Why are there lenses with different focal lengths

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1 size doesn’t fit all

7620mmco2lens-300When you are shopping for laser machine parts, you’ll find that focus lenses are sold in different models/focal lengths.

The most common focal length is 2″, but some other popular focal lengths are 1.5″ and 4″. Why? Read on to find out…

Inside our machines, the lens makes sure that the (almost) coherent laserbeam is modified to make the smallest possible point. In an absolutely perfect world and with a very big wallet, we would be able to buy “perfect” lenses and be able to get a very small focal point, but… in the real world we have to settle with “acceptable” results.

focal-lengths

In the picture above you see the result of a laserbeam going through 3 different focus lenses.

On the left, the light is focussed to it’s smallest point exactly 1.5 inches below the center of the lens. 2″ lenses need a little more distance and 4″ a lot more. At first sight they all do the same thing, right? Not at all. Let’s have a look at the focal point in close up view.

dof

In theory all 3 lenses should focus to the same size of smallest point, but in reality (too difficult to explain here) the 1.5″ lens is capable of focussing to the smallest dot and the 4″ lens will focus to the largest dot. Not hugely large, but still a lot larger than the spot of the 1.5″ lens.

On the other hand, I have drawn 3 grey rectangles with the same width. As you can see, lens 1 might produce the smallest dot, but… it does not produce a very high rectangle. This means that this lens is very good at engraving small dots, but not so good for cutting though thick materials.

The 4″ lens will produce the largest dot sizes (not so good for engraving), but look at the height of that rectangle. This lens will help to cut thicker materials when using the same laser tube.

  • 1.5″ lenses are often used in engraving-only machines. They look stunning and the other lenses can’t beat them in engraving quality (but they don’t cut very well)
  • 4″ lenses can cut through thicker material, but because the smallest point is quite big it will also produce the largest kerf in the material (and very ugly engraving).
  • 2″ lenses are the nice guys here. Mister (or miss) average. Making acceptable small dots and depth of cut.

i-want-you-jpg_9025_20150411-983

Therefore, most laser cutters are by default equipped with a 2″ laser lens and the LaserBeaver cutter will not be different. 2″ for president!

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