Different focal lenght at cheap, low power 40W 50W 60W laser cutters or engravers

[minervalo4]

I'm considering to buy and use laser as described in title mainly for cut materials and mostly for low - power vector engraving on wood/veneers.
Somebody mention on one from Internet forums about replacing 1.5 ich focal length optics to longer one - 4 inches for better cutting materials. I can imagine how beam is focused (physics), but how it performs in practice?
Can anyone point samples of same laser work before/after such change?

[Thosikbuh]

The cutting portion of the laser beam is not cylindrical but slightly tapered. It is narrower at the focal point and wider below and above it. Therefore the cuts made with a laser machine are never perfectly straight-perpendicular as with the rotary-CNC machines.

Shorter FL Lens will give you: smaller, more precise focal point, more laser power at the focal point, the tapered angle will be more pronounced, the laser power will fade quicker if the beam is not perfectly focused.

Longer FL Lens: wider focal point (wider cut and engrave lines), less power at the focal point meaning you have to combine them with more powerful tubes, the cut-walls will be straighter (perpendicular), the laser beam width will be more uniform and it will cut well even if the material surface is slightly out of focus.

2" FL is considered standard and it is recommended for most applications.

My advice is to buy a set of cheap lenses and experiment. The price of the standard lens is between $20 and $35. Most cheap (Chinese) machines support 1.5" FL and above. For 1" lens you may need a shorter lens-nozzle. Usually the nozzle is around $10-$30.

[Timothycals]

Russ at Sarbar videos ( https://www.youtube.com/user/SarbarMultimedia/videos ) has done a basic lens video and the latest one on power density is the best I have seen