# Aspherical focal length



## snimmer (May 12, 2010)

<-----new member, first post. First of all, I ran across this site about 6 weeks ago and it inspired me to mod a gordon 3D halogne and offbrand 6v halogen into LED lights. Now I find myself looking at how I can squeeze a few funds out of future paychecks. How can I be 39 years old and just now running across a hidden addiction.

Enough about that, I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works? I have two other lights with short heads that I would like to convert if this might fly. Thanks for any assistance you could provide.


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## DM51 (May 13, 2010)

Welcome to CPF, snimmer.

I'll move your thread to the H&M section, where modifications are discussed.


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## gcbryan (May 13, 2010)

snimmer said:


> <-----new member, first post. First of all, I ran across this site about 6 weeks ago and it inspired me to mod a gordon 3D halogne and offbrand 6v halogen into LED lights. Now I find myself looking at how I can squeeze a few funds out of future paychecks. How can I be 39 years old and just now running across a hidden addiction.
> 
> Enough about that, I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works? I have two other lights with short heads that I would like to convert if this might fly. Thanks for any assistance you could provide.



Welcome by the way! I can't quite picture that or how that would be useful. I have two aspherical lenses and I don't see how you could stack them in the first place given their shape but I guess you are saying that if place them in front of each other it has the effect of shortening their focal length?

So if I had two 10mm focal length aspherics you are saying that I could put one 10mm in front of the emitter and another one perhaps 5 mm in front of the emitter and the pair together would focus the emitter in the beam?

I don't know a lot about optics theory but it doesn't sound right to me (that doesn't mean much however). I don't have both of the aspherics where I can try this out right now or I would.

I'm sure you'll be more responses soon. By the way, the issue at DX would be even knowing what the focal length was of the aspheric you were looking at. They only seem to list diameter.


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## TorchBoy (May 13, 2010)

snimmer said:


> I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works?


It does.

In practice it might be a bit hard to get just right, but if the two lenses you use are identical your new focal length will be half that of using a single lens. The power of a lens is measured in dioptres, which is the inverse of the focal length in metres. So if you use two aspheric lenses each of 50 dioptres (20 mm focal length), together they'll be 100 dioptres (10 mm focal length).

FWIW the human eye is about 60 diopters, only about a third of which is the lens (the rest is the cornea). You could argue that both the lens and the cornea are aspheric; if adding them together didn't work we'd have a lot more car accidents.

:welcome:


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## 007adan (May 13, 2010)

No, you would lose too much light with 2 aspherical lenses. In order for an aspherical lens to be effective it needs alot of light that can be focused into a tight beam and it needs to be properly collimated with the led. I know that the DEFT uses two lenses the first is a pre-collimator pcx lens and the second is an aspherical lens.


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## snimmer (May 13, 2010)

gcbryan,
to clarify, the DX comment was to use copper wire to build a bracket to hold the two aspherics close together at the end of the head (not where one aspheric was close to the led and the other at the end of the head.

007adan, I think you are right about losing too much light

Thanks for the responses everyone, guess I'll try to replace the current aspheric with an aspheric condenser to shorten the focal length


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## TorchBoy (May 13, 2010)

Actually 007adan and snimmer it depends what you're doing with your two lenses. I've used two lenses to make a tighter beam than anything a small single lens is capable of, and even though there was much less total light in the resulting beam (the second lens was capturing less than 10% of the light from the first lens), because it was so tight it had a much better range.

As I said, in practice it can be hard to get right, but it's worth a try.


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## gcbryan (May 13, 2010)

snimmer said:


> gcbryan,
> to clarify, the DX comment was to use copper wire to build a bracket to hold the two aspherics close together at the end of the head (not where one aspheric was close to the led and the other at the end of the head.
> 
> 007adan, I think you are right about losing too much light
> ...


 
Where have you found to be the best place for sourcing your lenses? I'm new to this myself.


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## snimmer (May 13, 2010)

I have only dabbled with a couple of glass aspherics from DX. Recently read about the aspheric condensers but having trouble finding them in 23mm diam. They supposedly have shorter focal length. Kaidomain has a few as well.


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## 007adan (May 14, 2010)

Dx has some really nice aspheric lenses, they call them glass optics however. The 66mm is about 2.5 inches which is the biggest lense they sell $4 what a steal. To get a 3 inch lense you need to go to ebay and buy a HID kit that come in pairs only.


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## 007adan (May 14, 2010)

Torch boy,

Any luck using a pre-collimator lense like saabluster uses on his deft? It consists of a pcx lense that must capture close to 100% of the led light before it hits the aspheric lens. It sits almost directly over the led. I think the eye lense from a pair of binoculars would be perfect for this.. Most binoculars have a 50mm lens which condenses down into the eye piece of a pair of binoculars. I'm guessing the reverse would be true in making a killer thrower. The smaller lens concentrates and captures the light from the led and the larger aspheric takes all of this focused bright intense light and makes a nice tight resolved beam.


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## Walterk (Jun 1, 2010)

Anyone tried this yet? Should work.

Someone on CPF made a dive light with a Macro lens as aspheric. 
Think that should work also. At least they have short fl.


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## TorchBoy (Jun 1, 2010)

I have one of the DX 66 mm aspheric lenses on the way that I'll do some playing with. It might arrive by the end of the week.


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## Nos (Jun 2, 2010)

TorchBoy said:


> I have one of the DX 66 mm aspheric lenses on the way that I'll do some playing with. It might arrive by the end of the week.



I threw this lens right away. It produced 7 hotspots of one emitter :shakehead


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## gcbryan (Jun 3, 2010)

Just for what it's worth (not much) but in playing around with some aspherics today I noticed that they work when reversed at a different focal length.

I'm not sure if they are just as bright when reversed...it was somewhat difficult to tell.

I had one 28 mm aspheric that was focused in a 6P clone when it replaced the front lens. I had another 28 mm which has a shorter focal length and is therefore "overfocused" at that same point.

I reversed that aspheric so that the curved surface faced the emitter. It was now in focus.

I tried this with several other aspherics as well.


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## TorchBoy (Jun 6, 2010)

Nos said:


> I threw this lens right away. It produced 7 hotspots of one emitter :shakehead


I wonder if it's a bit of a variable quantity. After a small amount of testing I've found that mine (which did arrive on Friday) has what I presume is a very subtle nipple lens on the top, meaning the lens has two focal lengths. Apart from that it works very effectively for focussing a Cree XR-E; even with the LED run at 1 W it throws 200 metres easily. I've made a 2x D cell focusable torch with it that should last about three hours on a pair of Eneloop AAs in D cell adapters.

To belatedly sort of address 007adan's question: Using another smaller aspheric lens between the LED and the 66 mm DX aspheric lens (I actually used a C78 1x AA zoom torch on widest zoom) produces a very short focal length (the 66 mm lens is only ~4 mm off the front of the C78) and the projection of the LED die is larger and correspondingly dimmer.


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## Paul Baldwin (Jun 6, 2010)

TorchBoy said:


> I wonder if it's a bit of a variable quantity. After a small amount of testing I've found that mine (which did arrive on Friday) has what I presume is a very subtle nipple lens on the top, meaning the lens has two focal lengths. Apart from that it works very effectively for focussing a Cree XR-E;



I've got one of these lenses and I just cant get it to focus properly, I put it down to it being a second/reject. Others seem to have recieved better ones but it's a lottery, you can't expect too much for the amount they cost. I took a picture of my mc-e with a dead die in my projector thread IIRC.







I'm sure my results would have been better if I had received a better lense? I played about with various dx lenses but their quality seemed to be a bit suspect too. In the end I found I got my best results using a 50mm lense as a pre-collimator.











This is still far from ideal as my best results were achieved with the larger lense touching the 50mm.
When funds allow I'll get some higher quality lenses from somewhere like Surplas Shed and try an SST-90 and have another play.


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## Walterk (Jun 6, 2010)

Interesting setup!

What f-number have the lenses?
(or diameter and focal length for that matter)

For a narrow beam you need the largest lens to have a relatively long focal length. The first lens is there to catch light from the Led, the second determines the beamangle (and thus intensity) of the outgoing beam. 
The f-number of the second lens better can be 4 then 0,5. 

Also, I think the use of a pre-collimator is better once there is a substantial difference in diameter. Say 1:3 or 1:5 is great. If the diameters are to close together, the losses of rays going through two lenses is not made up with the tighter collimation.

I am working on a similar setup.


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## Paul Baldwin (Jun 6, 2010)

Walterk said:


> Interesting setup! I am working on a similar setup.



I hope it's more professional than mine lol. 

I'm afraid I haven't any information on the lenses apart from which ones they were. I didn't take any measurements of focal lengths etc as I was using a light meter on the projector output to measure the difference overall. This was my first play with lenses and I was just trying to get a watchable image from a projector with a dead bulb. I liked the fact the led could potentially last 50,000 hours instead of 2,000 for a bulb!
I'll have a look and see if I can find links to the lenses I used.


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## Paul Baldwin (Jun 6, 2010)

http://www.dealextreme.com/details.dx/sku.13618 
66mm-24mm 2-90 degree.

http://www.dealextreme.com/details.dx/sku.4584
22mm 5~80 Degrees (focal length 25mm according to the comments)

http://www.dealextreme.com/details.dx/sku.5297
28mm (someone in the comments reckons 16mm focal length to get an image of the die)

I wouldn't really recommend any of the above lenses for this sort of application.

http://www.kaidomain.com/ProductDetails.aspx?ProductId=1753
Specifications: 
Focal length: 34mm
Diameter (overall): 52mm 
Inner lens diameter inside the bezel: 46.8mm 
Material: Optical glass, double sided Anti-reflection coating, 
light transparency 98.8%+ (UCL grade) 
Center thickness: 25mm 
Back focus length: (I measured 22.5mm)
I think this lense is pretty good quality to be honest, it focuses the dies very clearly. I wish the 66mm had been equivalent as I'm sure my results with better smaller lenses would have improved my results even more.

I ordered all the above without really researching anything much tbh, they were just cheap. I did however find one of these which may be a slightly better holder for my MkII version? http://www.dealextreme.com/details.dx/sku.34509

I did find a usefull nugget of info here quite recently tho https://www.candlepowerforums.com/threads/248766 Check Saablusters post #3

electrodacus has also got some usefull info, photos and results in this thread http://lumenlab.com/forums/index.php?showtopic=24619&st=0 which got me started in the first place 

I think I'd go for one of these next http://www.surplusshed.com/pages/item/l3788.html with an SST-90 and possibly a pre-collimator but they don't list international shipping costs and in my experience of firms that don't it's usually because they are totally unrealistic compared to the likes of KD,DX etc.


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## Walterk (Jun 6, 2010)

No expert, but did a lot of reading, the results posted in:
Aspheric lens tricks? post 12
What is the f-number of your aspheric lens?

From my experiments, a f#0.5 to 0.6 in combination with a larger lens with f#2.85 performed very very well.
The distances inbetween the lenses being: (the focuslength of the large diameter lens) minus (the focuslength of the small lens), AND the Led was closer to the small lens then its focal length.

As you described I experienced that there are two distances at which the first lenses works well as precollimator. I found the longest focal length of these two significant better working. A test-bench adjustable for different sizes of lenses is gold here.


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## Th232 (Jun 6, 2010)

Paul Baldwin said:


> I think I'd go for one of these next http://www.surplusshed.com/pages/item/l3788.html with an SST-90 and possibly a pre-collimator but they don't list international shipping costs and in my experience of firms that don't it's usually because they are totally unrealistic compared to the likes of KD,DX etc.



In my experience, shipping has been very reasonable with Surplus Shed, $5 for my last order for 5 lenses.


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## Paul Baldwin (Jun 8, 2010)

I'm no good with numbers so I did a little experiment. Thats an xpg R4 and an xre P4 both mounted in X2000's. The xpg has the standard X2000 lense and the other my daft temporary idea, what do you think? I could do with seeing what the outputs of both are with my light meter. A DEFT it isn't you'll probably laugh when you see how I did it?


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## gcbryan (Jun 8, 2010)

I'm not quite sure what we're looking at?

Is this a wall shot a few feet from the camera? If so the XP-G looks huge and the XR-E looks about like what I'd expect.

When you say the XP-G is in the X2000 is it set on max throw?

Thanks.


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## TorchBoy (Jun 8, 2010)

That pic reminds me of the double beam I got from my 66 mm DX lens yesterday when I pointed it at the edge of a table, pointing down. (I mentioned in post 16 above that it has two focal lengths.) A tiny die image was projected onto the table, and a larger die image onto the floor, both in focus.


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## Walterk (Jun 8, 2010)

@PaulB: The difference is remarkable ! 

Are this the two lenses in the frame with heatsink fom the photo in your earlier post? 
If so, what lenses are they? (you posted so many links to lenses I am lost) What is the distance between the two lenses plane surfaces?
What is the distance between the Led and the small lens?

Seems good, pretty effective!, but need more information to expirement with a similar assembly...!

First measure the distances 
Second try take beamshots at larger distance (outdoors?) , thats the best testcase to see how good it works.
Oh, and three, post the beamshots


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## Paul Baldwin (Jun 9, 2010)

Sorry I was in a rush when I posted yesterday. The xpg is on max throw but doesn't focus totally crisp. I need to shim it to achieve that but it doesn't really effect my real life usage of it so I haven't bothered. 
Both torches were 7' from the wall. The xpg die is 7.5" across and the xre is 1". The xre die image with the standard X2000 lense at the same distance is 6" across.
It really isn't a practical design for outside use and there will be an awfull lot of transmission losses I'm sure. 
I did it because the projector project I have needs about a 5 degree light beam to work effectively according to electrodacus. This wasn't really achieved with my mc-e setup. I had out of interest shone a standard X2000 into the projector and got a fairly bright image considering it's relative low power in the past. I idly wondered that if I could narrow that beam further it may improve results more? I haven't had chance to test this out yet tho.


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## Walterk (Jun 9, 2010)

A spot of 1 inch at 7 feet distance means the beam has an angle of 0.7 degrees, thats neat.


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## gcbryan (Jun 9, 2010)

That's tight for sure. Just as another comparison the tight center spot in my Uniquefire HS-802 reflectored thrower is 3 inches at that same distance.


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## Paul Baldwin (Jun 10, 2010)

TorchBoy said:


> That pic reminds me of the double beam I got from my 66 mm DX lens yesterday when I pointed it at the edge of a table, pointing down. (I mentioned in post 16 above that it has two focal lengths.) A tiny die image was projected onto the table, and a larger die image onto the floor, both in focus.



That sounds like quite a strange flaw in that lense, quite different from the one that I received!
As for the 0.7 degree beam I did it with this lol






Its specs are 15-60x60 and fully adjustable*  I simply removed the head of the x2000 and adjusted the eyepiece, zoom and focus rings until I got a crisp image of the die  While playing about with it I did note that reflected light bouncing off the other corner of the wall increased at some points when I had a nice diffuse round hotspot showing. 

* It's a bird spotting scope or for wildlife/perving with a camera if you have a very fast film  15-60X magnification.


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## gcbryan (Jun 10, 2010)

Sweet! But does it have a clip and can it tail stand?


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## Paul Baldwin (Jun 11, 2010)

Lol, I'm sure a tailstand would be possible but if it ever fell over I doubt it would do it much good. The clips a no go I think but it attaches nicely to a tripod. :thumbsup:
I don't know how much these sort of scopes are now second hand on ebay etc, this ones 20 years old so I would think they are a relatively cheap source for quality lenses?
Still havent had chance to try it out with the projector yet, wish I had some spare cash to buy a few more lenses to play with and an SST-90.


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## Tally-ho (Jun 11, 2010)

Walterk said:


> A spot of 1 inch at 7 feet distance means the beam has an angle of 0.7 degrees, thats neat.


Just to try it, I put a flood to throw flashlight with head in flood position behind the lens of a 4x5" camera chamber. I made the focus and it projected a perfect image of the die that measures 10.5 cm by 10.5 cm on a wall at about 3.10 meters (310 cm) from the die.
That is about 4.13 inch at 10.2 feet

Once the aspheric lens removed from the flashlight, the projected image of the die is about 1.6 x 1.6 inch at 10.2 feet :naughty:

Flashlight is (sku.18697) C30 flood to throw, XR-E.P4
Lens is a fujinon.sw 105mm opening at f:8


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## gcbryan (Jun 11, 2010)

I tried something like that as well. I got the sliding front lens from an old slide projector and took the head of my flashlight. Due to the focal length of the projector lens I had to hold the flashlight about 10" away but the emitter image 18' from the wall was less than 2".

I'll have to try it outside tonight.


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## Paul Baldwin (Jun 12, 2010)

gcbryan said:


> I tried something like that as well. I got the sliding front lens from an old slide projector and took the head of my flashlight. Due to the focal length of the projector lens I had to hold the flashlight about 10" away but the emitter image 18' from the wall was less than 2".
> 
> I'll have to try it outside tonight.



What's the verdict? Have you tried your setup outside too Tally-ho? It's rather more compact than my effort lol.


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## gcbryan (Jun 12, 2010)

Paul Baldwin said:


> What's the verdict? Have you tried your setup outside too Tally-ho? It's rather more compact than my effort lol.



It didn't work so well due to the low illumination level. In the house it looked bright but very small. Outside it didn't get that much bigger (proportional of course) and by the time it had any real distance it was just hard to see.


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## Tally-ho (Jun 13, 2010)

Paul Baldwin said:


> Have you tried your setup outside too Tally-ho?



I tried from my window to light the wall of a building that is about 110m away but the projected image of the die was not bright at all.
To make the focus, the LED is about 3.54" from the back lens so a part of the brightness is lost because of the distance between the LED and the lens.
And brightness is also lost each time the light goes through a glass lens, this fujinon is made of 6 elements (6 glass lens), maximum aperture is f:8.


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## gcbryan (Jun 13, 2010)

That was another factor with my projector lens...it was made up of at least two lenses and maybe more.


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## Paul Baldwin (Jun 13, 2010)

I'll have to try this one out one of the evenings then. The fully focused version wouldn't have any practical use I can think of at the mo but I'm interested to see what it would be like as a diffused spot. I definately noted a sweet spot where there was a noticeable amount of reflected light.


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## gcbryan (Jun 13, 2010)

I've found the quick way to test out individual lenses if you have any loose lenses laying around is to just use a cardboard tube (from toilet paper) and shove a flashlight that uses a XR-E R2 in one end and hold the lens over the other end.

I did a quick test of a 50 mm lens from DX last night using that method.

It throws well of course!

For anything larger in diameter than that I'll have to make a larger cardboard tube.


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## Walterk (Jul 7, 2010)

Lol !

And thats probably how we all started messing with lenses.

I started with a 300mm zoomlens, but found it was not the solution, too litle lumen to be visible in a distance.


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## gcbryan (Jul 7, 2010)

Walterk said:


> Lol !
> 
> And thats probably how we all started messing with lenses.
> 
> I started with a 300mm zoomlens, but found it was not the solution, too litle lumen to be visible in a distance.



Not our friend Dr. Jones...he started out with a cat food can.


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