laser cutter lens/mirror decisions

heya, quick question, what difference does lens diameter make in the laser cutter? there are 25mm and 20mm for the mounts. I was thinking of getting a copper gold coated lense for durability and decent reflection. and for the lens, I'm not sure just yet. also, 2.5 focal length seems perfect for 3-5mil plexiglass cutting as well as decent engraving.

-Dave

... with a bigger lens you can get a better focus and cutting depth, if you scale up the optical setup (distances, beam diameter) accordingly ...

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Viktor
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hmm well then, I guess 25mm mirrors is what I should go for then right? I do need it mainly for cutting although engraving would be nice later on with the same machine. what size lens would you go for? any recommendations? thanks

... if it's a CO2-laser, then a single lens won't change much - you'll need a lens-pair or telescope to expand the beam before focussing.

With a diode-laser with expanding beam this is possible with only one lens ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

You use a lens diameter that fits your laser head. Unscientific research on aliexpress gives a 3:1 ratio of number off offerings in the favor of 20mm lens 934 vs 287.

gotcha, I was gonna buy some Cu but after reading for a bit I noticed that most machines use Si. plus theyre significantly cheaper. any thoughts? 20mm will be the ones I'm goin for along with some znse lens.
should I buy a 50w tube instead of the 40w I was going for?. ive been reading that companies inflate the price just because the tube can hit 50w despite it being damaging to the tube in the long run and that the tube power peaks at about 40w even though the 50w advertising on the listing.

@VDX yea good point. guess it wont matter much anyways. thanks man!