The things are even more fuzzy - the lens systems we're talking about are being sold as "collimating lens" with different front and back focal length. The focus in a spot at given distance is accomplished by changing the distance between the L.Diode and the lens system....I'll leave to Cyparagon to explain his thoughts regarding a whole system :shhh:
If shorter focal length makes smaller spot...
On Ebay, you can get 3-element coated lens for 405..465nm LASER at prices starting from $3.50 (to around $15). There are also uncoated acrylic lens, but I'm not sure how they perform at P>0.5W.
Which lens would make smaller spot at 30cm (300mm=30cm). It might be the most expensive, but it might be the cheapest - no guarantee here. Talking about smallest spot, the best known are the 3-element glass lens. But the brands and quality of the lens are highly mysterious.
Also, I've found 3 kinds of lens with specified something called "F" (focus distance?):
F=8.3, F=9.8 and F=11, they are being sold at ~ $7.8 on ebay.
I guess F=11 stands for F=11 mm.
As for the Sanwu's lens, they re here: Sanwu shop - lens.
The Gxxx ones are single-element and more expensive, $15 at the time of writing vs $8 for his 3-element lens. Their pros are they reportedly allow for up to 10-20% more energy to pass. Their cons - higher price and slightly inferior focus than 3-element.
I asked staff from techhood about the difference between their $13 UV lens and another, unknown $3USD glass coated 3-element UV lens, sold on taobao. What they know is that they are from different factory in China and different coating. So the information is next to nothing :can:.
I have bought the $3.5 ones from another ebay supplier (eama/nanma) but they are in transit for the last 20 days...
I can compare the spot only with acrylic uncoated (when they arrive), but I don't know how to shoot it - on white paper it has lot of glare, probably on black paper or best - direct expose the 1/2.4 inch CMOS matrix (of web camera).
There is some additional stuff like beam expanders, which may help you to maintain low divergence at distances at the cost of expanded beam, but then you'll need additional focusing lens near the subject to focus the low divergence expanded beam in a tiny all-vaporizing spot :yh: