whoa cool!
for the record, i am NOT just a former academic… i believe in hands on testing just as much.
hands on measurements are good science too. its testing predicted results. this one is your pick…
https://www.fasttech.com/products/1606/10002523/1583700-replacement-100mm-optical-borosilicate-plano
a couple practical things i like about your pick? assuming we can use the 135mm FL as a reasonable estimate…
F 1.35 is no slouch… especially since the higher focal lengths “fool your eye” so WELL when the focused emitter pencil beam “appears brighter”. Booruit’s 11mm focal length lens with .3x F number? is NICE, but… when the emitter is too “big” to illuminate anything? my eye gets “fooled” into thinking i cant see anything at distance, lol…
we CAN “quantify” all this, and with no heavy ray tracing software and meter readings that are about as useful as white wall hunting for outside practical terms.
one “muff up” in a lot of the chinese specs? is that there are several “focal length” speccs, any one of which COULD have been picked by the SELLER to put on the web sale site as the “spec”
with a THIN LENS, you can accurately measure FL for yourself… simply draw up an image of the MOON to perfect focus on a piece of white paper held square to the lens. the exact distance from the edge-middle to the paper straight back? is the FL in mm. simple as that.
once you get into aspherics, and really you get into 7 pounds of molten glass blobs a foot thick, it gets ridiculous… you no longer have that easy measurement to hold the web sales spec feet to the fire… the actual FL cant be measured so easily with such a weird blob of lens.
the actual FL becomes the measurement (once again focused on the moon, a nice bright object at “infinity”) from the “datum point” or something to the paper you draw up the image on. for ease of use in practical life? on the night vision site, we started using “back FL” as if it were focal length… it was “useful” but… can throw you with really thick blobs of glass.
but 20mm is 20mm… and out of a 60mm diameter lens? its a LOT to calculate a difference when arriving at the calculated focal length… which now “isnt”.
so, we are screwed royal? well, you might be, i aint… fortunately theres another way to skin this cat. you are taking a certain size emitter (dedomed for trial purposes, the dome acts like another lens and ruins it) and project the emitter well focused at a KNOWN DISTANCE (i recommend 100 yards or 100 meters) with two lenses…
1) a very thin lens of measured FL (moon)
2) the new suspect lens. of any size shape and thickness your heart desires.
you see, the insanity of math, is that nothing except focal length, and focal length alone? will determine the size of that same size emitter at THAT distance. you walk up, you measure the emitter, you are done.
the numerical aperture equations, and simple “chief soh-cah-toa” application of basic geometry? will back calculate the actual focal length of the unknown lens. you just need to no crap your pants when solving a sin or cos equation, and be comfortable with an answer stated as “degrees” and half angles… which is actually directly transferable to “projected size” (which is where the chief soh-cah-toa comes into play, not hard)
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then, theres no lying about the diameter of the lens, we know the “actual” focal length… and we can calculate, from then on out? the predictive SIZE of the dedomed emitter at a known distance… which as we all know, allows us to fool our eyes into thinking we can see things at a great distance.
but wait! theres more!
since the focal length and the focal length alone controls the exact size of a known sized emitter at a known distance? if anyone starts playing with multiple lenses to make one compound lens out of it? its now JUST as easy to calculate the actual FL of your new compound lens you just created.
but wait! theres more!
then, when someone insists on using a different size emittter? say… an xpzl70.2 that makes 9994 lumens at 16.7 amps???
see, normally everything goes wacked, because all of a sudden the same lens is projecting a gigantic emitter onto a much larger area at distance, and the eye gets fooled into thinking it cant see anything (even though the law of “absolute throw” is not being violated) simply because the everything is apples and oranges?
not so fast, slick…. what you WANT is to illuminate the SAME SIZE PROJECTED EMITTER size at the same yardage? just with a bigger brighter emitter… we can calculate without too many cups of black coffee and hand rolled cigarettes, the NEW focal length we WANT, in order to “paint” the same size emitter.
chief soh cah toa gives us the keys to the kingdom to predict and control everything.
this is all well proven, and well established. its all perfectly quantifiable. none of it violates any ESTABLISHED laws of math.
now, the last time i did this? on my night vision site? everyone groaned and said “i’m a hands on builder, you make my head hurt”. I wrote software to do the calculations for them, so they could click click click… not one download.
that laptop got stolen, and i’m not writing it again for no downloads.
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anyone want to accuse me of making it all up? because “throw” and “beamshape” mean exactly nothing.
and when a certain size emitter, makes a certain size focused emitter at a certain distance? everything else becomes easily calculable, and i can make the simple software on visual basic AGAIN, this time to “model” whatever emitter we all decide to pick. i COUKD resolve the equations to get OUT whatever focal length prediction, would “paint” a different size emitter, to the same size, at the known distance.
i am NOT doing all this again, with the new work on top of it? if no one will use it…
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look, i’ll tell you a secret, if you know where to keep it, and you promise not to tell (my apologies, laura, RIP)
back on my night vision site? the boys were basically trying one lens after another, and getting incremental, though slow, progress. it was impressive real world results.
some guy wandered in, he had come from CPF, and he was here to help us! whoa cool, we finally had attracted an expert who could save us from ourselves.
all he knew was, “focal length does nothing, it only affects beam shape… only lens diameter increases throw”
HE wanted us to use various lenses, which we already had trialed… and our sensors were showing dimmer images than what we were using… after a “some period of time” arguing… you know what happened?
turned out, ironically… HE had a Gen2+ night vision for boating, and HE wanted a better moustrap too, that didnt cost 300 dollars for a 50 dollar zoomie like everyone wants for them at the time…
he built OUR at the time “best lens unit” to our specs, and you know what? he was excited to post pictures proving he had “beat” his 300 pound (600 dollars atthe time, wow) commercial illuminator, with our 90 dollar build information. he went away happy as a clam, and hes probably still yachting somewhere, hes obviously rich.
all i am saying is, i was reminded of the phrase “i’m from the government, and i’m here to help!” and in the end? none of what he claimed “whoever” taught him his un-defined terms on CPF did anything. he built our unit, and he was tickled pink.
all i’m saying is… i’m willing to do all the work, i’m willing to boil t all down in the end, to simple “cookbook” equations, simple plug ins, that anyone can use a calculator if they press the right keys. i’ll even model it for each emitter one by one, if and only if, someone somewhere does something resembling a little legwork for me. it wont be hard, i’ll do all the heavy lifting… when its all done, if we verify the predictive usefulness? i’ll put it into a simple visual basic program which anyone can use to play what if.
i will start a thread, i will reserve several posts up front for my work. everyone can say whatever they want to in all the ongoing posts, go on and talk about “moon bat units” and laws which dont seem to exist all you want. when i am done? we will predict almost everything, and we will easily measure the exact FL of chinese lenses with a click of a mouse.
because i am throwing down the gauntlet, and i am willing to follow thru… and prove it with a real predictive math model.
just think, how much fun you can have, poking fun at me if i fail, forever time immemorial. tasty tidbit to dangle, there, eh? lets get to the bottom of this, once and for all.