of the spot is shown relative to the chief ray landing coordinate in the Image Surface. You can also use Centroid to refer the spot diagram to the centroid (centre of mass) of your spot.

Perhaps, another way to look at this is to use a Refer To: Vertex, in that case, the reference for the spot grid is the surface vertex and is likely to be the same for all spots. Have a look at this system:

This is what the Spot Diagram looks like with a Vertex reference:

As you can see, because both spots land on the same surface, the Vertex is the same, and IMA stays at 0.000 mm. One spot (on the left) is in the centre, and the other (on the right) is at the very edge of the grid (towards the top). If I change to Refer To: Chief Ray, this is what the Spot Diagram looks like:

On the left, the spot stays in the centre and the chief ray is co-linear with the optical axis so the grid stays at 0.000 mm (IMA). On the right, the off-axis chief ray is at the centre of the grid, and IMA just tells you what height this corresponds to: 0.989 mm. You can further very this in the Merit Function, if you use the operand REAY (local real ray y-coordinate in lens units at the specified surface) on the chief ray of the off-axis field, this is what you’d get:

As you can see, the value is exactly 0.989 mm. Does that make sense?

The cursor coordinates are something different, it shows you the cursor coordinates in the grid in micrometers!

I hope this helps, and take care,

David