# Asperhic lens? What is it?

#### whitenoise

##### Newly Enlightened
This is probably a really dumb question, but I'm clueless when it comes to such things and I'm new here! I have seen people talk about asperhic lens' but I'm not actually sure what they are. I've seen some beam shots here where the lights seems concentrated into a 'square' shape with no spill, is this the lens?

Maybe I've got this backward, but surely if you concentrated all your light into a small area wouldn't you have a massive hotspot on what you were trying to look at/take a photo of? Say you were looking at a wreck or something, wouldn't it be better to have more of the wreck lit up than say a tiny area?

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#### bretti_kivi

##### Enlightened
an aspheric will accurately reproduce the die of the LED on the wall. Very bright hotspot, very little spill. Great for throw.

Choose the right light for the use. That's why some reflectors have orange peel or stipple, to increase the hotspot. I'm sure this is in some beamshots somewhere...

Bret

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#### Z-Max

##### Newly Enlightened
with aspheric lens you can do any beamshot. If distance between lens and LED big (focus lens)- you have very hotspot, but if you make distance smaller - you may take foold beamshot with Uniform luminescence without artefacts.

example reflector:

aspheric hotspot:

aspheric flood:

#### jspeybro

##### Enlightened
actually, a 'normal' lens is a spherical lens. This means that the curved surface of the lens is a part of a sphere. If you would draw a line over the surface, this would be a part of a circle.
This is very interesting from the point of view of the manufacturars, since this is easier to make.

an aspheric lens has a surface that is not a part of a sphere. there is some more complicated mathematics involved in describing the surface mathematically. Also, this is more difficult to manufacture.
Since we see a lot of plastic lenses lately, these are easier to make because you just make a mold and extrude the plastic in the mold. You just need to make the shape of the lens.

aspherical lenses give you the advantage that, when designing the lens, you have extra parameters to compensate for unwanted effects like abberations, but this only works good if you don't deviate too much from the optical axis of the lens (here, we would say when the beam angle is too big, an aspherical lens wouldn't be very useful or better compared to a normal spherical lens). This is also a reason why these lenses are, as far as i know, only available for small angle beams.

hope this makes a little sense to you guys :thinking:

#### 2xTrinity

##### Flashlight Enthusiast
actually, a 'normal' lens is a spherical lens. This means that the curved surface of the lens is a part of a sphere. If you would draw a line over the surface, this would be a part of a circle.
This is very interesting from the point of view of the manufacturars, since this is easier to make.

an aspheric lens has a surface that is not a part of a sphere. there is some more complicated mathematics involved in describing the surface mathematically. Also, this is more difficult to manufacture.
Since we see a lot of plastic lenses lately, these are easier to make because you just make a mold and extrude the plastic in the mold. You just need to make the shape of the lens.
This is correct. Most of the aspheric lenses we are interested in for our flashlgihts are parabolic, rather than spherical. If you mathematically look at a parabola (the desired surface for focusing light) and a sphere, they are approximately same for lenses where the focal distance / lens diameter, or f-number is large. For lenses with a high focal length / diameter, a bigger section of the parabola is required, which will start to differ from a spherical surface.

aspherical lenses give you the advantage that, when designing the lens, you have extra parameters to compensate for unwanted effects like abberations, but this only works good if you don't deviate too much from the optical axis of the lens (here, we would say when the beam angle is too big, an aspherical lens wouldn't be very useful or better compared to a normal spherical lens).
Beam angle and being on-axis aren't really related. Staying on axis means that 1) your LED die should be well centered underneath the lens, and 2) the die size should be small compared to the size of the lens. If the die has a large extent, then the frignes off the lens will be off-axis and tend to be distorted.

This is also a reason why these lenses are, as far as i know, only available for small angle beams.

hope this makes a little sense to you guys :thinking:
The beam angle exiting the flashlight isn't really the issue. The bigger issue is the acceptance angle, which relates to how large of an angle of light the lens can capture from the LED. This is described in photography by the f number (f/d, or the focal length divided by the diameter). Low f-number indicates more light gathering power.

One lens I have in one of my maglites is an apsheric lens that is 52mm diameter, 33mm focal length (about 0.6 f-number). Because this lens can be placed close to the LED, it can capture light from a wide range of angles.

Now if I were to use a 50 x 200mm spherical lens (f number of 4), I could still project an image of the LED de on the wall just like I could with the aspheric lens. Beam angle (output) would be identical. The difference is with the lens 200mm away from the LED, the vast majority of the light from the LED is being lost as only the light from the LED going "forward" initially will be collimated.

Beam angle is related to the size of the LED die compared to the diameter of the lens. Beam angle can be larger if there are defects in the lens or reflector -- including intentional defects such as diffusers / stippling etc. but that isn't what determines the actual angle itself.

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#### FlashInThePan

##### Enlightened
:welcome:

If I'm reading your question right, it sounds like you're looking for a simple, practical answer. An aspheric lens is a type of lens that - because of its shape - helps focus a flashlight's output into a narrow beam that throws much farther than ordinary lenses.

You're absolutely right: it won't help you much if you're trying to look at nearby objects (the wreck example you mentioned). However, if you're trying to spot an object really far away - say, at 200 meters - a normal flashlight might not be concentrated enough to illuminate that object. (In other words, all of that light gets "used up" illuminating the close objects.) An aspheric lens gives up some of the flood that lets you work effectively at close range in order better throw the light further downfield.

It's a very specialized type of light, but it's very effective for its purpose! Just another tool in the flashaholic's arsenal.

Hope this helps!

- FITP

#### Packhorse

##### Flashlight Enthusiast
Think of an aspheric lens like a magnifying glass. When its in focus you get a nice sharp image ( an aspheric will give a tight beam usually projecting a image of the LED die). When you defocus it the beam angle increases.
So by moving the aspheric in relation to the LED you can get either a narrow or wide beam or anything in between.

The question is what is better, a wide beam or narrow?
Well that depends on what you want it for. For taking pictures/video you want a wide beam.
For signalling and looking at objects at a distance you want a narrow beam. Narrow beams are also great for low viz conditions as you dont get nearly as much backscatter from illuminated particals in the water.

Keep in mind that if a light has a beam angle of 4 degrees it will 4 times as bright as the same light with a 8 degree beam. It will be 16 times as bright as a light with a 16 degree beam and 64 times as bright as a 32 degree beam. For photography and video you will probably want at least 64 degree beam. That will be 256 times less intense as the same light focused into a 4 degree beam.

#### Umibuta

##### Newly Enlightened
Would a reflector still be needed when using an Aspheric Lens? Or can you just use a bare emitter with the aspheric? Btw, I am trying to achieve a flood for video use.

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#### Norm

:welcome:
Lots of threads on aspheric lenses
Use the google search box at the top of the page, it restricts your search to CPF.
How to search CPF

Would a reflector still be needed when using an Aspheric Lens? Or can you just use a bare emitter with the aspheric? Btw, I am trying to achieve a flood for video use.
No reflector is needed. An aspheric lens when not sharply focused gives more of a beam like a theatrical follow spot.
Norm

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#### Barbarin

##### Flashlight Enthusiast
In addition to the very good answers I must say that a narrow angle with very little spill coming from a aspherir lense can be usefull on murky waters.

Javier

#### whitenoise

##### Newly Enlightened
Thanks very much to everyone whose replied, it's much appreciated. Excellent responses as always, thats cleared things up nicely. I could thank specific people, but then all your replies have been helpful so :thanks:

I'm liking CPF more and more, even if some (ok most!) of things talked about are way over my head! =D

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#### busylifemeto

##### Newly Enlightened
with aspheric lens you can do any beamshot. If distance between lens and LED big (focus lens)- you have very hotspot, but if you make distance smaller - you may take foold beamshot with Uniform luminescence without artefacts.

example reflector:

aspheric hotspot:

aspheric flood:

Hi great information here and most helpful. I have been messing around with some aspherics including the 52mm used in the Maglite and with XRE Q5 and the MCE-E and unable to get the great spot beam you have here on you beam shots, could you let me know how you did it ?

#### Z-Max

##### Newly Enlightened
It is easy

You can change distance from led to lens for changing angle of the beam. more distance = bigger angle.

If you want to get big angle you may use lens built in reflector (with very small distance)

#### whitenoise

##### Newly Enlightened
Thanks for the information and pics Z-Max

#### Z-Max

##### Newly Enlightened
You are welcome.

Sorry for mistace in my previous post:
more distance (form lens to led) = smaller angle

If distance = focus length of the lens then you take max. focused beam.

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