Why do 2 lenses of the same diameter and focal length behave VERY differently

Question about replacement lenses … The Laguna owners manual for the PL12|20 CO2 40Watt Laser is inconsistent about specifying the type of lens needed. In 2 places the manual says the lens is 20mm diameter 50.8mm focal length. However, in the specifications chart, it says 20mm diameter 25.4mm focal length. The 25.4mm focal length is 1 inch. The lens is up in the adjustable head tube much more than an inch. So the 25.4mm cannot be right. Right?
I got a replacement lens that’s 20mm diameter 50.8mm focal length. It behaves totally different than the original lens. At the same focal spacing (7mm) and power & speed settings as the original lens, it produces a wide burn mark and a wider beveled cut. It obviously has a different focal length than the origin lens. If both lenses are 20mm diameter 50.8mm focal length, why would this be? What am I missing?

Two things come to mind. Either your original lens was a 1.5" / 38.1mm focal length, or you flipped the lens over.

The focal length changes slightly depending on if you set your lens curve up or curve down, and the shorter the focal length, the more that matters. I run strickly curve up for my cutting, gets me as close to the nozzle for air assist as I can short of adding some spacers to the nozzle thread.

Also, that 1" may be to the nozzle tip, not the actual focal point. We all know how excellent the Chinese to English translations are… Do a ramp test to see what your actual focal point is using 1.5" and 2" as your starting point.

I have the domed side up on both the original and replacement lenses. The original lens performs flawlessly. Maybe the owners manual is wrong all together and the original lens is 20mm diameter with 38.1mm focal length?

I’ve also noticed that all the 20mm diameter 50.8mm focal length lenses I’ve found seem to be meniscus (i.e. concave on one side and convex on the other side). The 20mm diameter with 38.1mm focal length lens I’ve found is convex. Amazon says it has free returns, do you think this would be worth a shot?

If you have the tube length, or can add an extension to get the tube length, I’d keep the 2". The longer the focal length, the better you cut thicker material.

EXAMPLE: With my original set up and a 1.5" lens, I was cutting 3mm Premium MDF easily, and barely cutting 6mm. Switch to a 2" and the 6mm is cutting okay, but the 9mm is a bit iffy. If I get the new mount in place for the 4" lens, the 9mm will be no problem, and I may take a shot at 12mm.

Nice to know. Thanks for your insights.

Quite often the factory set up will handle both a 1.5" and a 2" lens. Usually the 1.5" will sit in a notch in the top of the nozzle, and the 2" will be held up in the lens tube by a retaining ring.

The zinger comes in when you determine if you have an engraving set up or a cutting set up. If you have an engraving set up and a 2", you can get a bit better performance by dropping a 1.5" lens in to get closer to your work and drilling out the air elbow fitting. It won’t be a true cutting set up, but it’s better than an basic engraving set up.

Trying to cut with an engraving set up puts you at a rough 40% disadvantage.

My lens is up in the tube with a retaining ring

That’s about as long as you can go with your stock set up. Is that with a 1.5" or a 2"?

Better question, how much distance do you have lens to nozzle tip?

2-1/8 Inches from lens to tip of nozzle. I guessing a 1 inch or 1-1/2 inch focal length lens would not work?

If you put it all the way up, you are going to need a 2.5" lens. On my first set up, the lens sat on top of the nozzle with the retaining ring and an o-ring in the tube above it to hold it all together. If you can do that, then I would imagine you are in the 2" or maybe 1.5" territory depending.

If you have the room, you can space the lens down with o-rings and such.

I use and keep a 4" lens in my machine for everything. The engraving is excellent. The cutting is amazing. Best tip I can give you is to stick with Two - Six lenses (II-VI).

Thanks … I’ve just ordered a 20mm Dia. 63.5mm Focal Length lens (2-1/2"). I’ll see if that does the trick. The owners manual is probably all wrong about the lens … if the distance from the lens to the tip of the nozzle is 2-1/8" a 2" or 1" lens would focus before it made it out of the tube.

Next thing, have you determined of you have an engraving set up or a cutting set up?

What Jeff said. II-IV are great. Personally I use Ronar-Smith lenses from Singapore, but that’s because they have been my geo for more than ten years.

I use a 3" for everything, but have spare tubes with 2", and 4" lenses, should I need them.

99% of Chinese lasers come with a 2" lens.

Thanks Dave … The 20mm Dia. 63.5mm Focal Length lens (2-1/2") works great and matches the performance of my original lens. I’m cutting and engraving with no issues. The owners’ manual is totally wrong about the lens specs.

Glad to hear you have that part figured out. Have you determined of you have an engraving or cutting set up? I have pics of the nozzle and air elbow differences.

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