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Old Oct 6th 2016, 05:08 AM   #1
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The effect of focal length on collimated light output

I was trying to get an idea of what effect if any different focal lengths for the same diameter lens had on a collimated beam?

The conclusion I have come to so far is that it has no effect on the final collimated output, only the distance the light source must be from the lens in order to achieve collimation. Is that correct?
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Old Oct 6th 2016, 07:42 AM   #2
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I would say it's correct as long as the output from the light source is 100% directed at the lens. If the source is not directed, such as a standard light bulb, then from the geometry of the situation the closer the bulb is to the lens the greater the percentage of its light output hits the lens, and hence the greater the throughput of light through the lens. Thus a shorter focal ratio may be better.
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Old Oct 7th 2016, 04:58 AM   #3
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Theoretically this is correct,
however there are some practical issues that could be considered.
The theoretical perfect lens has zero thickness,
the practical fact that a real lens does not have zero thickness produces some issues.
For example Spherical_aberration and Chromatic_aberration

A thinner lens will generally exhibit less of these issues than a thick lens, and so might be preferred if a short focal length is not mandated.
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Old Oct 8th 2016, 01:22 AM   #4
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Thanks for the answers so far.

Chromatic aberration is not an issue as I'll be working within a fairly narrow wavelength (within infrared). Spherical aberration shouldn't be much of an issue either as i'll be working with pulsed light where the exact shape or focus isn't relevant.

All that really matters is that the beam has as minimum spread possible over distance (which could vary between 2-20 meters).

There are a dazzling array of lenses to choose from in many different focal lengths and I wasn't sure which would be most fit for my purpose.

ChipB's advise on a shorter focal length to capture as much light as possible seems like good advice.
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