You have a tiny spot for photo engraving. the ‘sweet spot’ for cutting is a matter of ‘depth’. I’ll give you my laymen explanation, and you can cuss me out later.
You have the perfect ‘spot’, but if that was the only place you could cut, the only thing you could cut with a laser would be tissue paper. While the ‘spot’ is the hottest, you have X amount of distance above and below the ‘spot’ where the laser still has the energy to cut. That energy along with power ( Wattage ) and good air assist combine to give you a usable cutting tool.
So, now on to this X distance. Say you have an 8mm laser beam hitting the lens. That lens takes that beam and focuses it down to that ‘spot’. If you have an HD lens, the angle on that bend is pretty sharp, and that X distance is barely above of below the ‘spot’. On the flip side, a 4" lend has 4" to make that same ‘spot’. With a much shallower bend to the beam, that X distance is now quite a bit above and below that ‘spot’ giving you a much greater usable cutting depth.
EXAMPLE: I can cut 6mm Premium MDF with my 1.5" lens and only get about 90% of the way through 9mm. Drop a 2.5" lens set up in and now I can cut that 9mm, and get about 80% of the way though 12mm. I’m waiting on the holder for my 4" lens to see if i can cut the 12mm… All this with the same machine power, and speed… Only the lens has changed.