# Aspheric focal length question



## gcbryan (Aug 17, 2011)

I'm not exactly clear on what determines the focal length of a lens. I assume it's curvature or something like that?

In an event I have two small aspherical lenses. One is made of plastic and one out of glass. Both have a diameter of approximately 20 mm. The focal lengths are not the same however. The plastic lens has a shorter focal length and therefore projects an image of a larger emitter. The glass lens with the longer focal length projects the smaller image which of course will take longer to diverge.

The longer focal length lens should receive less light from the emitter than the shorter focal length lens. As far as throw is concerned however isn't the advantage generally going to be with the larger focal length.

My reasoning is just that increasing the diameter of a reflector or increasing the focal length of an aspheric (which generally comes with an increase in diameter at some point I think) has a direct (linear) relationship to throw whereas an increase in light output has a squared relationship to throw.

So, I would think the loss of brightness from being further away from the emitter would always be more than offset (only considering throw) by the direct increase in throw from increasing the focal length.

Is this factually correct or not?
Thanks.


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## Walterk (Aug 24, 2011)

Hey Bryan,

This topic passed the forum a couple of times.
Focal length has no effect on throw. 
It does affect Lumen output of the system due to losses from viewingangle of the led and distance to the lens.

The links that I can find easiest are:
Optic theory , post 115 page 4 focal length and throw pictured
Formula for calculating throw using aspheric lens

Would be nice to hear some insight on plastic versus glass for lenses from anyone though?


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