Focus Lens.

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Focus Lens.

Postby Tweakie » Wed Apr 28, 2010 6:47 am

My existing focus lens is convex / convex (equiconvex) but I have just obtained a new lens (which should give me a slightly greater focal length) which is convex / flat (planoconvex) and my question is which way up should it be fitted?.
Should the flat surface be towards the tube of towards the work ?.
Anyone have any ideas ?. :?:

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Re: Focus Lens.

Postby lasersafe1 » Wed Apr 28, 2010 11:51 am

Flat toward focus, which is also flat toward your work piece.
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Re: Focus Lens.

Postby bdring » Wed Apr 28, 2010 12:22 pm

It is a collimating lens. See...Wikipedia
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collimating_lens.JPG
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Re: Focus Lens.

Postby Tweakie » Wed Apr 28, 2010 4:43 pm

Thanks Guys - it is always nice to check these things with someone else first.
Pretty sure it is just a standard focus lens, no doubt I will find out if it is collimating or not by trial. :D

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Re: Focus Lens.

Postby lasersafe1 » Fri Apr 30, 2010 10:44 pm

You should realize that the beam coming out of the tube is already collimated as well as it can be. In reality it is slightly divergent. Short tubes have more divergence than the long tubes, it's a physics thing... This is why it is best for people building big systems that expect a 4ft by 8 ft XY travel distance to go ahead and buy the 80 or 100W tubes.

The lens can actually go in either way and it will produce a focus, but the focused spot will have fewer abberations if you put the flat toward the desired focus.
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Re: Focus Lens.

Postby Tweakie » Sat May 01, 2010 11:12 am

Hi Lasersafe,

Thanks for the extra info.
I had, incidentally, tried my new lens (on the bench) using a small HeNe laser in order to compare focal distance with my existing lens and can confirm that planoconvex will focus either way round but it is good to know which is the right way. 8-)

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