Lenses and their benefits?

[Mike Audleman]

Ok, I have seen reference to different focal length lenses, for example someone commented they have a "1-1/2in" lens. I am sure that refers to the focal length.

So, what is the advantage of having a different focal length from another? On the surface (no pun intended) it would seem that a longer focus length would be advantageous as it would keep the cone well out of the smoke zone and less likely to strike holddowns, weights or other protrusions from the cut item. Would having a longer focus length also be an advantage of having a longer more tapered cutting beam? Shorter wouldn't it have a fatter cone and thus affect straightness of edges in thicker material?

[Scott Shepherd]

Smaller focal length = smaller spot size. If that's important to you, it's something to consider when buying them.

[Mike Audleman]

I wasn't really shopping for a lens. I just ran across several comments about focal length. And began wondering if I should be looking for additional lenses to increase flexibility or accuracy. Basically I was wondering what it was all about.

[Scott Shepherd]

It's just the spot size. It all depends on what you are doing. If you are doing tiny scale models and you need a smaller spot size, then the 1.5" might be good for you. If you are doing 1" tall letters, then a 4" might be good for you. It's all based on what you are trying to accomplish. Different spot sizes do better/worse on different things, that's all. If you're happy with what you have, then you probably don't need another one with a different focal length :)

[Gary Hair]

One other benefit, besides spot size, is focal range. A longer lens has a longer focal range. That may come in handy when you are lasering something that's not very flat. You pay for that extra range with a larger spot and lower power density, but adjustments can be made to overcome that problem. I use a 4" lens on sandblast stencil if the substrate isn't flat, that eliminates partial cuts that I would get with the 2" lens.

[Ross Moshinsky]

Basically the smaller the lens, the smaller the spot size but also the "shorter" the sweet spot.

I'll post a diagram later since my simple text one didn't seem to work.

[Mayo Pardo]

I thought a longer focal length lens allows you to cut thicker material?
I've been told that the narrowest part of the hour glass shape of the beam is larger in the larger focal length lenses. This results in not only a larger spot size or beam diameter but also allows the beam to be in focus over a greater depth. This is my limited understanding of it, being a newbie to lasers in general.

So this poses additional questions:
When set at the same power and speed and ppi settings on the same material, would a 2 inch lens cut faster and deeper than a 1 1/2 inch lens?
Is the beam size the same from one lens manufacturer to another, within the same size lenses?
I've seen reports of cheap imported lenses performing equal to original manufacturers lenses.
If this is true, it makes it difficult to want to spend $400 + versus less than $50.00

Should a lens last forever if it is cleaned routinely and properly?

[Gary Hair]

I thought a longer focal length lens allows you to cut thicker material?
Maybe. Even though the focus area is longer, the beam diameter is larger which means you have lower power density. If you can increase power or decrease speed then it might make up for the difference.


So this poses additional questions:
When set at the same power and speed and ppi settings on the same material, would a 2 inch lens cut faster and deeper than a 1 1/2 inch lens?
No, the exact opposite would be true. The longer lens would take more time because you would have to slow down the speed - unless you aren't cutting at 100% power anyway.

Is the beam size the same from one lens manufacturer to another, within the same size lenses?
In theory, it should be the same.

I've seen reports of cheap imported lenses performing equal to original manufacturers lenses.
If this is true, it makes it difficult to want to spend $400 + versus less than $50.00
I agree completely!

Should a lens last forever if it is cleaned routinely and properly?
I am using 8+ year old lenses, the 2" gets used several hours a day, 5 days a week. It's not in perfect condition but pretty darn close! So, yes, a lens should last a long time (don't know about forever...), if cared for properly.

[Richard Rumancik]

Mike, you might want to search for some older posts on this subject. Here is a link that explains probably more than you want to know. There is some math but it is explained in a very informal way and I think it is helpful to understand the concepts of focal length and depth of field, and the implications on cutting/marking.

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

[Mike Audleman]

Being an avid photographer, I am acutely aware of what depth of field and focal length is. Its where it comes into play here in laser land that I am unsure of. Thank you for the link, I will parse through it and see what I can glean from the information there.