We have done allot of photo engravings with the J-Tech lens in there 2.8W laser kit, but the focused beam is not exactly round. We had it set at the 3" focal height. We always had to engrave only in one direction using the narrowest side of the focused beam. It may not make any difference with Vector burning. J-Tech also recommends a shorter focal distance then what we use, but we never tried that to compare any. They promote Vector cutting more so with there laser kits then photo engraving, so that may be why they recommend such a short focal height.
We tried a different single element lens recently and it appears to focus a little smaller then the J-Tech lens with the 6W Laser Diode we are 3D engraving with. This is at a 2" focal height because we are using an air assist we developed. The focal point size is not as critical as photo engraving, but we did notice a small increase in the feedrate we were able to set PEP5 at to get the same depth of cut in Poplar. This is because we were able to set a smaller size Pixel Resolution. http://www.ebay.com/itm/261445531997?_t ... EBIDX%3AIT