I Shorten the lens distance while having same focal length?

[Horizont22]

Hello Everybody,
I'm David from Spain, and happy to be here.

We have a LED that needs a FL50mm collimator which is focused to infinite, because we want parallel rays.
So the lens is at a distance 50mm (approx) from the led.
However, 50mm is too much distance for our design, so the question is:

How we could get the same beam results as with a FL50mm lens, but with a lens (or several) which is much more closer to the LED??
(For example at 10mm from the LED instead at 50mm)
Should we add another lens and if so, how we could calculate equivalent FL50mm with 2 or 3 lens system?
Or maybe there's another way?

Many thanks for your Help.
David.

 

[Keith_McClary]

LED that needs a FL50mm collimator

Is 50mm just a suggestion? If you put a source one focal length from any lens you will get an approximately parallel beam. You only get a perfectly parallel beam with a point source (and a perfect lens). With a finite source, the beam will diverge with an angle proportional to the dimension of the source and inversely proportional to the focal length.

 

[Horizont22]

Thanks for your reply. Yes, a suggestion and Yes, I know it will diverge. But this don't ask my question.
Question is: Could I place some lenses (or something?) to get a much smaller setup distance than 50mm, and get the same light behaviour than with a FL50mm lens? thanks! In other words: remove that FL50mm lens for example for shorter FL pair of lenses...and get the same results but with a shorter beam path?

 

[hutchphd]

Why do you want a pair of lenses? Just use a shorter focal length lens. Are there other requirements you are not sharing?

 

[jbriggs444]

I have next to zero expertise, but would something like this satisfy the requirement? A 10 mm focal length in a 13 mm diameter fresnel lens.

https://www.knightoptical.com/stock...-standard-range-10mmf-lx13mmdia-aperture.html

 

[Horizont22]

Why do you want a pair of lenses? Just use a shorter focal length lens. Are there other requirements you are not sharing?

A shorter focal lens will not provide the same beam properties than FL50, I'm thinking more towards a Barlow lens with a 3 lens system...

 

[Horizont22]

I have next to zero expertise, but would something like this satisfy the requirement? A 10 mm focal length in a 13 mm diameter fresnel lens.

https://www.knightoptical.com/stock...-standard-range-10mmf-lx13mmdia-aperture.html

But..does this FL10mm lens provide same beam specs than FL50mm lens? I'm afraid not...Again, idea is, same beam properties than with a FL50mm, but with shorter distance than 50mm (or near)...any ideas? thanks guys anyway

 

[hutchphd]

You need to specify the "beam properties" you need. To my knowlege, the term FL50 just means a lens (simple or compound) that has a focal length of 50 mm.
What specifically do you need to do?

 

[Drakkith]

Should we add another lens and if so, how we could calculate equivalent FL50mm with 2 or 3 lens system?
Or maybe there's another way?

What exactly are you trying to do? Adding a 2nd lens to the system should be identical to just using a different single lens in the first place. What 'beam properties' are you concerned about? Collimated beams of light don't have many properties to modify. Beam diameter is just about it unless you're doing something very elaborate or precise.

 

[Drakkith]

We have a LED that needs a FL50mm collimator which is focused to infinite, because we want parallel rays.

What do you mean when you say that the LED 'needs' a FL50mm collimator? Most light sources work perfectly fine with collimators of any focal length. Is there something about the way this LED emits its light that doesn't work with a collimator of a different focal length? Could you place a diffuser directly in front of the LED? Then you could use a collimator of any focal length you desire.

 

[Horizont22]

Thanks Drakkith, but it doesn't matter my setup. The question is simple, I want to know if there's a way to get the same FL but with a shorter distance than with a single lens. For example, in a telescope, you can add a Barlow lens in the middle of two, to extend magnification, so in fact, you're 'virtually' enlarging the total FL of the system without increasing its size... just want to know if such setup or system, (or any other you know) could work for collimating light sources, and, purpose is to get shorter optical paths, that's all.

 

[sophiecentaur]

For example, in a telescope, you can add a Barlow lens in the middle of two, to extend magnification,

Yes. The Barlow itself is just a concave lens and 'magnifies the image' whilst keeping the focus in much the same position.
This link gives the sums for a Barlow. But what you describe needs a shorter focal length and that would need a second convex lens. The same formula as in the Barlow reference would apply but the added lens would need a + focal length.
In the same way that the Barlow reduces the effective aperture (increased f number), I'd expect the negative lens (a 'focal reducer' for astronomers) to reduce the f number and let more light through. That could be good value for a light source. But I can't think of an obvious reason for not just replacing the original lens. The existing hardware would need modifying in any case.

 

[Drakkith]

Thanks Drakkith, but it doesn't matter my setup. The question is simple, I want to know if there's a way to get the same FL but with a shorter distance than with a single lens. For example, in a telescope, you can add a Barlow lens in the middle of two, to extend magnification, so in fact, you're 'virtually' enlarging the total FL of the system without increasing its size... just want to know if such setup or system, (or any other you know) could work for collimating light sources, and, purpose is to get shorter optical paths, that's all.

You can certainly add a 2nd lens to bring the focal length down. The simple thin lens equation is $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2}$ where $d$ is the distance between the lenses, $F$ is the combined focal length, and $f_1$ and $f_2$ are the focal lengths of the two lenses you're using. So if you want a 30mm focal length, plug 30 in for $F$, 50 in for $f_1$, choose a distance for $d$, and then solve the equation to get the requisite focal length you need for the other lens. Or start with a focal length for $f_2$ and solve for $d$ if you already have a selection of lenses you want to try.

Of course, you could just use a 30mm lens to begin with and get the same results.

 

[hutchphd]

Thanks Drakkith, but it doesn't matter my setup.

How we could get the same beam results as with a FL50mm lens, but with a lens (or several) which is much more closer to the LED??

Use a shorter focal length lens.
For instance FL49.

Thanks Drakkith, but it doesn't matter my setup. The question is simple, I want to know if there's a way to get the same FL but with a shorter distance than with a single lens.

This is a different question. The answer is yes. But you don't want the same FL. You do understand that FL means focal length of collimating lens?
https://www.edmundoptics.com/knowle...ation-to-determine-optical-lens-focal-length/
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