Maximum useful focal length

This is my first laser modification. The goal is to keep the dot as small as possible for as long as possible... if even possible because I wasn't sure if I could focus such a cheap laser. I opened my only working laser <5mW red that came with an airsoft pistol. Funny thing about that pistol is it had terrible precision, the non lasered one was spot on. Anyway, It does have an adjustable lens which takes pliers to adjust since it looks like thread lock was used, and I was able to get the focus to change from a tiny dot at 5ft to a large but very slowly expanding dot: 3mm point blank to blurry 8-10mm at 25ft. I assume the focal point is less than 25ft because the dot appears to have changed from a horizontal oval at close range to a vertical one, so it probably isn't worth further adjusting. I guess the question is have I found the ideal long range focus for this laser/lens combo? I assume the farther the focal point is from the source, the larger the dot is at that point because of divergence. Either way it is much better, and since I can't burn stuff with it, long range is the next best thing. It used to make a huge dot at 40ft around a few inches wide, now it might be an inch.

I should put those innards in a nice pen.

Batteries for my pocket laser should be in tomorrow. I'm curious to see it's beam quality.

What you are looking for is to focus it to infinity. Since it was glued in place, I suspect it was already there before you messed with it. That is a very cheap 5 mW laser that would cost less than $2.00 anywhere in a store. It runs on button batteries and is most likely a 660 nm wavelength, as I have one and that is where it spectrometers out to be. You might be better off getting a new laser and replacing it.

It was focused to 5ft as mentioned above. I tried unscrewing the lens and it focused to <1ft. I screwed it in just past where it was and got a wide beam, so I tweaked it out untill it stayed pretty even in size with what is possible for this quality of laser. Will real lasers (not theoretical ones) focus to infinity with good results? I would think the beam will still get wider even after 10-50ft.

Yes, the beam will continue to expand. It is called that because it is the farthest one can focus the beam with the single lens that, in your case, came with the laser. Now, you can focus it out further if you install a beam expander, but it will make the beam wider close up.

Nutball said:

It was focused to 5ft as mentioned above. I tried unscrewing the lens and it focused to <1ft. I screwed it in just past where it was and got a wide beam, so I tweaked it out untill it stayed pretty even in size with what is possible for this quality of laser. Will real lasers (not theoretical ones) focus to infinity with good results? I would think the beam will still get wider even after 10-50ft.

Click to expand...

Yes grasshopper, the beam will get wider that is true. Even if the lens (on any laser) has been place into position for infinite focus the beam will expand just like the beam of a flashlight does once the beam passes the Rayleigh Length(RL). RL's can be short or very very long it all depends on how much the beam is expanded before it is collimated to infinite focus. "Good results" is too vague of a term to make any sense enough to give an answer.

Right, good results as defined my the picture in my head. I guess what I meant to ask was does focusing to infinity provide better results on our lasers than focusing to 50ft? or 300ft, 1 mile or 100? Because eventually the beam will expand to the point of being useless for most purposes or very undesirable to most people, which means that there must be an ideal average focal length range which very well could be infinity or lightyears away, or just 100ft.

I'd assume it depends on what results you're looking for, it's going to depend on the task you want to do.
If you want close up burning, or maybe popping a balloon across the room, or even a small dot on the other side of the lake, all will require a different focus point.

I think one could assume I was talking about just plain pointing with any power, not burning, but I should have been more specific.

Nutball said:

I think one could assume I was talking about just plain pointing with any power, not burning, but I should have been more specific.

Click to expand...

Good. Now that you've made your goal clear it's still not possible to answer that question specifically. It comes down to what you like. I can say this though, no matter the power outputted, if the beam is expanded too much you'll decrease the brightness of the spot at great distances. Remember what I said? I said the laser beam behaves just like a flashlight's beam once the beam passes the RL.

I got my batteries just now for the pocket laser. I'm surprised how it has a MAXIMUM focal length of 3ft. That's shorter than my cheap laser was set at originally, but at least it could be adjusted to... just tested now:<1" maybe 1/2" at 50ft While at it's longest focal length the pocket laser is 5". Is there any way I can adjust the lens in its housing to get a longer focal length out of the pocket laser? The whole reason I got it was for a high quality long range narrow beam pointer, what I thought a cheap laser couldn't do. Now it seems that by spending nothing I could have gotten what I wanted from the cheap one, except its power is a bit week for outside in the daytime.

I must admit the pocket laser instantly sparks brown paper at 4" with the exact right focus, and that's after the battery voltage sags. Looking at the almost non existent burn mark I see one could write a message in font as small as you can read, or smaller. A pin prick would be much bigger than the burn mark, that's pretty cool

steve001 said:

Yes grasshopper, the beam will get wider that is true. Even if the lens (on any laser) has been place into position for infinite focus the beam will expand just like the beam of a flashlight does once the beam passes the Rayleigh Length(RL). RL's can be short or very very long it all depends on how much the beam is expanded before it is collimated to infinite focus. "Good results" is too vague of a term to make any sense enough to give an answer.

Click to expand...

Exactly.

Nutball said:

Right, good results as defined my the picture in my head. I guess what I meant to ask was does focusing to infinity provide better results on our lasers than focusing to 50ft? or 300ft, 1 mile or 100? Because eventually the beam will expand to the point of being useless for most purposes or very undesirable to most people, which means that there must be an ideal average focal length range which very well could be infinity or lightyears away, or just 100ft.

Click to expand...

The problem is that lasers and optics for them do not behave the way they do in your imagination/ the way you imagine them to be. Is not the way they work in reality---so your question is not really possible to answer.

BowtieGuy said:

I'd assume it depends on what results you're looking for, it's going to depend on the task you want to do.
If you want close up burning, or maybe popping a balloon across the room, or even a small dot on the other side of the lake, all will require a different focus point.

Click to expand...

Exactly

Nutball said:

I think one could assume I was talking about just plain pointing with any power, not burning, but I should have been more specific.

Click to expand...

Every laser has different beam quality and characteristics to begin with and optics are used to collimate and focus that beam can only do so much with any given beam quality within the limts of what those optics can do.

The problem is a lack of fundamental understanding of lasers, both DPSS laser which havethe best beam quality and laser diodes which all have less than ideal beam quality compared to a perfect round gaussian beam ---diode all have rectangular output not dot, generally see: https://en.wikipedia.org/wiki/Laser_beam_quality
See also: https://en.wikipedia.org/wiki/Laser_beam_profiler

All lasers exihibit "divergence" means the beam leave the laser at an angle--the spot size growing with distance no matter what the optics are. You can use a beam expander to reduce divergence but you can't eliminate it.
See: Laser beam divergence and spot size (Theory) : Laser Optics Virtual Lab : Physical Sciences : Amrita Vishwa Vidyapeetham Virtual Lab

steve001 said:

Good. Now that you've made your goal clear it's still not possible to answer that question specifically. It comes down to what you like. I can say this though, no matter the power outputted, if the beam is expanded too much you'll decrease the brightness of the spot at great distances. Remember what I said? I said the laser beam behaves just like a flashlight's beam once the beam passes the RL.

Click to expand...

Exactly,Steve

Nutball: Lasers, laser beam characteristics, and optics for them are each large and complicated technical fields fields that are entire areas of education. There are no all lasers do this kind of answers other than all laser produce coherent light. Really you have go use google to investigate whatever aspects you want to know about ot buy textbooks on same. LPF search will also give good information on many questions.

Here is a list os laser related terminology: http://directedlight.com/laser-components/catalog/glossary-laser-terms/

I already know all of what you said Encap, I probably didn't show it with how I wasn't specific and how even though I know the info you mentioned, I'm not experienced with applying it. I think based on the information I provided you could safely assume I'm for the most part talking about laser diodes similar to the one in the first post. I wasn't trying to get too specific of an answer as I know there is some variation between different ones.

It looks like the lens in the cap of my Sanwu pocket laser is threaded, so I could possibly adjust it to where I want provided there is clearance between the lens and diode.

According to Sanwu's pocket laser page on his site, the pocket laser is definitely focusable, hopefully you can get some better results after some re-focusing.

The problem is that with the cap screwed all the way on the focal point is 3ft, I can unscrew it until the point is 3" or less from the laser. The lens in the cap has it's threads glued I PM'ed Podo about getting another cap and lens without glue. I am so happy right now with my 5mW laser, it holds such a tight beam for so long. I just aimed it at a tree around 200ft away and it is still a small dot, maybe 5", but it looks tiny.

Is it safe to assume the focal point beam size of the pocket laser is the same as the tiny holes in the paper? Just so I can get an idea of the true diameter, not what I see.

Nutball said:

Is it safe to assume the focal point beam size of the pocket laser is the same as the tiny holes in the paper? Just so I can get an idea of the true diameter, not what I see.

Click to expand...

I would say that's a pretty close estimate, the diameter may be even smaller than that due to the paper burning a bit more after the beam passes by. This is a total assumption on my part, I'm sure someone else give a more definite answer.

Nutball said:

I already know all of what you said Encap, I probably didn't show it with how I wasn't specific and how even though I know the info you mentioned, I'm not experienced with applying it. I think based on the information I provided you could safely assume I'm for the most part talking about laser diodes similar to the one in the first post. I wasn't trying to get too specific of an answer as I know there is some variation between different ones.

It looks like the lens in the cap of my Sanwu pocket laser is threaded, so I could possibly adjust it to where I want provided there is clearance between the lens and diode.

Click to expand...

If it can do what you want.

The Sanwu Pocket series comes with either a 3-element / G2 / G7 lens whichever one you choose and is focus adjustable.

OK whatever you say, you "know all" that I mentioned and everyone else has been telling you/saying but ask unanswerable questions as if you don't comprehend in any functional way what you say you already know--go figure :banghead:

Sounds like trial and error is the only thing you can do to determine what can and can't be done with your laser. If you don't mind a wider beam you could add a Sanwu 3X bean expander to reduce divergence resulting in a smaller brighter spot over a greater distance if one can be fitted to a pocket series

Re:the laser in your OP---As paul1598419 said in post #2: "What you are looking for is to focus it to infinity. Since it was glued in place, I suspect it was already there before you messed with it. That is a very cheap 5 mW laser that would cost less than $2.00 anywhere in a store." What do you expect to be able to do with a laser diode, module, driver and lens that cost half the price of a good cup of coffee?

Nutball said:

I got my batteries just now for the pocket laser. I'm surprised how it has a MAXIMUM focal length of 3ft. That's shorter than my cheap laser was set at originally, but at least it could be adjusted to... just tested now:<1" maybe 1/2" at 50ft While at it's longest focal length the pocket laser is 5". Is there any way I can adjust the lens in its housing to get a longer focal length out of the pocket laser? The whole reason I got it was for a high quality long range narrow beam pointer, what I thought a cheap laser couldn't do. Now it seems that by spending nothing I could have gotten what I wanted from the cheap one, except its power is a bit week for outside in the daytime.

I must admit the pocket laser instantly sparks brown paper at 4" with the exact right focus, and that's after the battery voltage sags. Looking at the almost non existent burn mark I see one could write a message in font as small as you can read, or smaller. A pin prick would be much bigger than the burn mark, that's pretty cool

Click to expand...

As you have found out, the reality of any given laser is not a hypothetical thing.
As I mentioned in post # 11:"The problem is that lasers and optics for them do not behave the way they do in your imagination/ the way you imagine them to be. Is not the way they work in reality"

Here are a few laser calculators by Ophir Photonics who make excellent world class Laser Power and Energy Measurement equipment and also excellent Laser Beam Profilers that you might find interesting and useful:
Focal Spot Size Calculator for Gaussian Laser Beams http://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/services/focal-spot-size-calculator-for-gaussian-beams and
Laser Power Density Calculator http://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/services/power-density-calculator and
Laser Peak Power Calculator http://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/services/peak-power-calculator and
Laser Focusability (Gaussian Beam) http://www.ophiropt.com/laser--measurement/laser-power-energy-meters/services/laser-focusability-calculator and
Laser Power Through Aperture Calculator (Gaussian Beam) http://www.ophiropt.com/laser--measurement/laser-power-energy-meters/services/power-through-aperture-calculator


In any case good luck in your experimenting with lasers---since it has not been said >>>remember , safety first--be sure and use laser goggles to be safe and not risk damaging you eyes.