determining lens parameters
Lone_wolf_84:
You can determine the most important lens characteristic using the sun, a yard stick (or meter stick for you in countries that use sensible measurements), and an ant:
Hold the lens up so one side of it is facing the sun and lower it toward the ground until the ant is vaporized. Then measure the distance from the lens to the ant carcass. That is the focal length. That tells you how "strong" the lens is. Window glass has a focal length approaching infinity. A very strong magnifying glass may have a focal length of 200 mm. Then flop the lens over and measure the focal length on the other side. They may be the same, or they may be different.
This only works for positive lenses, like a PCX, DCX, or positive meniscus. If there is a negative lens in your OHP lens assembly, then another method is used.
If you can dissassemble your OHP lens assembly, then you can measure the focal lengths of any positive lenses. You can also measure the focal length of the whole thing. This is the critical number to know for calculating how the projection half of your system will work.
Negative lenses (ie. PCV, DCV, negative meniscus) have a negative focal length. That means they have an imaginary focal point on the same side of the lens as the light source. To measure one of these, use the sun around noon, two yard sticks, and a patch of clean concrete: Have your assistant hold the lens a couple of feet above the ground, so one side faces the sun. (No ant required. 😀 ) Find the outline of light coming through the lens by moving it up and down a bit and watching the circle shrink or grow. Then put one yard stick on the one edge of the light circle and against the corresponding edge of the lens. Put the other yard stick on the opposite edges of the light circle and lens. Where the two yard sticks cross is the focal point. Have your assistant measure the distance between the lens and the intersection of the yard sticks.
Or you can just combine a negative lens with a stronger positive lens, measure the combined focal length, and calculate back to the negative lens focal length:
1/fl(pos) + 1/fl(neg) = 1/fl(combined)
Don't forget the negative lens focal length in that equation is a negative number!
Your OHP fresnel is probably actually two of them solvent welded together. That's okay. Measure both sides focal lengths.
As for UV protection, the biggest danger is that you will fry your eyes working with a MH bulb. I found a paper on this that said most of the bulb envelopes filter out very little of the UV. Don't look at or work near a lit MH bulb. Or else wear lexan safety glasses: if you search the web for the absorbtion spectra of lexan and acrylic, you will see they absorb about 90% of the UV light. If your fresnels are acrylic, you already have pretty good UV filters! You can also buy Lexan-XL at home improvement centers with even more UV absorbtion. A piece big enough to use as a panel or fresnel support costs only a few dollars. There are also some better UV filter sheets you can buy here and there for <$10. it would be interesting to measure to see if they add much more filtering to a system with a piece of lexan and an acrylic fresnel between the bulb and the LCD.
Lone_wolf_84:
You can determine the most important lens characteristic using the sun, a yard stick (or meter stick for you in countries that use sensible measurements), and an ant:
Hold the lens up so one side of it is facing the sun and lower it toward the ground until the ant is vaporized. Then measure the distance from the lens to the ant carcass. That is the focal length. That tells you how "strong" the lens is. Window glass has a focal length approaching infinity. A very strong magnifying glass may have a focal length of 200 mm. Then flop the lens over and measure the focal length on the other side. They may be the same, or they may be different.
This only works for positive lenses, like a PCX, DCX, or positive meniscus. If there is a negative lens in your OHP lens assembly, then another method is used.
If you can dissassemble your OHP lens assembly, then you can measure the focal lengths of any positive lenses. You can also measure the focal length of the whole thing. This is the critical number to know for calculating how the projection half of your system will work.
Negative lenses (ie. PCV, DCV, negative meniscus) have a negative focal length. That means they have an imaginary focal point on the same side of the lens as the light source. To measure one of these, use the sun around noon, two yard sticks, and a patch of clean concrete: Have your assistant hold the lens a couple of feet above the ground, so one side faces the sun. (No ant required. 😀 ) Find the outline of light coming through the lens by moving it up and down a bit and watching the circle shrink or grow. Then put one yard stick on the one edge of the light circle and against the corresponding edge of the lens. Put the other yard stick on the opposite edges of the light circle and lens. Where the two yard sticks cross is the focal point. Have your assistant measure the distance between the lens and the intersection of the yard sticks.
Or you can just combine a negative lens with a stronger positive lens, measure the combined focal length, and calculate back to the negative lens focal length:
1/fl(pos) + 1/fl(neg) = 1/fl(combined)
Don't forget the negative lens focal length in that equation is a negative number!
Your OHP fresnel is probably actually two of them solvent welded together. That's okay. Measure both sides focal lengths.
As for UV protection, the biggest danger is that you will fry your eyes working with a MH bulb. I found a paper on this that said most of the bulb envelopes filter out very little of the UV. Don't look at or work near a lit MH bulb. Or else wear lexan safety glasses: if you search the web for the absorbtion spectra of lexan and acrylic, you will see they absorb about 90% of the UV light. If your fresnels are acrylic, you already have pretty good UV filters! You can also buy Lexan-XL at home improvement centers with even more UV absorbtion. A piece big enough to use as a panel or fresnel support costs only a few dollars. There are also some better UV filter sheets you can buy here and there for <$10. it would be interesting to measure to see if they add much more filtering to a system with a piece of lexan and an acrylic fresnel between the bulb and the LCD.
You where one of them disturbed little boys wernt you? 😀 😀
ANYwho even though in the middle there you lost me, after the whole any frying thing.. ""WOOSH"" i guess i will have to read it a few times, and give a couple attempts at controlling the ant population around my place.. I will let you know how it works out, I have a huge junk of lexan that I was using as a windshield this past race season, and its not to badly scratched up, so i will defiently give it a try.
ANYwho even though in the middle there you lost me, after the whole any frying thing.. ""WOOSH"" i guess i will have to read it a few times, and give a couple attempts at controlling the ant population around my place.. I will let you know how it works out, I have a huge junk of lexan that I was using as a windshield this past race season, and its not to badly scratched up, so i will defiently give it a try.
recycled lexan
If you can find an area that is still perfect, then that would be ok. But if this is going in your light path close to the LCD, you will see ANY imperfections. Another reason to spend a couple of bucks and get a new piece: Lexan-XL has better UV filtering.
I was just kidding about the ant, in case any PETA members are reading. No animals have been harmed in the production of this forum thread!
If you can find an area that is still perfect, then that would be ok. But if this is going in your light path close to the LCD, you will see ANY imperfections. Another reason to spend a couple of bucks and get a new piece: Lexan-XL has better UV filtering.
I was just kidding about the ant, in case any PETA members are reading. No animals have been harmed in the production of this forum thread!
LMAO so what did you use then?
I was also wondering, this mirror thats in there inbetween my two lenses... is it important?
I was also wondering, this mirror thats in there inbetween my two lenses... is it important?
it took 21 inches or 635 mm for the hypothetical ant to vaporize... same on teh back side, and i got the exact same from the other lense.. I couldnt get it to work with the fresnel though...
It takes under 2 seconds with a blow torch
Guys im sure we are aware of the saftey in here, please dont go and start fires with lenses, this is not only dangerous but foolish. The various posts above was only intended for a joke. The safe way to measure the focal length is doing it inside on a table with a room light on. We simply focus the light bulb to the table and then measure the distance from the lens to the table to get the estimated focal length of the lens.
Trev🙂
lol I did the indoor light thing. as of an hour ago i got a 17" 400:1 lcd with 15 ms(R), 10ms(F). With the lenses i got and the fresnel i got should I have a good start? i wouldnt mind to get a fairly decent imagine to keep my spirites high during the rest of the tweaking. SOOooOo after lots of reading and what not can i expect a good image?
measuring focal length vrs projection
>focus the light bulb to the table and then measure the distance from the lens to the table to get the estimated focal length of the lens
I don't know how big your room is (and how powerful your light bulb is), but under normal conditions this will give a VERY bad estimate! Focal length measurement needs a parallel beam, like sunlight. I just measured a lens with a light bulb about 10 feet from the lens, and got 680 mm. Then I took it outside and used the sun: 480 mm
This is consistant with the lens equation:
f = focal length
u = the distance of the object from the lens
v = distance of the image from the lens (may be negative)
1/u + 1/v = 1/f
When you use the sun as your object, 1/u is near 0 so it simplifies to:
1/v = 1/f and then to:
v = f
If they don't have a sun where you live, then you can use a light bulb, but you need to plug the distance between the bulb and the lens into the lens equation.
>focus the light bulb to the table and then measure the distance from the lens to the table to get the estimated focal length of the lens
I don't know how big your room is (and how powerful your light bulb is), but under normal conditions this will give a VERY bad estimate! Focal length measurement needs a parallel beam, like sunlight. I just measured a lens with a light bulb about 10 feet from the lens, and got 680 mm. Then I took it outside and used the sun: 480 mm
This is consistant with the lens equation:
f = focal length
u = the distance of the object from the lens
v = distance of the image from the lens (may be negative)
1/u + 1/v = 1/f
When you use the sun as your object, 1/u is near 0 so it simplifies to:
1/v = 1/f and then to:
v = f
If they don't have a sun where you live, then you can use a light bulb, but you need to plug the distance between the bulb and the lens into the lens equation.
As I look outside and see nothing but darkness and small drips of moisture i doubt i ill be able to test my focal length today.. Mr.Weatherman says we are going to get some sun today, but he as been wrong all week. So i will just sit here and cross my fingers.
the biggest room in my apt here is about 12 feet X 12 feet.
The OHP uses a EYB 360 watt 2000 lumans bulb,
Oh.. i found this on a website regardin gmy OHP:
Lens Speed & Focal Length: 14" Doublet
and ill kill more hypothetical ants when the sun comes out.
the biggest room in my apt here is about 12 feet X 12 feet.
The OHP uses a EYB 360 watt 2000 lumans bulb,
Oh.. i found this on a website regardin gmy OHP:
Lens Speed & Focal Length: 14" Doublet
and ill kill more hypothetical ants when the sun comes out.
Guy it works for me spot on everytime. Lets face it, the sun is dangerous and its not a practise that should be used.
Ive got over 100 lenses here and all work fine with the light bulb trick to their spec. If i go and get a 200mm fl lens out now and do the light trick, the distance from the table to the lens when the light bulb is focused on the table is just about dead on 200mm, give or take 1mm. Its exactly the same measurment with the sun.
The only problems may arise is if we have a 1000mm focal length lens because we run out of room and the room light becomes too close.
Have you tried this in reality and compared the 2 or are just relying on theroy?
Frensels also need to be split to have their focals measured acuratley, you need to measure each focal length on each lens to get any form of an acurate result.
Trev🙂
Ill add if your going to measure a frensels make sure its turned around the right way first, this also goes for PCX lenses or any lens that is plano.
Trev🙂
Trev🙂
well um, i only have one fresnel.. and if there are two, then there sealed tgether under the acrylic/plastic layers.. I did get that number off the corner though... still dont know what it means though, i think its porbably just a part number.
Yeah its 2 cemented together around the edges, my advice would be to go to your OHP's manufactures website and look up that part number, it should state the specs, if not hit them with an email. You will then have your acurate results for each frensel, the top frensel ( normally with the sticker on it) will have a different focal length from the rear, hardly any OHP's have frensels that have the same focal length on both sides.
Trev🙂
"Eppur si muove" ... Galileo
>Have you tried this in reality and compared the 2 or are just relying on theroy?
Let me quote my post:
"I just measured a lens with a light bulb about 10 feet from the lens, and got 680 mm. Then I took it outside and used the sun: 480 mm"
When I said "I just measured a lens", I mean that I really did before I made the post. I wouldn't post it unless I was pretty sure it was true.
But the lens equation is the fundamental description of converging lens behavior. The very definition of focal length is the distance from the lens to the focal point when a parallel beam strikes the lens. If you want to verify this, pick out one of the longer focal length lenses from your pile, measure it with a light bulb (and measure the distance from the bulb to the lens), then take it outside to measure it with the sun. If the sun is unavailable, then you can just measure it with the bulb far and the bulb near (ie. 10 feet versus 6 feet). Your measurements will be radically different. Since you will also have the bulb to lens distance, you can plug it into the lens equation to get the real focal length.
Then start remeasuring those 100+ lenses!
So lone_wolf: You don't have to wait for the sun! 😀
>Have you tried this in reality and compared the 2 or are just relying on theroy?
Let me quote my post:
"I just measured a lens with a light bulb about 10 feet from the lens, and got 680 mm. Then I took it outside and used the sun: 480 mm"
When I said "I just measured a lens", I mean that I really did before I made the post. I wouldn't post it unless I was pretty sure it was true.
But the lens equation is the fundamental description of converging lens behavior. The very definition of focal length is the distance from the lens to the focal point when a parallel beam strikes the lens. If you want to verify this, pick out one of the longer focal length lenses from your pile, measure it with a light bulb (and measure the distance from the bulb to the lens), then take it outside to measure it with the sun. If the sun is unavailable, then you can just measure it with the bulb far and the bulb near (ie. 10 feet versus 6 feet). Your measurements will be radically different. Since you will also have the bulb to lens distance, you can plug it into the lens equation to get the real focal length.
Then start remeasuring those 100+ lenses!
So lone_wolf: You don't have to wait for the sun! 😀
yip i luckly caught a break and just finished before it clouded back over and started raining, and same thing my best guess is that these are 600 mm, (im averaging inside, outside, then rounding)
So now that i know the focal length, what calculations need to be done to get specs of the box i need to build?
So now that i know the focal length, what calculations need to be done to get specs of the box i need to build?
If i didnt tap my credit card right out on my lcd purchase, id fork out the money for the lumalab forumn and design. but since i cant, i wont, and this place is better anyway 😎 
lol
so if anyone wants to give me some rough hel on an enclosure, id be extatic.
Also does it matter how far away the fresnel is from the light source? and does it matter how far away the lcd is from the fresnel? and the mirror inbetweem the lenses in the ohp head, does it do anything special?
lol
so if anyone wants to give me some rough hel on an enclosure, id be extatic.
Also does it matter how far away the fresnel is from the light source? and does it matter how far away the lcd is from the fresnel? and the mirror inbetweem the lenses in the ohp head, does it do anything special?
ONE more thing, ROUGHLY from one of the lenses to the mirror is 2" and same from the other lense to the same point on the mirror.
So does that mean when i put these lenses in my box, that one lense should be 4" away from the other?
So does that mean when i put these lenses in my box, that one lense should be 4" away from the other?
Well from my 2 years hands on experience i didnt beleive it for a second, you also didnt state what lens you are trying to measure or how you had it placed/turned ect.
That kind of depends on the focal of the lens in the first place doesnt it? I measure mine with a diffused light sorce btw and i dont have any problem, if you turn your lenses around the wrong way you will. 0 focal barrier is atleast 2x the focal of the lens, having it within this barrier will give you inacurate results. The sun is also a point source not just a beam of paralelle light. Manufactures measure lenses with precision lasers.
Why? they all have their correct focal measurment from their manufactures lol and that how i know the light trick works. Try using a energy saving light, i dont use a normal bulb.
Trev🙂
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