# Thread: Laser lens focusing question

1. ## Laser lens focusing question

What is the usual practice for focusing your lens do you focus too the top or the middle of what you are cutting?

2. I dont claim to know what the industry standard is but I would say you'd shoot for the middle. Think in terms of maximizing the power density of the beam -- smaller beam is always better. The beam comes through the focusing lens and forms an hour glass shape. The most power is at the cross-section of the beam waist -- the area of highest power density. I think you want that beam waist to be at the middle of the part, because that makes the beam size at top and bottom of the part at their minimum diameter.

picture included.

owen

3. What about focal length? and depth of field?

4. I used to use a piece of 304 stainless on low power then raise the tip untill a blue color started and after that you new where the center was drop it back down .12 and that was the middle of the focus for 1/4 inch. Not sure if that helps.

• well, I can tell you what I think focal length and depth of field means.

Lets call the cross over point where the beam is at its minimum diameter the beam waist. The cross-section of the beam waist is the minimum spot size. The focal length is simply the distance between the lens and the beam waist.

Depth of field (DOF) refers to the side profile, like we see in my picture of the "hour glass" shape that is formed by the beam crossing over above and below the beam waist. Saying again the path the beam takes through the lens converges like a cone, then it diverges away from the beam waist as another inverted cone.

So, from what I know, DOF refers to the portion of the hour glass where you can place your object and it still has enough power to cut. That's one way to think of it. But another definition is the following:

"DOF is the range along which the size of the beam is no more than 1.4 times the minimum spot size." This is useful because if you define DOF this way, you can compare expected DOFs between different focal length lenses.

Note. There are two relationships concerning focal length and depth of field.

1) Longer focal length lens produces a longer depth of field.
2) Longer focal length produces a greater spot size at the beam waist.

One implication of this is that while it would be great to have a really long depth of field because you could use it to cut through a whole 2by4, the downside is that your spot size gets too big and then the beam isnt powerful to cut through anything.

I couldnt tell from your question "What about focal length? and depth of field?" exactly what you were wondering about, so I hope this helps.

There is a great file that formalizes this stuff here:

http://www.parallax-tech.com/faq.htm

owen

• Thanks for the link very informative, I'm buying a 4" FL as the 2.5fl does not of the DOF I need. I've learned a lot I wish their was more info on building a cnc laser, but I guess I truly have been the ginnie pig.

• I have actuall operated production lasers, and we focused to the top of the material you were cutting

• Originally Posted by endmill
I have actually operated production lasers, and we focused to the top of the material you were cutting

If the material is thin yes, but if you do so on thk material and a short DOF you will see large kerf angle from the beam. If you use a longer FL your DOF is longer so focusing to the top will maintain the same kerf cut.

• We were cutting thin material. actually were drilling plastic about 50mil thick also a 5000 + watt co2 laser

• The point at which the hourglass is its smallest,the most concentrated energy is, is known as the focal point. Where the focal point is placed in the material is based on the material you are cutting and more importantly, the type of cutting assist gas you are using. For carbon steel, oxygen is mostly used and the focus is placed towards the top of the material. For most other materials, Nitrogen is used, therefore, the focus is directed towards the bottom.