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DTR G8 lens

Shotgundrums

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Hi,
I just got a DTR 20mm NDB7875 with a G8 lens and I notice an “artifact” ( mostly on the right side).
Here are pics...View attachment 71029
View attachment 71030

Any ideas ? I’d this normal ? I just got it and powered it up and this appears and changes with lens adjustment.
I’d say it’s normal. Probably a left over aberration from the diode guts collimated into the spot. I have a couple G8 lenses. One in front of a PLTB450B, and it casts something similar. Hell, I have a G7 in front of an A140. Has extra bits in it, too. When focusing the lens in and out, does the artifact go wide to narrow on the same axis? That’ll be the diode. You’re probably good.
SGD
 



RedCowboy

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Multi mode laser diodes can be pretty clean or not, however you may want to make sure your lens is clean, the thinnest layer of finger oil that's hard to see with your naked eye can cause all kinds of aberrations.......reading your other thread about this I expect you would be happier with a NDG7475 with G8 lens, maybe use a G2 with your NDB7875 and make a desktop burner, don't forget your laser safety glasses.

For the wider cast " overspray " like you can see in my yard in my multi beam pic, I like to use a shroud, I often use a simple rolled up length of cardstock or a threaded length of tube with baffles, I like to use empty lens barrels. However usually the G8 is really clean.
 

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Wakrah

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Techhood on ebay sells a G8 lens, but the G8 has nothing to do with the focal length, it is really quite short at under 3 mm. Just pointing this out so others won't think they are getting an 8 mm focal length lens.
 

RedCowboy

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Techhood sells more than one lens as a ( G8 ) but his 8mm FL G8 is a fantastic lens, it's the 1st of the 3 below which are all different.



 

Wakrah

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G means glass, 8 should mean FL, why would they offer short FL lenses and call them a G8? Wow.
 

RedCowboy

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Why do the Chinese even bother with technical specs when they are usually wrong/flawed, it's up to us to understand what's what and make use of what we can, both the DTR G8 and the Chinese copy are very good but know that there are other Chinese lenses sold as G8 that are not the same, if you have any doubts as to what's what then buy your G8 from DTR
 

Unown (WILD)

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My best guess is that they will do anything that will encourage the sale, even if that means making it seem somewhat legit. They don't care if it's wrong, they just want sales.
 

Wakrah

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Robert,

The first G8 lens you linked to in your post has this in the description:

"Working Focus: 2.357mm (Normal Glass lens this data is around 7-8mm)"

Huh? 2.357 mm focus specified which means it is NOT a G8. When I check the back focal length by focusing a ceiling light upon a piece of paper on my desk, I measure about 5 mm, which the listing says it will have, but they also show a working distance of 2.357 mm which means the focal length is more like a G2 lens, not a G8.

When I write Techhood about this, he says his engineer confirmed the FL is 8 mm, WTH? If it were 8 mm, I would have a lot of threads hanging out the end of the DTR diode mount, but when collimated at infinity, I am estimating the lens is about 2.5 mm from the diode which means this lens is really a G2.

Another BS listing on ebay from China! G8 lenses are probably in demand, so they put G8 in the header for people to find the listing. Ebay allows it, there is no way to report sellers for doing that, I'd rather report it to ebay to force them to change their listing than give bad feedback, but I think I will for this one.
 
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RedCowboy

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I have a bunch of techhoods G8's and DTR's G8's, they are both 8mm FL, the advertisement/specs techhood has on ebay is wrong as are many of their ads/specs, but the DTR G8 copy techhood has is a very good lens as is DTR's G8 lens......you have to identify techhoods G8 by the pics THIS is techhoods G8 or just buy DTR's G8.
 
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Wakrah

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The AR coating might have burned to look like tiny metal particles, I suspect.
 

FireMyLaser

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I've already shown you what may cause this issue. Also, why you get more crap on one side suggests that the diode may not be pressed in straight.
 

FireMyLaser

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Sandpaper will mess it up. Before doing anything, show the spot on a wall and unfocus it as much has you can before the threads run out. Keep the laser host stationary. Preferably show it on video as you turn lens. Use a camera setting that dont blow out the details of the artifacts.
 

Wakrah

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Gatrezal Don't be so out of hand dismissive to such, I've burned the AR coating on a G8 lens just last week. All AR coatings have a maximum amount of power density they can take in a given area, all of them. That, and I wouldn't trust some of the Chinese AR coatings to be high quality, these are just cheap pointer lenses.​

 
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FireMyLaser

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Its just a $10 lens, a lunch. Get a new one and learn from mistakes. When dealing with optics, you'll only make it worse if you try to fix small imperfections when you don't know what you're doing. Either don't fix it or learn what to do first.
 

Wakrah

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Yep, if you burn dust on the lens by the beam, it will also produce artifacts which may appear to be little bits of metal or something. If a multi-element lens, they may appear to be inside.
 




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