I would like to get some help on re-optimize an existing multi-element lens (object space field height +/-7.3mm), output collimated (image plane at infinity) to increase its EFL and reduce FOV by about 20%.

I used DENC 95% and RWCE, but the DENC 95% is not good enough for the edge field. I varied lens prescriptions and thicknesses, not sure if any advice/trick is available for doing this type of re-design/optimization. 

Is there any articles/resources available for me to learn how to improve the design?

Thanks very much.