well, i started at th beginning, knowing about nothing… heres the readers digest version…
1) every lens has 2 main criteria… focal length (FL) and diameter… the stuff i worked with? not much else seemed to matter, at least with the basic math.
2) size of the projected well focused emitter? is entirely a function of the FL, and the size of the light source. if you have an ACCURATE measurement o the light source? an the focal length of the lens? you can fairly accurately predict the size of the projected emitter, at any give distance. (note: i am not 100% sure the “dome” does not SKEW the mathematical prediction, i think it magnifies it a little)
3) focal length is not a CONSTANT, it actually varies a great deal, depending on the distance to the target. If you pick up a “135mm FL lens”, if its an accurately described “spec”?? if you are aimig i against the WALL 3 feet away? you will measure a lot more than 135mm at that short distance… it will FALL to 135mm at “infinity”.
for this reason? if you pick up a random lens, say one you bought off the internet? and it did not have a “spec” for focal length? you cay just focus it at the wall, take a measurement, and proclaim it “thats the FL”
theres actually an short easy formula for that, where you enter the distance to the “target”, then it spits out the ACTUAL focal length.
4) a “simple” lens? which here would be something, like, say… a physically thin biconvex traditional lens? FL is measured from the MIDDLE of th edge, back to the source.
ONE problem with the internet “flashlight asphericals” that a bunch of sights are selling? is that many people measure from the REAR edge of the lens, back to teh source, and call THAT the focal length… compounding th problem is that many people aim it at the WALL across the room as the “target”… further compounding the problem.
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all that aside? heres the basic formulae….
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the FIRST equation is the most critical… but its useless in its given form. We want “f” (the resultant FL for the combined system…) by ITSELF on the left side of the equals sign, dont we? (yes, we do, lol)
i sat for a couple nights, using the super high technology that is a paper and a pencil… i just kept doing basic algebra, fairly at random, until i got closer and closer to getting “f” isolated like i wanted. Finally, i got the ratio of the gawd-d@mned airgap distance to product of focal lengths to “drop away” from all sides… which was a pain in the arse, but after about 30 or 60 “attempts” i finally got pay dirt.
made errors on several early “false successes”, then i finally got a solutio that worked, IE, i was able to go back thru, and plug in my answer, and everythnig was still equal.
i solved it several times, several different ways… and after getting the same resultant equation a couple times in a row, that checked out… i pronounced it “done”.
THAT produced a “cookbook” equation… IE, you just “plug in” the two focal lengths, and the airgap (even if zero airgap (touching…)) and it “spits out” the resulting focal length.
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the SECOND equation is eminently useful, and as providence would favor the irish? it did not need solved… its a simple “plug in” and it spits out the “BFL” (back focal length), whhich is the distance from the middle edge of the rear thin lens, to the light source.
in other words, how close to the LED emitter th rear lens is (which is where the efficiency comes from)
after playinig “what if” with about a couple hundred “this lens FL, and THAT lens FL… what does THAT make?” I had the idea i should just write a little piece of software, to do this FOR ME, so i can just sit and play “what if” all night…
which is where these 2 screen shots came from…
you flashlight guys? you get a free pass on sm stuff i had to sweat about, lol… all YOU are concerned about, is “how does it LOOK?” and “can i SEE things at what range?”
then, you all sit around, ad offer “qualitative” things, like “beam quality”, and complain or compliment on tint and such…
in MY project? i have serious CONSTRAINTS on my emitter image… i am watching a SCREEN, which is hooked up to my sensor, which is receiving a IMAGE from a lens system. If the emitter size projected at distance is too BIG, i have wasted precious IR… if its too SMALL?? I can only see a tiny portion in the middle of the crosshairs of my view.
you ca mathematically describe a “field of view” wither thru a camera lens, or, thru a gun scope… many ways. You can specify the field of view by WIDTH and HEIGHT at a certain distance… or, you can express it as a DEGREE, or even as a radian measurement.
also, with a 3-9 variable power gun scope? this is dynamic… i have to be able to go from 13 degrees to 2 degrees, perhaps.
sized perfectly to the gun scope view.
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before i worked out all the MATH, i was just “playing” with 2 lenses… it quickly gets frustrating, and you give up.
when you have run the MATH a coupe hundred times, you get a “feel” for what the two lenses are going to DO… and i began to not hav to run the math…
i can either set the front lens and find the BFL i need by inspection. OR, i can SET the BFL (distance to the light source) and move the front lens to focus.
or, i can set the airgap fixed… and move the light source to focus.
BUT, all this became possible only after getting familiar with the math.
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this all ended up, wheere i realized i SHOULD be able to pick up random lenses… and MAKE my OWN “gun scope” or “camera lens”…
and, this worked out too.
I also take camera lenses APART, and add or subtract lenses t the camera lens… to adjust the FL of the lens, and also to set the “F” number (light transmissionn) of the camera lens to where i need it to be.
too little light transmission, no night vision… too MUCH light, or, off axis light nearby? and i “wash out” and its unuseable…
here’s me with the japanses lens system salvaged off of an old camcorder, and resetting it to focus up on a sensor…
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that was just an early “proof of concept” for the later “hand held night vision spotter” unit…
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heres some before and after and “working” photos of the little screen wth a cell phone. MIND you? this is a low res cell phone pic, of a little screen… what i see in “real life” is MUCH crisper and more detailed. Its like watching cable TV channel, lol…
150 meters… control shot (no illumination)…
same 150 meters, with illumination…
same 150 meters… defocused for wider more even illumination…
heres a “line of sight map” i made, because i crushed my distance my laser ragefinder will work at… only way could determine true distance… (and? this is flat distance on the map… TRUE distance is greater, because i’m up on a hill, and the angle makes it even further… cosine of the angle, is the adjacent divided by the hypotenuse… remember “chief SohCahToa” from Trigonometry class? lol… the true distance is the hypotenuse… the adjacent is the map distance…
and heres an image, of the far river bank, 800+ meters… no moon out. pitch black. (light has been defocused for even illumination)
heres about 350+ yards… control shot, no illumination…
same view, with illumination…
heres a “practical” view… of a live GAME ANIMAL… so you can get a feel for if it “works in real life” or not, lol…
heres the updated handheld spotter unit… this puppy is so sensitive? in most situations, it doesnt need ANY ir illumination, heh heh… its about as far as the eye can see in the daytime? except at NIGHT when there is NO MOON out at all, ha ha… i took a real camera lens apart, and added a lens in the rear of the unit, behind the iris mechanism… and adjusted the focal length to suit my needs.
and, heres the “big” illumniator, mounted on a gun scope… prototype naturally… needs to be metal lathe’d out, and a real windage/elevation mount added… and thats not a “real” gun scope… thats a night vision scope, built completely inside what was formerly a gun scope… THIS is the unit that you saw 800+ yards across the river to the far riverbank, on a moonless night in pitch black…
lastly? (finally, whew you say, the end?) heres what i mean by “pitch back” with no moon out… this is what “across the river” looks like, without night vision…
