View Full Version : I donít understand what the lenses do

Frank barry
01-20-2013, 7:27 AM
I am new to lasers and am trying to learn and find the forum full of imfo
I am in the process of buying a laser engraver from China and am down to 3 manufacturers as I see it, it will come down to after sale service

I donít understand what the lenses do am I right in thinking they can change the thickness of the beam ??

What lenses should I get with the engraver

Thanks in advance for your help

matthew knott
01-20-2013, 12:58 PM
the lens focuses the laser beam down to a spot, the shorter the lens the smaller the spot, (generally a good thing) but also the shorter the lens the shorter the depth of field you have, i think 2 inch is a common best fit for all lens, but as they are pretty cheap why not get a selection and see what suits you.
Short lens = fine engraving Medium = cutting and engraving Long = Cutting thick or uneven material. They swap out pretty easy and you just need to adjust focus, which you would need to do for each job anyway.

John Frazee
01-20-2013, 1:22 PM
I already understood the way the lenses worked but if I didn't, I would have learned a quick lesson this week. I was working on the alignment of the beam. I accidently put my hand in front of the beam with it at 100% power. I was lucky I put it in before the beam went past the lense. I got a big bad blister that hurt for hours. If the beam would have got me after it went through the lense, it would have left a serious deep cut. I would have been on my way to the hospital. My blister is about 1/2 inch across while if the beam would have passed through the lens it would had been a small focused entrance into my hand. I would think it would be the same as a magnifying glass. Hold it out of focus using the sun, it doesn't burn. Focus it to a small spot, the smallest spot you can make and it's lethal.

AL Ursich
01-20-2013, 1:25 PM
Picture 2 pyramids stacked point to point, one up side down. Tip to Tip they are in Focus, converting say a 1/4 inch bean of light down to a pin point of burning power. So picture drawing a line at the intersection and call that your name tag looking at the table level. Now move that line up or down and it goes out of focus as the beam gets thicker at the tag surface. Wider Beam, More Surface Area, Lest HEAT being produced. Sometimes Engravers focus slightly out of focus for cutting so the point of the bean is deeper into the item being cut.

I believe Standard Lens is a 2 inch focus but they also sell 4 inch that let you laser things like a coffee cut where the handle moves toward lens but not hit it in a Rotary.

To practice this in Theory get a Magnifying Glass and Use the Sun to focus and burn WOOD.

Good Luck,


Laser Bits dot Com has some Laser How It Works stuff to read.

Michael Hunter
01-20-2013, 1:26 PM
Picture to go with Matthew's explanation.

Glen Monaghan
01-20-2013, 2:15 PM
The problem with that picture is it shows each of the beams focusing to an actual non-dimensional point with zero depth of field. The danger is that it's too easy to mistakenly conclude that the longer focal length lens is finer or "sharper". Perhaps this is because the "V" at left is more narrow (akin to the way a knife is sharper if its beveled edge is narrower, like a scalpel or filet knife, and less sharp if the edge is broader like a butcher knife or axe). Drawings like these are too simplistic/inaccurate because the beam never focuses to a true point, only a circle with non-zero diameter which is larger for longer focal length lenses and smaller for shorter focal length lenses.

To see the proper relationship of beam width, depth of field, and focal length, you have to plot at least two rays from each edge of the lens, not just one. Then you get a "diamond" shape centered at the focal point, with the height of the diamond showing the depth of field (by definition, the distance around the focal point where the beam width is within 40% of its minimum) and the width of the diamond showing the minimum beam width, or spot size at focus.

Frank barry
01-20-2013, 2:44 PM
thank you all that is very interesting i do know a little more now i guess when i get my engraver it will help me to understand

thanks again guess ill be back with more ??