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DTR G8 lens

RedCowboy

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Thought I would test a G7 against the G8 and a DTR G2 as a reference point then I went ahead and ran a couple different clean new 3 elements, I used a seasoned nubm44.
Note: Even with a large heat sink it's important to adjust the next lens to be tested then let the laser return to room temp before testing, also 3 element lenses are not created equal.

DTR G2 = 6804
DTR G8 = 6249
San G7 = 5896
3 ele #2 = 5361 Half threaded 3 element.
DTR G3 - 5315
3 ele #1 = 5137
8mmFL = 4953 8mm FL ebay lens AR coated 450nm in DTR G2 lens barrel. Tested several times 4.9w each time.
 

logsquared

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No, the back focal length of a G2 is actually 4.8 mm at 445nm. The 2.39 mm distance is from the center of the back surface of the lens to the laser die, not counting the diode window. This also changes with the WL of the diode. I believe at the design WL of 405nm for the G2 this distance becomes 2.357 mm. Usually the EFL and the BFL of any asphere increase with the WL the farther it is used from the design WL. Conversely, it decreases in the opposite direction.
G2

Here is the lens china ripped off and became G2.

You can see the FL = 4.02mm. Also, WD = BFL

You should go back and edit your posts so people in the future don't get the wrong info.

Note: Even with a large heat sink it's important to adjust the next lens to be tested then let the laser return to room temp before testing, also 3 element lenses are not created equal.
I too have noticed the 3E are not all the same. The half threads always seem better regardless of where i bought them.
 
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DTR

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G2

Here is the lens china ripped off and became G2.

You can see the FL = 4.02mm. Also, WD = BFL

You should go back and edit your posts so people in the future don't get the wrong info.



I too have noticed the 3E are not all the same. The half threads always seem better regardless of where i bought them.
This^

There are actually so many of them no way to be consistent. Different specs, different AR coatings different mounting. This lens will be called a DTR-G-8 and I have used the DTR in front of all the G lenses I stock so you at least know you will get consistently the same lens. Does no good for applications that are tailored to a specific lens element and have all these guys out there with different lenses calling them the same thing. That is just the least of the misrepresentations out there in this field due to it being dominated by gray market sellers in Asia.:(
 

steve001

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I am not talking about the diameter of the input aperture of the lens. I am talking about its EFL. I suppose I could take the lenses out of both the housings for the G3 and G8 and reinstall them in an empty housing. Then, it would be very easy to see if their EFLs are the same. But, I won't be doing that today as it will take some time to get it done properly and I don't have that kind of time right now.
There's a non destructive way to measure focal length. Place a lens in the beam of a collimated laser. Measure the distance from the surface of the lens to the smallest spot size. That's the focal length. The Sun works too.

You may recall on and off I've speculated a wider diameter lens might eliminate or make less pronounces the box artifact. From what I've read in this thread that seems true.
 
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paul1598419

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The EFL of these lenses is not taken from any surface of the lens, but from a point between both surfaces. The working distance is measured from the center of the surface closest to the diode to the emitter face on the die. That is likely what you are talking about.
 

logsquared

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I don't think you understand what I'm trying to say. I was saying, if you have a know FL (EFL) lens you can use it to find the approximate FL (EFL) of an unknown lens with decent accuracy.

You can't use the BFL or WD to do this, its meaningless for this calculation.

Another example. If you have a known 8mm FL (EFL) lens and the spot is 10mm wide at 20 feet and you put in a unknown FL (EFL) lens and the spot is also 10mm wide at the same distance, then you know the unknown is also 8mm FL (EFL). Similarly, if the spot is 2X wider then the FL (EFL) is 1/2.
 

RedCowboy

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The EFL of these lenses is not taken from any surface of the lens, but from a point between both surfaces. The working distance is measured from the center of the surface closest to the diode to the emitter face on the die. That is likely what you are talking about.

It's an easy way to get a rough idea, granted the multi mode spot can be irregular, so set your target 10 feet away or more and measure the spots, this is if you know the EFL of one of the lenses, the G2 is 4.02mm EFL 2.39 BFL but don't worry about BFL

63701
 
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paul1598419

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I don't think you understand what I'm trying to say. I was saying, if you have a know FL (EFL) lens you can use it to find the approximate FL (EFL) of an unknown lens with decent accuracy.

You can't use the BFL or WD to do this, its meaningless for this calculation.

Another example. If you have a known 8mm FL (EFL) lens and the spot is 10mm wide at 20 feet and you put in a unknown FL (EFL) lens and the spot is also 10mm wide at the same distance, then you know the unknown is also 8mm FL (EFL). Similarly, if the spot is 2X wider then the FL (EFL) is 1/2.
I understood it fine. My reply was to the post just prior to mine.
 

steve001

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[/QUOTE]
It's an easy way to get a rough idea, granted the multi mode spot can be irregular, so set your target 10 feet away or more and measure the spots, this is if you know the EFL of one of the lenses, the G2 is 4.02mm EFL 2.39 BFL but don't worry about BFL
I don't think you understand what I'm trying to say. I was saying, if you have a know FL (EFL) lens you can use it to find the approximate FL (EFL) of an unknown lens with decent accuracy.

You can't use the BFL or WD to do this, its meaningless for this calculation.

Another example. If you have a known 8mm FL (EFL) lens and the spot is 10mm wide at 20 feet and you put in a unknown FL (EFL) lens and the spot is also 10mm wide at the same distance, then you know the unknown is also 8mm FL (EFL). Similarly, if the spot is 2X wider then the FL (EFL) is 1/2.
You're both making this rather complicated. To find the focal length use a collimated light source like a laser or sunlight. Shine the beam or sunlight through the lens. Measure from where the spot is back to the lens. As Paul pointed out the distance starts ends in the middle of the lens.
 

RedCowboy

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You're both making this rather complicated. To find the focal length use a collimated light source like a laser or sunlight. Shine the beam or sunlight through the lens. Measure from where the spot is back to the lens. As Paul pointed out the distance starts ends in the middle of the lens.

So if I focus an acrylic lens in front of a Gaussian beam I can find the focal length HOW? by looking for the smallest point of waist or by looking for the sharpest focus against a stop ? That's not practical.
 
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logsquared

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I understood it fine. My reply was to the post just prior to mine.
I see. Sorry.

You're both making this rather complicated. To find the focal length use a collimated light source like a laser or sunlight. Shine the beam or sunlight through the lens. Measure from where the spot is back to the lens. As Paul pointed out the distance starts ends in the middle of the lens.
Have you tried this with a tiny short FL aspherical lens? Not easy to do with any accuracy.
 

Miya

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So.. G8 lens is similar to a 3element lens point?
 

steve001

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So if I focus an acrylic lens in front of a Gaussian beam I can find the focal length HOW? by looking for the smallest point of waist or by looking for the sharpest focus against a stop ? That's not practical.
Have you tried this with a tiny short FL aspherical lens? Not easy to do with any accuracy.
To avoid further doubt watch this short, how to, video by Edmund Optics.
 
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RedCowboy

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To avoid further doubt watch this short, how to, video by Edmund Optics.
Again, not practical with a 4 - 8 mm EFL tiny 6.3mm lens in a 9mm lens barrel.
Try to differentiate the difference between say a 4.7mm EFL and a 6.3 EFL tiny 6.3mm lens in a 9mm lens barrel this way.........( IT'S JUST NOT PRACTICAL ) I did not say impossible, just not very practical at all.

Now if I had a pair of 3 inch wide lens, one a 25cm FL lens and a one a 35cm FL lens I could hold them each over my desk top and see where the desk lamp's light comes into focus making an image of said light bulb so clear I can see the print on the light bulb like an picture on my desktop, I have done this, but a laser spot at 4 - 8mm holding a tiny lens is just not practical, I even tried it and as expected it's not practical.


---edit---


BTW did I mention .............
The G8 is Great. :D
There's no need to wait.
So hurry now and partake.
I can put an end to all debate.
Because the G8 is Great. :D
 
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paul1598419

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If I were trying to measure these lenses with any frequency I would make a jig using a micrometer and a light source. This should work fine to measure the FL of even tiny short focal length lenses.
 




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