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How to infinite focus?

A

Arsenic

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I just purchased a 150mw 405nm focusable laser. i like that it can focus, but it is annoying since i have to constantly focus it based on what i point it at. I googled around and heard about "infinite focus" but I cant seem to do this. I read somewhere that you adjust the focus on a very far away target, then everything close will burn.

When i adjust the focus for something far away, it does not do anything of interest like burn things (when it usually can, in focus.) The beam is still very big when it is on close objects.

So my question, how do you infinite focus, and what am I doing wrong? many thanks
 
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SerpantSlayer

SerpantSlayer

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you always need to change the focus depending on how far the object is to burn things. by adjusting the focus youre moving the distance of the beams focal point, so the beam diameter will be different at different distances.
 
RA_pierce

RA_pierce

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"Infinite focus" does not mean every point in the beam is focused to the smallest possible point. It means that the beam is focused so it maintains the minimum diameter at a distance.
In other words, focusing your laser "at infinity" means focusing the laser for the lowest possible divergence.

If you want a laser to burn without focusing, you need:
(a) More power
(b) A different lens
To explain (b), using a lens with a shorter focal length will make a thinner beam out of the aperture, therefore more power density and it will be easier to burn without focusing the laser on the target. However, to get a thinner beam at the aperture means you are going to sacrifice power density at longer distances.
Beam diameter and divergence are inversely related, so if you shrink one, you increase the other. There is no way around it that I know of so you'll just have to live with a compromise.

Anyway, why is it so irritating that you have to adjust focus to burn stuff? Are you really that impatient that you can't wait to turn the lens a couple times to see some smoke?

Hint: try moving the target or taking a step back or forward. Adjusting the lens is not the only way to get the focal point on the right spot.
 
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A

Arsenic

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RA_pierce said:
"Infinite focus" does not mean every point in the beam is focused to the smallest possible point. It means that the beam is focused so it maintains the minimum diameter at a distance.
In other words, focusing your laser "at infinity" means focusing the laser for the lowest possible divergence.

If you want a laser to burn without focusing, you need:
(a) More power
(b) A different lens
To explain (b), using a lens with a shorter focal length will make a thinner beam out of the aperture, therefore more power density and it will be easier to burn without focusing the laser on the target. However, to get a thinner beam at the aperture means you are going to sacrifice power density at longer distances.
Beam diameter and divergence are inversely related, so if you shrink one, you increase the other. There is no way around it that I know of so you'll just have to live with a compromise.

Anyway, why is it so irritating that you have to adjust focus to burn stuff? Are you really that impatient that you can't wait to turn the lens a couple times to see some smoke?

Hint: try moving the target or taking a step back or forward. Adjusting the lens is not the only way to get the focal point on the right spot.
Click to expand...


Thanks for your help, such a nice explanation. I was unaware that the beam diameter and divergence were inversely related, and thanks for explaining infinite focus. Well I'm just used to my 50mw green laser (which I pot modded and broke) because it always had a nice dot at whatever i pointed at, regardless of the distance.
 
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proud2deviate

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It's also worth noting that most people's eyes don't focus well on violet lasers, so the dot might look blurry or out of focus at a distance, regardless of tightly focused it really is. You might try viewing the distant dot through some sunglasses (note that sunglasses are not protective eyewear for lasers!) or bouncing the dot off a distant mirror so that it projects on a wall near you. The closer you are to the dot, the less blurry it will appear, and the easier it will be to get a tight focus at a distance.
 
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BShanahan14rulz

BShanahan14rulz

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theoretically, the energy density of the beam will be the same on all parts of the beam when focused to infinity. In other words, if a perfectly ideal laser were focused to infinity in a vacuum, you would be able to burn just as well at 0.1 meter as at 100 meters
 
justinjja

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BShanahan14rulz said:
theoretically, the energy density of the beam will be the same on all parts of the beam when focused to infinity. In other words, if a perfectly ideal laser were focused to infinity in a vacuum, you would be able to burn just as well at 0.1 meter as at 100 meters
Click to expand...

even focused to inf a laser still has a divergence, this still applys in a vacum.
I believe to have no divergence you would need a laser with an emitter that is infinitly small (not possible)

OT
Is your lens easily removable?
You could upgrade to a 405 G2 lens and it may be enough to burn when focused to inf.
 
chipdouglas

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also note, that if divergence bothers you, then you will need to get a beam expander.

michael.
 
Blord

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To have no divergence you need to have infinite diameter of the exit beam.

I believe to have no divergence you would need a laser with an emitter that is infinitly small (not possible)
Click to expand...
There is something with the airy disc. Even with a infinite small emitter you still have a divergence.
 
tsteele93

tsteele93

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RA_pierce said:
Anyway, why is it so irritating that you have to adjust focus to burn stuff? Are you really that impatient that you can't wait to turn the lens a couple times to see some smoke?

Hint: try moving the target or taking a step back or forward. Adjusting the lens is not the only way to get the focal point on the right spot.
Click to expand...

I have a survival 1.3 that burns great, and that is nice. But I would trade burning for a better beam. Like the OP, I have a 60mW green with a great dot and I would love to set my survival laser to infinite focus and have a smaller beam overall.

ts
 
chipdouglas

chipdouglas

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tsteele93 said:
I have a survival 1.3 that burns great, and that is nice. But I would trade burning for a better beam. Like the OP, I have a 60mW green with a great dot and I would love to set my survival laser to infinite focus and have a smaller beam overall.

ts
Click to expand...

That's not exactly what the op is saying. I understand the op to want to never have to focus the laser to burn. And he doesn't really know what focus to infinity is...

You seem to know what focus to infinity is. So set your survival laser to infinity and Loctite your lens in place.

Michael
 
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shintashi

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I've never heard the "infinite focus" expression until today, but I usually focus using as much range - such as a wall - as I can, so that my beam is more like a pencil lead and less like a snow cone - at nearly any range. If you can burn a match across a room, I think you've got a pretty well rounded focus.:thinking:
 
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