Planning an aspheric scratch build

I’m planning to do a wavien style scratch build. I’m currently trying to decide on a lens and am considering three I found. Two are cheaper moulded lenses. One is a commonly found 78mm lens with a 50mm focal length. The other is a 100mm lens with a 80mm focal length. The third is a surplus precision ground coated 79mm plano convex lens with a 105mm focal length. Currently inclined to go with the 100mm but would like opinions. Since it’s a scratch build I can tailor it to the hemispherical reflector and lens I select.

Following, sounds good already!

the raw power and efficincy of transmitting MORE LIGHT per square inch? is the F number…

the F number is focal length divided by diameter… lower F numbers are “better” than higher F numbers, everything else being equal (which it rarely is, but still)

if you know that the F number is “unit-less” because the units canceled out? you get a cookie…


50mm fl / 78mm dia = .64 F

80mm fl / 100mm dia = .80 F

105mm fl / 79mm dia = 1.33 F

the 78mm lens is the most EFFICIENT, and also the closer you are to the light emission, you lose less that falls outside the lens edge… since its the shortest FL also, it gets points that way too.

this is all a dance though, and the 80/100 F.8 ? isnt far behind it… and the higher FL will make a smaller focused emitter, brighter smaller image projected.

still some “judgement call” though… but at least i tried to quantify and help.

@sedstar Thanks. You confirmed what I was thinking based on what I read on blf and elsewhere. I’ll go with the 78mm 50mm fl. I was inclined to go with the 100mm for a tighter beam but the 78mm would result in a shorter narrower head compared to the 100mm.

edit: decided on another lens in comment below

I was looking at a lens I have last night, thinking, I wonder if this would be a good lens for a thrower?

I have a convex 114.3mm lens (with flat back) that I was messing with in front of my flashlights.

The focal distance seemed to be about 3" in front of an LED.

Not meaning to go off topic. I just thought I would share.

11mm focal lenght, 31.5mm inner effective diameter, a tad less than 0.35 F.

Bought at Yajiamei Optics Co. Ltd.

You may want to check out their bigger models.

Cheers ^:)

Light collection efficiency is one thing, absolute throw is another. The beam intensity is proportional to the area of the lens, so the 100mm lens will provide the most throw, 1.64x that of the 78mm lens.

Also since you plan to use a reflecting aperture the F number is not the only thing determining the light collection efficiency. A larger F number typically means less light collection efficiency, but if using a RA, more light can be “recycled” in this case which increases the efficiency some.

OK, after more searching I found the lens I’m going to use. Final decision. 87.5mm diameter, 50mm focal length. I doubt I’ll find anything better at a reasonable price. For this lens 50mm / 87.5 = .57 F and the increased size should increase throw a fair amount compared to the 78mm lens.

The best possible lens is the fasttech 100mm lens, which has a focal length of about 145mm.
I have bought pretty much every cheap flashlight lens between 70-120mm and the fasttech one is the best for throw.

Just calculating F does not tell the whole story because some lenses are not very aspherical, some have an outer rim, some do not focus well, etc etc etc
What I did is buy all the options and test each of them to find the best one, which was the 100mm diameter ~19-20mm thick lens.

If you want more light collection then sure you can go with a shorter focal length (larger thickness) lens, but it will not get as much throw.

easyB? i am not being rude, nor do i think i am “right”…. hey, lets have fun dinner conversation. dinner tonight? coffee, cigarettes, a big bag of lemon cough drops and as many acetaminophen wont make my liver quit overnight, lmao. sick as a dog, keep me honest…

i still have “nightmares” and PTSD from way back when, i followed a gigantic, epic, turd of a thread really… on CPF years ago. it was supposed to be “the math of optics”, but… other than dr. hoozits (whose an optical engineer) chiming in and correcting everyone, before he gave up…

good lord, i read every post, every many long page, dutifully. Took hours, got me exactly nowere… it was a train wreck, someone needed to “clean it up and sticky it”. they never did. would be a small feat in itself. everyone is talking “beamshape” and “throw”, and these are “qualifications” not QUANTIFICATIONS. the engineer in me gets “queasy”.

my thing is… okay, i didnt do anything monumental or anything, i just introduced the easy “camera math” for f-number… but i know its valid quantification.

you state that beam intensity is proportional to the diameter of the lens. and i BELIEVE you… theres no replacement for displacement. but… lets assume i believe that “only larger lens diameter increases… ‘absolute throw’ ” (absolute throw is a term i need defined, it sounds like “beamshape” and other unquantifiable terms i hear a lot)

but, going with the given plot line… cool…. i can now ignore F number, it doesnt matter. I wonder why camera people worry about it so much, since they are focusing an image (light) onto film or sensor, and need a reliable quantification from lens to lens to determine the brightness of the image to accurately set shutter and stuff. F number is INDEPENDENT of changing lens diameters and focal lengths. you KNOW the brightness of the image.

before you say “oh, we’re doing the reverse… its different, were projecting light, not collecting it…”?

projector lenses have the same F numbers on them. you can take a “projector lens”, look at the f number, focus it up onto a sensor or film… and it works the same.

could you define this “absolute throw” term?
could you explain exactly how focal length does not “do anything”?

my crystal ball says, you will say something like “only lens diameter increases throw, focal length only affects beam shape”

on that definition? a 200mm flat piece of glass, should provide “higher absolute throw” than any lens on this OP he is entertaining, he should just go with that. whats the definition of “absolute throw”, and why dont the people that use camera lenses and projector lenses know anything about this much easier to use formula, where focal length can be effectively ignored, and only diameter does anything for focused brightness?

i cant understand the “why”, but some how, some way? camera and projector lens people only use increased diameter as the only factor when the focal length is the same… as soon as you change both? you have no working quantification other than F number. camera people never use the term “absolute anything” nor do the projector lens peole say anything about “beamshape”. they both use the F number.

if you want to get into the “dance” of comparing the smaller or larger focal lengths and diameters, and the relative brightness of the larger or smaller projected emitter image? thats already done thru numerical aperture equations which arent much harder than the other stuff. no complicated fourier transforms or ray tracing software.

what is the definition of “absolute throw”? whats the equation?

a happy meter reading is one thing, but in practical terms? i was focusing up IR emitters at long distance, to be picked up by IR sensors for my night vision projects. my eye cant be fooled by apparent brightness, since the same sensor and the same camera lens is picking up the image… its equal and fair.

the furthest imaging (at one time i had the “record” for distance on my night vision site, i might not still have it, i dont know) always happened with the lowest f number… but, the dance was, i had to get the focal length high enough the “spot” was small and intense enough to be visible to the sensor.

it was a dance of “hand tuning” of the 3 and 4 lens systems, but… in the END, i measured the f number of the overall lens?? sure enough, the “winners” were always the ones with the lower f numbers.

all i was REALLY accomplishing? was raising the F number (numerical aperture, really) by getting the rear lens closer to the light source, and keeping the large main lens for diameter. but, i had a “larger. but brighter” focused IR emitter… which seemed strange to me… until i measured my overall focal length of my new multiple tuned lens (one lens in the end) and came up with… a lower F number thru the dance of it all.

a lot of people throw a lot of terms around, that sound very authoritative, but… when no math supports it? i get queasy…

look… i am NOT knocking practical build styles and knowledge, far from it, i LOVE to get that too.

but…even if i “know” my eye is being “fooled” by the cheap charlatans trick of focal length combined with big diameter lens? (f number, basically)… i LIKE my eyes being fooled… because i can SEE THINGS farther away.

i am NOT saying my lens pick is best, nor anything else… i’m thinking the guy should “trial” both lenses, by making a apparatus to hold them square and be able to focus them on the bench, out the window at night… see which one dances better… if the emitter gets too big and too faint? its not worth “cool light meter readings” because it violated he “law of absolute throw”

With the amount of variables and lack of accurate specs on these cheap chinese lenses, I gave up trying to calculate stuff and guess specifications and I just bought every option and measured the lux myself xD
Still have not found any option better than the fasttech 100mm.
Next thing I plan to try is a custom made professional quality lens, which will cost about $150.
These are all the ones I have bought and tried so far:

This is the lens I plan to use.

It will work fine, gives a decently large spot which is useful.
A lot more light collection than the 100mm lens, but less throw.
Also a lot more compact with the short focal length.

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…

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)


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.


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…


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.

Exactly my thoughts. Almost the mid-point between the 78mm and the 100mm.


possible problem being, once again… the shorter focal length, is going to paint a bigger emitter at distance, which is going to fool our eyes into THINKING its a dimmer image than it really is…

boy, is that F number useful… it combines the dance of focal length and lens diameter…

i’m still waiting for the definition of “the law of absolute throw”, too… but i like the equation it yields. diameter is everything, focal length is nothing.

but thats cool, i’m still waiting for a definition of “throw” for about 3 or 4 years now running…

i’m honestly not trying to be rude? but, endermans “i cant beat this lens here…” is much more meaningful, than the unitless, meaningless “absolute throw laws”.

i wan to calculate the exact FL of weird shaped chinese aspherics.
i want to use it to calculate actual F numbers, which actually mean something quantifiable.
i want to predict the size diameter, and focal length “needed” to project the big super emitter onto the same size ground at the same distance?
i want to be able to replicate the known F number, to KNOW what FL i need to paint the same size ground with a different size emitter, and to KNOW what diameter to find in what FL? because the units will be expressed in F numbers…

all my work? will be replicable, and will be repeatable. i want my damned math model. i’m starting a thread, i dont care if no one follows it, and no one cares. i care, i want my work cast down on clay tablets so a crackhead doesnt run off with it like last time, lol.

i’m reserving several posts up top, anyone and everyone is free to say anything they want, its a free world. if it gets too long and starts to resemble the “trainwreck cpf thread” that still gives me ptsd? i’ll simply copy my work onto the new thread, name it #2, and go right back at it.

the sooner begun, the sooner done, and i think we will all benefit form meaningful work that doesnt involve moon bat laws that dont really exist. this isnt rocket science, and i not only allow, hell i encourage 16 other people to do their own threads, their own way… we’ll see in the end who can predict WHAT.

gauntlet down. i’m in for a penny and a pound. i got buddies in real life who cant spell simple words and cant perform math to a 7th grade level as required to gain entrance into CDL school… for some weird reason, i have to tutor them to get them from 6th grade math education level, into 7th grade math level on the CDL entrance exam? then they magically make it.

same guys though? claim i’m “so smart i am stupid”

same guys claim “my university degrees made me stupid”

same guys claium that the fact they cant spell simple words and cant perform basic math? but they make 40 dollars an hour, somehow translates into they are smarter than me, simply because they make more per hour. which i have trouble following the logic train of…

i am going to se if i can build the perfect beast. (apologies, mr henley)

Here is some information regarding throw:

The important relation is I=LA, where I is the luminous intensity measured in candela, L is the luminance, in cd/m^2, and A is the apparent area. So with a given emitter at a given current the beam intensity is proportional to the area of the lens or reflector of the flashlight. Of course, if there are imperfections in the lens or reflector you will get less throw than the relation predicts.