How to get the focal point close to the lens ?

[Dawinzi]

After almost two hours research i'm still not brighter then befor.
I'm sure there is already written something about that, so please don't freak out. Just guide me nicely to the helping Thread. Thank you.

 

[Mohrenberg]

....unscrew the lens as far as it goes without falling out. I'm assuming you want the apex close to the lens for burning?
or perhaps i misunderstood the question.

 

[Dawinzi]

You got it right Mo. I unscrewed the Lens but i need the Focus closer to the Lens. About 10-15 mm would be perfect.

 

[Ashortdude]

You got it right Mo. I unscrewed the Lens but i need the Focus closer to the Lens. About 10-15 mm would be perfect.

do you have any another lenses?

 

[HIMNL9]

Focusing distance is related to magnification (and FL) of lenses ..... for obtain a focused dot very near the lens, you need a lens with high magnification and short FL, like single-element lenses ..... as alternative, you need to use a second positive lens, for increase the magnification of the lens assembly and shorter the FL.

I'm not at home at the momentand cannot post pics, sorry.

 

[Ashortdude]

Focusing distance is related to magnification (and FL) of lenses ..... for obtain a focused dot very near the lens, you need a lens with high magnification and short FL, like single-element lenses ..... as alternative, you need to use a second positive lens, for increase the magnification of the lens assembly and shorter the FL.

I'm not at home at the momentand cannot post pics, sorry.

reading my mind ey:spank:

 

[Dawinzi]

Focusing distance is related to magnification (and FL) of lenses ..... for obtain a focused dot very near the lens, you need a lens with high magnification and short FL, like single-element lenses ..... as alternative, you need to use a second positive lens, for increase the magnification of the lens assembly and shorter the FL.

I'm not at home at the momentand cannot post pics, sorry.

Do you mean something like that?

Blue Laser Collimating Aspheric Lenses - Edmund Optics

 

[HIMNL9]

^ high prices, but basically yes ..... now i'm at home and can post a pic, maybe it's more clear this way, than with my bad English

Basically, imagine a lens just as a device that bend light with a certain angle ..... more the lens is magnificating, more the angle is big ..... and the angle don't change, when you displace the lens (i know this is not totally true, is just for try to simplify how it work )

Now, with a lens of, say, 10mm, you must place the light source at 10mm for have a parallel beam, cause this is the "turning angle" ..... if you want to get the focusing point close to the lens, you need to place the lens far from the light source (in our case, the LD chip), and there is a limit about this, due to the "large turning angle" of this lens, as in the first diagram.

Now, imagine to have a 1mm FL lens ..... the "turning angle" is bigger, so for close the focal point to the lens, you need to place less far the lens from the light source than with the previous lens.

Ofcourse, laws of phisics cannot be broken, so with a single lens, these is a phisical limit that you cannot overcome (the approaching rate of the focusing point is not linear, and for reach the LF distance, you need to place the light source at infinite distance, so the focusing point can NEVER be close than the FL ..... in fact, much far than this) ..... for do this, you need to add a second lens, that change the optical parameters of all the assembly.

(Sorry to the opticians presents, i know it's not totally correct, the draw is just to give an idea about it, i don't think this need formulas )

 

[Dawinzi]

Thank you for that man.
I just wonder if its no Lens at the market wich is adjustable for 445nm.

 

[HIMNL9]

Don't know specifically for 445nm, but for the band 400 - 500nm i think yes ..... also, if you really need the burning spot very near the lens, is better to use an add-on lens in the front of the first lens assembly, also for another reason ..... pulling the lens far from the LD chip, you take the burning point near the front of the lens, but at a certain distance, part of the light from the LD fall outside the lens, and you loose power with that ..... instead, using the main lens for focus, like, parallel beam, then a second lens for re-focus the beam close to it, you still loose a bit of power, but less than before.