Focus lens and mirrors

So I am new to all of this and have tried to locate advantages of focus lenses based on their size. When I say size I mean diameter, examples 18, 20, 25 and so on. I currently have 18’s on a 100 co2 laser. What are the advantages or disadvantages of larger size focus lenses. I understand the focal lengths just not what makes the sizes better or worse. I was told based on the wattage I should have 20’s but they did not give me a reason or explanation.

Hi @firemine,

Tell us more about your machine.

Here is an excellent write up on understanding different size lenses on a laser engraving machine.

Have you seen this video fromRuss from Sarbar Multimedia?

It explain the importance of a small burn dot in engraving.

This is the lens set he uses

I’m assuming you are referencing engraving and not cutting.



Thanks for the reply. I understand the focal length of the lens I just do not understand if there is an advantage to the size of the lens and mirrors. Is there a point to upgrading to a 25mm lens over the current 18mm. I have been researching the stacked lenses and also looked at this one as well.

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I may be off base here, but as long as the whole of the beam is hitting each mirror and passing through each lens is shouldn’t matter. Higher wattage tubes have a large beam diameter leaving the tube and thus might need a larger diameter lens to give you “wiggle” room for making sure the beam isn’t getting clipped on the edges due to poor alignment.

There is a great difference in quality between the cloudray compound head and the Beam Buddy high resolution laser head. First of all lens quality. The Beam Buddy head uses custom manufactured high quality USA ZnSe CVD AR coated lenses that are rated for 150W from ii-vi infrared as opposed to the cheaper China ZnSe lenses that cloudray ships. As well, the cloudray head is stacked one lens right on top of the other which causes a great deal of heat buildup that can lead to premature lens failure. The Beam buddy head has the lenses spaced a good distance apart. The Beam Buddy also uses a variable air assist valve so that you can dial in the precise amount of air that you require. Our Beam Buddy head is made from aircraft grade aluminum and has different models for different lasers, it is not a one size sort of fits all like cloudray, but a custom fit for your machine. (I am one of the principals in Beam Buddy, and if you have any questions, please feel free to ask me)

I haven’t had the opportunity to try the beam buddy yet but I can tell you that the cloudray compound setup comes with both lenses from ii-vi.

Not to knock your product, I haven’t tried it, but I’m a pilot and also a hobby machinist. I always cringe a little bit when I see someone say a product is made from “aircraft grade” aluminum or billet aluminum. There are many different alloys of aluminum used in aircraft and I’m not aware of any specific alloy that’s only used in airplane construction. I would think that for the purposes of a laser focal tube most alloys would be suitable. I much prefer a company say it’s made from 6061 or 7075 or whatever the alloy is. When I see that, I know what I’m getting. When I see “aircraft grade” aluminum I immediately think a marketing person wrote it.

Again, I’m sure you’ve got a great product and I’m looking forward to trying it out one day. That’s just my 2 cents.

Oh, and billet aluminum? That just means a block of aluminum.

Lens diameter is mostly related to tube size and power. A higher wattage tube will have a wider unfocused beam, so it will need a larger / lens to collect and focus it. Larger area also allows for a little more heat dissipation.

When someone says “aircraft grade aluminum” I always get an image of Aloha flight 243 in my head.


This post reminds me of a book I read back in 2004
It’s was quite good.


Thanks, So would it be good for me to go from a 18mm to a 20 or 25 mm lens or is it just a waste of time and money.

Thanks Travis, my beam appears to be pretty well focus in the middle of the mirrors without loss so I am probably good with 18mm focal lens but wanted to make sure there was no advantage in jumping to a larger lens.

Have you done the 1 mm dot test that Russ uses to dial down his laser for sharp engraving ?


Not yet, I watched the video you provided and need to determine how he created the file for the test. I am assuming it is several pixels with a line and a dashed line but have not had a chance to create it yet.

Here is the file
It’s that small right click and save as



@firemine you are also going to need one of these


It will help out has a fixed scale that will help you identify how far apart the dots are in mm.

Take this as a personal testimonial I own one and absolutely love it. Look at all the positive reviews

All the best


If everything seems to be working as it should, you’re hitting the middle of the mirrors, hitting the middle of the lens, etc. then I can’t see why you’d want to change to larger diameter mirrors or lenses. I’m strictly speaking about lens diameter here, not focal length. I don’t think you’d see any appreciable change solely from going to a larger diameter lens. Curious what others think about this as well.

Focal length and compound lenses are a whole different discussion.

Thanks for the file. Still working through this process.

Thanks for the help.

I don’t suppose you have the file for the acrylic focal length gage do you?

By the way Billet aluminum is good compared to cast aluminum ! Billet is worked (hot or cold) to harden and basic forming the piece ,then machined. This will find flaws in the metal before it is machined. Cast, which is what a lot of things are , is cast then machined. Cast aluminum is not as good as billet .

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Don’t disagree. But it all depends on what you’re using it for.