Thank you for your reply.
First of all, my lens is a zone plate lens.
It has a special structure, so I can’t do it even if I want to do it with Zemax. (That’s a lot faster ^^,,)
By the way, I have checked the position of the lens focus through 2D FDTD. So we know the exact focal length of the lens, 250 um.

According to my understanding, the simulation should be carried out twice in total, 

The first simulation is to simulate a planwave propagating through an object,

In the second simulation, I think we should take the ZBF file from the first simulation and use the import source function to enter the import source into len for simulation.

Is this right?
If this is correct, I don’t think it matters if you keep planwave and object close to each other in the first simulation setting, but how far should you keep object and DFT Monitor apart?

Also, should I set the DFT Monitor to extract ZBF to 3D structure and proceed with the simulation?

In the second simulation, how much should the import source be separated from the len?

I’d appreciate it if you could answer.