CNC machining is complicated. Every tool path you program has hundreds of options and you can never really know what is going on.
I have milled 12 lenses, and have gotten some nice results.
3-axis milling is done with a ball-end mill from a thick acrylic sheet. (1/2" in image)
For your router bit, bigger is better. The bigger the radius of the tip, the less severe your scalloping will be. I usually go with a 1/2" ball end mill.
I set the step-over to 0.02", meaning that a 12" lens takes 300 concentric passes to cut! This can take a long time. I'm not going to get super-specific in this instructable, but I'll list some specifics.
Speeds and feeds are from this document, I take off about .01"in /tooth at 12000 rpm.
I do my CAM in Fusion 360, which is the friendliest CAM software I have ever seen. Parallel finishing, morphed spiral, and spiral are great tool strategies.
Here is a link to my latest Fusion file with a great toolpath applied. It is for the Big Eyes lens, which makes your eyes large like a cartoon character.