Focus distance in relation to speed

I’m trying to determine the optimal focus distance for my materials that I’m cutting or engraving.

What I’m finding is that at difference speeds, I need to set my focus distance differently.

Example is on 3mm acrylic.
I get the best focus distance at 10.5mm for me 2.5" lens at 40mm/sec when cutting
When trying to do a vector line at a higher speed of say 400mm/sec, I find I need to set my focus distance at 14mm.

I realized that we have a new feature in Lightburn to do focus tests, but it seems the speed for that is maxed out at 99.99mm/sec. Is there a way we can increase that to the max speed setting in the machine?

This is just a seat-of-the-pants answer, but if you are getting different results at higher speeds that are corrected by changing focus distance, there’s something else wrong with the machine. The physics of light won’t change enough in the small variation you suggest, which is a tiny fraction of the speed of light. (it’s not just a good idea, it’s the law)

400 mm/second is crazy fast and may be causing acceleration/deceleration flexing/distortion problems.

The speed for the focus test tool in LightBurn is just for cutting the test guide out of your material. Actually running the test you can use whatever speed you want.

But I agree with @fred_dot_u the speed the head is moving has nothing at to do with the focal length *. Focal length is determined by your focus lens and the wavelength of the laser - nothing else. (unless the laser head is moving at speeds close to C)

@Allen when I used the focus test tool to do a test to see my spot size, and tried to test for the fastest I can do some vector one line font marking.

I tried to use a setting of 30% power at speeds of 15mm/sec vs 99.99(max speed), my focus distance was different between the two.

At higher speed, I would have to move the focus further from the piece to get a better spot size.

If anybody don’t mind and try this to see if you are getting the same results that I’m seeing.

I understand what you are saying, but seriously the speed the head is moving is not a factor in the focal length. The dot size is typically measured with the head sitting still (pulse test). If the head is moving then of course the “dot” will be stretched in the direction of motion since the laser can only turn on/off so fast (about 1,000 times per second, depending on the speed of your high voltage power supply) so the faster the head is moving the more stretched the dot becomes. But this is only due to the laser switching speed, not the focal length.

If you are actually seeing a real difference in the focal length based on speed then you are having some type of mechanical issue with your motion system.

it is not really surprisingly that you need different focus heigts for cutting or engraving / vector line. But this has nothing to do with the speed.

For engraving / Vector lines the focus should be on top of the material.
For cutting the rule of thumb says in the middle of the workpiece.
In orther words, on a 3mm workpiece the focus for cutting should be 1.5mm lower than it is for engraving.


If you haven’t heard of or seen Russ Sadler’s videos on Youtube I highly recommend checking them out. Sabar Multimedia

He is currently carrying out some very extensive tests around focal length and it’s correlation to speed. Some of his findings so far are the same as @PeterN has found.

Also similarly he has found that for some materials the “rule of thumb” that @Roy has pointed out doesn’t always hold true.

I understand people saying laws of physics etc, aren’t being rude or disrespectful, as I’m not trying to be, but there is a lot of variables going on and an open mind can lead to some interesting discoveries. So I find myself agreeing with Peter that in fact the speed of the head can effect the optimal focal distance to set the material at.

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