Planning an aspheric scratch build

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: Flashlight Optics - Dome, Dedoming and 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.

It’s actually not that complicated :slight_smile:

If you keep the same diameter, reducing the focal length will increase the amount of light collected, making the projected die image bigger but not increasing the lux.
More lumens, more area, same lux.
If you keep the same focal length, increasing the diameter will increase the amount of light collected, keeping the same size die image and increasing the lux.
More lumens, same area, more lux.

Ideally, an infinite diameter and infinite focal length would give the best throw, however since we can’t use giant lenses in a handheld flashlight we need to find a good compromise between diameter and focal length.
Too large diameter with short focal length will make a very large spot, not very useful for long range.
Too long focal length and tiny diameter will make a teensy weensy spot that barely lets you see anything, as almost all lumens get wasted.

Other things that make it complicated are how accurate the asphere is, what material is used, what coating is used, what purity the glass is, and stuff like that.
A lot of cheap “aspheric” lenses I have bought are not perfectly aspheric causing a corona around the projection which doesn’t help at all with throw.
Also, bi-convex lenses are a no-go.

I’d prefer to avoid focal lengths of 100mm or more. A 50mm focal length would allow a reasonably compact head . The 100mm lens with an 80mm focal length would be the next option. Wouldn’t the hemispherical reflector and the size of the opening also affect the apparent brightness and beam? This won’t be just a lens and emitter.

It’s not bad at all.
Most of the farthest throwing flashlights in the world use lenses that are similar to the one you’re looking at.
The DEFT-X, rev victor enthusiast, mjolnir, black bullet, etc… all use lenses that are ~75mm diameter and ~40-80mm focal length.

Not a “definition”, but the concept is that it’s how much intensity is needed, that when projected at a certain distance lights up an object enough to be viewable.

The farther something is, the narrower the cone from you to that object. So your beam (that cone) has to be intense enough to light up the object X amount. That’s why a “thrower” has a narrower tighter beam, vs a “flooder” that has a wider more spread-out beam.

Just gotta watch out for those clayheads, then.

well, my PROBLEM that gets me so hopping fired up from a mathematical point of view? now tat i calmed back down and literally chewed up several generic tylenol, and drank a pot of coffee, dropping numerous throat lozenges into the coffee to open up sinuses that are kicking my @$$ soundly… and VYING with the rotten TOOTH infecting off and on downinto i suspect perhaps past the gum and into my very JAW… i honestly dont know if my sinuses kicked the toothinfection off this time, or, of the tooth infection kicked up into my sinuses? all i know is i have chills and fevers, i am hot and cold all at once, i am shivering and sweating… i have no longer “just” yellow and grees goo coming out of mu sinuses and nose and up from my lungs and throat? the goo now has scabe and blood in it as well… i mean really when you are coughing up blood, that just CANT be good. but, the radiating pain is coming from the whole head, not just my jaw and tooth, and not just from my sinuses and head… they have all joined forces to whoop my butt. LMAO.


MY position, is that the F-number means something. that the RATIO of lens diameter to focal length? does something. that it does something useful.

it means something to an “old hand”, its EASILY demonstrable. NEWBS can understand it all easily. an equation is an equation. not only THAT, its accepted in camera lenses, its accepted by photographers SWITCHING lenses, that might vary in diameter and might vary in focal length, inorder to set shutters and irises properly.

not only that, its perfectly accepted in the world of projectors, which use the same f number, the same way. and the SAME camera lenbses can be used as projector lenses if you really want, and the same projector lenses can be used as imaging lenses, again if you want to.

to the WHOLE camera world? F number MEANS something… and it means “brightness”

thats not really “my” position, thats pretty much the whole world of cameras and projectors and stage lighters, everyone.


now, what my “problem” is? is that each and every time i so much as MENTION the concept of an F number as a useful handy guide?

as soon as its in the fladhlight world? i hear the following…

“no no no, you dont know what you are doing. focal length is useless. focallength only affects BEAM SHAPE… only the diameter of the lens matters.”

and? they state it AUTHORITATIVELY. as if F-number as a measure of a lenses ability to transmit light is somehow some wacky flat earth theory.

then they start in with surface luminosity, and they start “estimating” lumens and kilocandela and luxes and all this stuff, and they start shining their ligts into these cool little meters and everything… and they somehow claim to “prove”, that the ENTIRE camera, projector, and telescope WORLD does not understand the “concept” that is so superior.


i get tired of this “beamshape” stuff too.

thats WHY camera people HAVE the F-number, it relates the dance between focal length and lens diameter, each of which contribute as per their ratio, to the final product.

and that waiven collar? i dont undersdtand what that has to do with the lens… if your putting “something” over and around the emitter that makes it brighter or whatever? fine…

but, staying on point?

1) my position was that lens diameter, focal length… which is the f-number… “mean something”. that it is a useful mathematical unit to assign relative ability of a lens to transmit light.

2) “they” position… and i mean EVERY SINGLE TIME i ever mention the f number, on this and 2 other sites? it gets stated WITH AUTHORITY that focal length is “meaningless”, that only DIAMETER means “anything”.


now, someone up there started himming and hawing around and i did notice the words “focal length” creep into the explanation THEY offered. THAT would indicate they were claiming that “option 1” was right.


but, newbs walk away from all the megalux candelabra estimated meter illuminance “thing” with one thing and one thing only…

“oh! sedstar doesnt know what he is saying… i just need the widest lens i can find, nothing else matters. thats easy. nothign else matters. everyone says so.”

and it AINT.

why does the entire camera world, the entire projector world, the entire telescope world… suddenly become invalid, when its a flashlight?

why all of a sudden does “diameter” claiming to be everything, somehow magically spring into life. why all of a sudden does anyone get to claim that F-number means nothing and is somehow invalid?

i listed the 2 options… i am #1. easy is #2. EVERYone always “corrects” me on my “supposed childish stupidity” when i dare mention the f number as if its some archaic concept that has no application.

the “interim” explanations up there? they are CONTAINING the word”focal length”…

heck, while were AT IT ? i think it was enderman? said the best observable lens he ever saw? to date? was a plano convex 100mm fast tech lens. how can THAT be? because according to “coghlan’s laws… of absolute throw”? (movie: “cocktail” joke, lol)

—- aspherical are ALWAYS better than ANYthing else. never any deviation.
—- only lens diameter does A-n-y—h-i-n-g, focal length is irrelevant, it only affects beamshape.


and newbs walk away thinking they can ignore focal length and that f-numbers mean “nothing” and that any aspheric, as long as it has a bigger diameter? will A-l-w-a-y-s win the day.

then what do we all do in REAL LIFE? we all do the same thing, and we all know it…

we pick the biggest diameter lens we can comfortably fit into our existing light, or feel comfortable fabricating, because i admit, theres no replacement for displacement… its true. but THEN we “trial” different focal length lenses, and with the “lovely asphericals” which cant be FL measured (i can, mathematically too) we pick different lenses, with different FLs… and we see which one “does better” and we can “see further” with…

but… you have your diameter, you ave your focal length which to ME clearly does “something”… and that “dance” is simply the F NUMBER…

and everyone waltzes in and smugly corrects me that “only diameter increases throw”… which is crap.

see, here we go… lightbringer has stated…

Not a “definition”, but the concept is that it’s how much intensity is needed, that when projected at a certain distance lights up an object enough to be viewable.

The farther something is, the narrower the cone from you to that object. So your beam (that cone) has to be intense enough to light up the object X amount. That’s why a “thrower” has a narrower tighter beam, vs a “flooder” that has a wider more spread-out beam.

ohhhhh. see, everytime someone and it happens constantly, mind you… everytime someone smugly CORRECTS ME, and makes me look STUPID, as if my “wacky flat earth conspiracy theory” is so off the wall as to be childish? as soon as i mention f-number, i instantly dont know what i am talking about. THEY are dropping knowledge, to get the correct information out, that never changes: “only diameter increases ‘throw’ ”

now its not a definition, its a “concept”? and this “concept” seems to be, that when you want a brighter image at whatever distance, if you need more… you up the amps. i dont disagree with this “concept”. you can call it what you want, surface illuminance, megacandelabras,whatever… more amps makes more kick. i wrap my head solidly around that “concept”. I firmly acknowledge it.

okay, next comes the “beamshape” argument, and the cone of light, and the QUALITATIVE (not “quantitative”) words thrower and flooder always come out. again, i have no problem with the concept. i fully acknowledge it.


WHERE in this “concept”, is anything that says “only diameter increases the ability of a lens to do its job… and that focal length changes do nothing. you are WRONG to use the ”concept” of the F-number”

because THATS what i keep getting constantly “politely corrected on”,m and i dont findit to be a correct statement.

the technically vague and meaningless “concepts” of flooder, thrower, cones of light….?

you are doing nothing but describing, in qualitative terms, the exact same QUANTITATIVE notion of the “humble f-number”

hm. let me maybe state my argument another way…

i take a LENS out of my pocket. I toss it on the table in front of you. and you say “what can this puppy do? how good is it?”

now, no talk of any surface intensity, because I now have to describe the capabilities of THIS LENS to YOU. I can state that “this lens has diameter x”. i can also state that “this lens has focal length y”. thats all you need to know, to understand mathematically, the capabilities of that lens. The ratio of x and y ? its called the F number.

now, surface brilliance, and megacandelabras, and all the authoritative sounding use of words like flooder/thrower/intensity etc… none of that means anything. For all i care? you can use this lens to transmit the image of a blue fart out of a cow’s @$$ you just lit on fire with a zippo lighter.

because as humorous as that mental image IS? its very very accurate.

you can talk about all the “concepts” you want to. NOTHING else is describing this lenses ability to focus how much light thru it? other than FOCAL LENGTH and DIAMETER. you can talk about surface coatings, and style of lens production as slightly more or less percentage of efficiency? youre only scootching the numbers up and down.

and we really COULD be talking about how bright an image we can transfer onto a camera sensor, of a blue fart coming out of a cows $$... because the image of that blue flame shooting out of that cows $$ will be BRIGHTER as the f-number goes down, and it will be dimmer as the f-number goes up.

period. end of story. no camera guy… no projector guy… no refractor telescope guy…not ONE of them will correct me, and tell me “no no, only diameter will increase the throw… of the image of a blue flame shootiong out of a cows @$$ onto whatever you wish to throw it onto”

i ask for a definition of “throw”, i get whatever i get

i ask for a mathematical formula? i always get that intensity formula… which means exactly SQUAT to the ability of the LENS to transmit a dimmer or brighter image of the blue fart flame coming out of the cow’s @$$.


now, i am correctly stating the lens’s ability to transmit how bright or how dim of an image? because nothing else in the way of a spec exists FOR THE LENS.

if you want to RELATE how many “moon bats” are going to be produced, per unit of time, per unit of area, per unit of whatever… all fine and dandy. But… i keep scratching my head, trying to understand how i am “wrong” using the f-number to describe the abilities of THIS lens o the table right here? before we build a light?

and everyone keeps CORRECTING ME, with the ring of authority… witha smug certainty they are conveying sage knowledge that i am too stupid to grasp? that “only diameter increases throw, focal length is nothing”.

which is DEAD WRONG, and i am DEAD RIGHT.

it doesnt matter how many “moon bats” ar going thru the lens out of the cows @$$… this lens, independent of WHAT you do WITH it? has 2 things that can mathematically describe it. The focal length, the diameter. period. thats IT.

the ratio is the F-number. done.

now… if you tell me the SIZE of the flame shooting out of the cows @$$? i can take the focal length, and tell you at what distance, exactly how tall and wide the projected image of that blue a$$ flame is… and with mathematical certainty. its not a “concept”… its a cold hard fact of geometry. Its honestly not open to mathematical argument.

the diameter and the focal length of the lens? and again thats all there is to mathematically decribe that lenses capcbilities… can be expressed AS a “numerical aperture”, and we can discuss what geometrical percentage OF the number of available moon bats get to fly thru the lens? but once again…

now… how in the &^%$ does it ALWAYS happen, that i get constantly CORRECTED, as if i am somehow WRONG… when i bring up the f number?

because you can make the cow fart harder and make the flame brighter. you can teach the cow to clench its @$$ cheeks together tighter? so the flame is smaller and more intensely bright of a blue flame… but… none of that has the slightest bearing on the capabilities of THIS LENS right HERE on this table.

i can accurately predict any number of things about what the lens can do, and it all flows from only 2 things… focal length and diameter. glass composition and shape are just scootching the percentages up and down a little.

because maybe the WAY you describe the “concept” of “what throw is” needs to be stated a little better… because ALL i keep hearing is “only diameter increases throw, dude…”. All your talk about intensity? is utterly meaningless.

i dont CARE how many moonbats you are making per square kilocandelabra, it doesnt describe the lens… you are describing wat you are going to shove thru the lens, and predicting how many moon bats are going to appear per square foot out the other end.

no one has said one single thing, that makes the f-number NOT a valid mathematical description of the lenses ability to transmit how bright of whatever image later when we build something with it.

and i’m sorry? but the following statement…

“only diameter increases throw”

exactly WHAT impression is that supposed to make in the mind of the new guy? and exactly how am i “wrong”? because i keep getting CORRECTED… as if i am wrong,when i state the f-numbers…. because focal length only affects beamshape, dude…

well, i got news for you? “beamshape” is everything! the f-number is critical! i’m telling you i can geometrically predict all kinds of stuff everyone wants to know, i can relate different size emitters to exact size patches of target, i can adjust focal lengths to illuminate the same size patches between different size emitters.

i can even put numbers on 2 lens compound lens setups… easily.

and all i get is…

1) your WRONG duuuude…. only diameter increases throw, duuuude. Its, like, all about the intensity of the throw, duuuude. Your focal length? its… like… way wrong, duuuuuude.

2) and i got, like, seventeen different kinds of math to back me up? and to accurately predict things? and all i get is told i am wrong. and i really dont think i am.

3) now? its a “concept” not a Law, or an equation? what ARE all you guys SMOKING? I want some of the good stuff.

sedstar, I didn’t mean to disrespect you. Sometimes my tone can be misinterpreted when I write on forums.

All I meant was that the throw (candela) and light collection efficiency can be considered separately, not that the F number is unimportant.

Hey, I was just trying to help out, describing by analogy vs throwing out technical terms, equations, other technobabble.

If I take 1000lm and use the chip as a mule, it’s got all flood and almost no throw. I won’t be able to see something 100yd away, let alone 1000yd away.

If I take 1000lm and focus it to a pinpoint, it’ll have lots of throw. Something 1000yd out will light up nice and bright.

I didn’t mention reflector size, lens size, f-number, anything of the sort. I tried explaining “throw” as a concept. Shine a 1000lm S2+, and a 1000lm C8. You’ll know the difference between throw and flood. How would you define it, then?

I wasn’t trying to “correct” anyone in any way. You said you were still waiting for a definition of “throw”. Question is, will you accept one, or not? I’m not sure what kind of acceptable definition you’re looking for.

Fwiw, a throw of a measly 10yds can be a floody S2 with OP reflector and diffusion film at 1000lm turbo mode, or a dedomed XP-G C8 at 30lm low-mode. The only thing that matters is how much light is reaching the target at said 10yd away. No mention of reflectors, lenses, f-numbers, nothing.

I ordered the 100mm 135mm fl borosilicate lens from fasttech and will also order the 100mm 80mm fl lens elsewhere. The longer focal length will require a longer head but since it is a scratch build I’ll just make it a big tube with a handle mounted switch. The size will allow mounting of three 18650s or two 26650 across the tube side by side rather than the typical position in line with the head. Additional factors I considered were presumed refractive and thermal properties of the unknown (crown?) glass and borosilicate. Before ordering the 100mm 80mm fl lens I will contact the seller again to find out what type of glass it is. I will also search for a shorter focal length 100mm lens (50mm fl if I can find one) and order a double convex lens and do as sedstar has suggested. It will be interesting how the claimed and actual focal lengths compare. It may take a while before I have all of the lenses here. An ‘everything else being equal’ comparison of 100mm 50mm fl, 80mm fl and 135mm fl lenses would show the influence of focal length and the related ‘f’ values on the projected beam. Maybe I should add a 100mm 100mm fl to the group.

The focal length values above are info provided by the seller. The actual values will be determined by testing.


let me try a different approach approach to this.

is anyone saying that the f-number equation… is not a valid equation?

yes or no.

Valid for what?
A 10mm lens with 10mm focal length has the same f number as a 1000mm lens with 1000mm focal length, however the 1000mm lens will throw further because the delta of incidence angles is smaller.

I think you guys are forgetting that an LED is not a point source of light, and therefore it is also important to have the optic as far away as possible from the LED (in order to minimize the delta).

@sedstar Not me. I just want to check the accuracy of claimed focal length and show for a given lens size the effect of focal length (and related f number) on the beam projected. As a former calibration tech (metrology) quantifiable data and related testing have a special significance for me.

If you know the significance of the 4:1 ratio in metrology you get a cookie.

The relationship between lens diameter and focal length are in a way (to me anyway) similar to resistance and capacitance in an rc circuit.

FWIW, the first stage of my plan is to use a set diameter lens and a range of focal lengths with the same emitter. The second stage is to use a set focal length and a variety of diameters with the same emitter. Third stage is lenses with the same (or as close as possible ‘f number’. To minimize cost the head tubes will be rolled aluminum flashing sprayed flat black inside. I’m not wealthy so it will take a while to accumulate lenses to illustrate the relationship between lens diameter and focal length and the resulting beam.