A bit of theory

to work on the focus it is important to understand a few things about the final focusing lens. Nothing here will require any fancy physics background to understand, it's all basic geometry.

First thing to understand is that in the real system the laser doesn't focus to a single point, but forms more of a very tight hourglass-like waist, like in top pictures to the right. With all else equal, a lens with a shorter focal distance will have a tighter focus (high power density) with a short length (can't cut too deep), a longer focal distance lens will be the opposite- larger focal point and longer length. Lens choice is a trade-off between the two features, and the 2" lens (usually the standard one you get) is a good general purpose lens. Other things can affect the focus size, for example making the beam broader before hitting the lens will make the focal point tighter, but usually none of these things are within our control.

The second thing to understand is how the beam position and alignment affects the focal point. The middle picture to the right shows 3 different beams, all going in the correct direction but entering the lens at different positions. Now since all 3 beams are going in the right direction they hit the same focal point. But because they are entering the lens at different positions they hit the focal point at different angles. If you were only doing engraving on the surface of your material you probably wouldn't notice, but when cutting through a material you will start to limit how deep you can cut when your beam is coming in at an angle. This is why when aligning the final mirror centering the beam becomes important.

The bottom picture shows what will happen with a very badly aligned mirror arrangement. If the beam is entering the lens at different angles, then the focal point will be in different locations. If you find that your cutting power seems to change as you move to different parts of the laser table, then this is most likely what is happening and your mirror alignment is off.

If your laser mirror alignment needs adjusting see my guide here.

Check laser focus height

So your laser cutter probably came with a little piece of acrylic cut for you to set the laser focus. It will be close, but checking it for yourself is a good idea.

I have the default 50mm (2 inch) lens installed. It is set up about 25mm from the bottom of laser head air nozzle, so the recommended height setting is another 25mm from the nozzle to material. What I am going to do is make a series of focus tests at different heights around this point to see what the optimum focal point really is. Using some wood as my target, I program the laser cutter to draw a line (200mm/s, 50% power). This will be the mark I make to consistently test each focus height. Then using gauge blocks, I set the bottom of the air nozzle to my first test height of 30mm, remove block, then engrave the line.

Next I shift the laser position down the wood a little, get the next set of gauge blocks (25mm), set the new depth and mark another line, repeating this process for the whole range of heights I want to test. At the end the experiment I have a series of engraved lines which can be analysed to investigate the focus profile.

So from looking at the wood, the sharpest lines are 22 and 21mm, so the center of focus is at a depth of about 21.5mm, so rounding off I have cut a new piece of plastic to 22mm which I will use to set my laser focus in the future.

Another interesting observation from the measurements is that the line looks mostly the same between a height of about 24mm to 19mm, and the line starts to broaden out either side of those values. This gives me an indication that about 5-6mm is the depth I can make a good clean cut through any material. 

Compare different lens focal lengths

I have acquired a 76mm (3 inch) and 38mm (1 1/2 inch) lens which I want to test as well. The only lenses I could find were 20mm diameter, and my laser head holds 18mm. So I had to machine up a new adapter, which is why the nozzle attachment might look weird in the following pictures.

Following the same procedure as before, I scanned a range of heights to build a focus profile. From the 3" focal length lens it looks like the focus width is at it's narrowest from about 50mm to 38mm. Notice though, the focal point is a lot thicker than for the 50mm lens. So while the depth is longer (allowing to cut thicker material), there will be a much weaker power intensity (so can only cut through softer materials).

The 38mm lens has a much tighter focus, going from about 17mm to 15mm, with 16mm looking like the optimum focal length. As expected this lens has a much shorter, but tighter focus.

Comparison of the 3 focal lengths

So below is the full set of results for comparison. The 50mm lens is the best compromise with a good tight focus and a reasonable potential cutting depth of around 6mm.

For thicker material up to about 10mm there are times when the 76mm focal length lens will be useful. Due to the broader, and therefore weaker power intensity, it may be limited to materials that cut fairly easy (such as foams, acrylic or softer woods), perhaps with multiple passes required. This lens should see a fair bit of use for some special applications.

The 38mm lens has very short depth so will be of little use for material cutting. The focal point only seems a little tighter than the 50mm lens, so I don't think for things like engraving wood or plastic there will be much noticeable improvement. Although when I do really need to engrave with really sharp lines (like anodized aluminium) and combined with high intensity (such a using CerMark, for engraving metals) I might give this lens a shot. It's less useful than the 76mm lens, but I still might find used for it. 

Comparison of laser cutter focus depth for 3 different lenses