Difference in focal distance after installing a new "same" lens

I’m fairly new to things laser, but have had some successful runs with my 100W laser. The lens that came with the machine was an 18mm 1.5" lens. However, I took the lens out for cleaning and noticed some blemishes. So I ordered another 18mm 1.5" lens (and another with a longer focal length to try later) and installed the new 1.5" lens.

I checked my alignment and ran a ramp test only to find that my “best” focal distance (distance between the laser head tip and my material) is now about 5mm away from the work surface! At this distance, it will cut and engrave with approximately the same settings it did previously, though I note somewhat more scorching on the material surface. Before – with the previous lens – my length between the laser head and material was at 17mm. If I try to engrave at the 17mm distance, the engraving comes out blurry. It seems to work at a very close distance of about 5mm.

Is this normal for the distance to be so close? What am I doing wrong here? It’s frustrating as I was in the midst of a gift project – but I don’t want to go back to using the old lens, either.

For now, I went back to my old lens. And it’s working as previous at its normal distance. Not sure what the issue is with the new lens.

That is odd! Lens focus is specified from a “primary focal plane”- a virtual point within a lens that can change with regard to the top or bottom surface depending on its construction. Note that this affects users differently if the lens mount uses the top vs bottom as the fixed mounting point.

But it’s within the lens’ thickness so it can’t be much- like a mm difference at most seems possible. 5mm can’t be explained this way.

I would have suggested “could it somehow be a 2” lens?" but that would be a 12.7mm difference, and you’re reporting 5mm. Never heard of a lens with a 1.75" focus by design, that wouldn’t likely be in inventory intentionally.

1 Like

Is it possible that you installed the lens in the opposite orientation? Convex side up vs down or vice versa?

Were the makes of the lens the same?

The makes were the same. I installed both the same. However, I think I may have had the wrong info on the original lens.

Well, I think that I may have had the wrong information on the original lens. I just installed the 2" lens and it focuses at exactly the same length as my original lens – which I was told was a 1.5" lens when I got the laser. Still, it’s strange that the new lens that I installed seems to focus so close. I’ve tried installing it both ways – convex side up and down. I’ve contacted OMtech for support as it was one of their lenses.

The bottom line is, Set the height where it works the best. Certainly the lens grinder should hit it closer, but a little different focal length is not a big deal, More important is to set the height where it cuts the way you want it, then measure it and make a guide so you can reproduce it.

Being that you have approximately 12mm+ difference in focal length from your ‘new’ 1.5” lens, that would indicate your original lens is a 2” lens. At least you now have a replacement, and a new 1.5” lens, too.

Yeah, that’s what I realized. What I read about the laser said it came with a 1.5, so that’s what I thought I was using. Thankfully, I ordered a replacement, as I (so far) prefer it over the 1.5. I’ll try the 1.5 again, though after the holidays and give it a chance. What I want to do is try some longer focal lengths for cutting thicker materials. Guess I’ll need to order some more lenses.

There are varying qualities of lens manufactured. A $10 lens and $100 lens while both claim to be the same focal length could vary in both actual focal length and in the quality of cuts. Another potential difference can be metric vs imperial. 36mm is close to 1.5" but not exact and 1.5" will never be exactly 36mm. The color or the lens material is also a clue to the quality of the lens. Many lower cost Chinese lens are amber in color where higher quality lens will be light yellow.

Although there is some quality difference in almost everything that comes from China, I have not heard yet that the actual focal length of lenses has been mixed up. But of course, there must always be the first time. Regarding the color of the lenses, it is the surface treatment, the material of the coating itself that determines it.

It’s not surface treatment. The cheaper low quality non-CVD lenses appear as a milky orange and a bit less efficient- due to the heat they generate, they are less suitable for higher power beams. They’re uncommon to even find now since CVD lenses are cheap and plentiful. CVD is a very clear light yellow. that @WoodpeckersHomeDecor describes

The high quality mfg is II-VI, and have a large share of the market, including China. II-VI does not make any low-quality lens. High quality II-VI are still quite affordable, there’s no reason to get other types.

The surface treatment is an antireflective coating that gives it a “sheen”. Reflection is a significant loss as the beam enters the znse material and this prevents it.

The other difference is plano-convex (totally flat on bottom, convex on top. top is the beam input side) vs concave-convex aka “meniscus” lens. Meniscus lens is superior, it yields a smaller minimum spot size at the best focal point. Universal Laser Systems machines use glue-in holders originally designed for plano-convex that don’t quite fit meniscus lenses smoothly, but still worth fiddling with the fit on a meniscus lens.

Again, the poorer-performing plano-convex are actually hard to find nowadays since meniscus lenses became cheaper to mfg. Plano-convex do show up in long-focus lenses like >=5" focus since it needs so little curve anyways.

I have not seen any “metric vs imperial” prob. The industry readily adapted by selling “19.05mm dia lens” for at least a decade. That unusual number is of course 3/4", they just standardized that as the metric name. In rare cases the ad may call that “19mm” but AFAIK the product still calipers at 19.05mm. A 20mm lens is 20mm.

A difference CAN exist in where the primary focal plane (PFP) is. That’s a technical term, a virtual plane inside the lens the focus is measured from. Lenses do sometimes vary in thickness, usually only by small amounts. Some mounts seat the lens on top, some on bottom, so the PFP moves a different direction if a thicker lens in used. Even though it is still a 50.8mm focal length from the PFP, this will shift the focal point, but it will be less than a mm.

1 Like

I find them much better than a ‘flat’ bottom lens. I have a 50 watt and it came with a 1.5" lens. I have a 2" that I use a lot. It’s also ‘further’ away from materials that cause problems, like the back of mirrors. They tend to vaporize and end up on the nozzle or on the lens if not enough pressure with the air assist.

Good luck


Hi Cheryl, I often use a 1.5inch lens and the distance between the ‘engraving’ nozzle and the material is ‘5mm’ which is great for engraving.
Just wondering if you had changed just the lens or swapped out the nozzles with the different lens?
i have found the 2-2.5inch lenses are great when doing engraves and cut-throughs.

Meniscus is indeed better. A II-VI CVD 20mm meniscus lens is $30 on Amazon, so I don’t see a reason to be trying anything else IMHO. I believe Cloudray’s “USA CVD meniscus” is also a II-VI but they’re not authorized to use the name. Looks like the same price anyways once you figure in Cloudray’s shipping.

I’m sure you already know this but for others that might not, when initially buying a lens with a significantly different focal length than the one(s) you have, look for a lens & lens tube combination or kit. In the case of a 4" lens, the included lens tube will be 1½" longer than a 2½" lens tube which allows you to maintain you’re usual distance between the nozzle and material while being in focus, which prevents the problem @jkwilborn mentions above.