​@mlisowski 

You could calculate the EFFL in the Merit Function Editor (and Target it to a specific value).

The case of a plano-convex lens is relatively simple. You can also find the general calculations there (I am not affiliated with Edmund Optics).

Getting the refractive index in Non-Sequential is somewhat more difficult than Sequential, and its something we’ve discussed here:

The bottom line is that you should use a ZPLM operand with the Numeric Function NPRO (code 200 for the refractive index).

Here is an example:

I hope this helps and take care,

David

Hey David,

Thanks for the tip about NPRO; this is better than NSRA with data=14 in many cases since you don’t need to know the segment number.  I learned something new today !

Another approach one could take regarding calculating the EFFL is the approach that sequential mode takes:

• Define an on-axis marginal ray from infinity in object space (the beam can be small so it’s still a “paraxial” calculation)
• Determine the marginal ray angle in image space 
• Back propagate the image space ray to intersect with the object space parallel ray
• The new right triangle is the EFFL

To make this work, you need to add a temporary Source Ray:

• Make the Y Position a small but finite number (I typically use 0.01 or 0.1 so the math is easy to verifiy)
• Make the # Analysis Rays 1

Then, the EFFL is simply:

(this is for the Double Gauss 5 degree field sample file which has an EFFL 3.99924 of  but works with any system that has a “well-behaved” optical axis...the results will start to diverge from sequential mode if you have a folded system since sequential mode ignores coordinate breaks when performing paraxial calculations but this method considers all tilts/decenters in the system)