How can you tell if the focal lens is in the wrong way ?

Lasers
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1

How can you tell if the focal lens is in the wrong way ?

2

If you flip it over & it cuts better! But seriously, the focal lens in general should be with the concave side facing down & the convex/flat side facing upwards.

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cheers :slight_smile:

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Bumps are up. Easy way to remember it.

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O shit really??? I think I have it backwards

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@Ariel_Yahni_UniKpty If indeed you do, you’ll be glad you swapped it over. Makes a huge difference.

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But Its weird since I’m getting cuts at 10mms with less than 10mA on 3mm

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@Ariel_Yahni_UniKpty Maybe you can get the same cuts @ 20mm/s with lens inverted? Or might just be a sharper cut.

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Will try it soon

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Curved side goes up or you get longitudinal aberration. https://1drv.ms/i/s!AtpKmELEkcL8imztsXWs-opsElTO
The only reason it would work better the other way around is if you had a crappy made lens. :slight_smile:

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@Nedman ​ “curved side” is an ambiguous term. Is the the one where the curve goes up (e.g. bump or convex) or the curve that goes inward toward the center of the lens (e.g. depression or concave)?

A bump is always away from the surface upward and a depression is always downward. Convex and concave respectively.

So bump or convex indicates a known surface and direction. Curved side is open to interpretation.

The planar lenses we use tend to (there are exceptions) focus to a narrower point convex/bump side up.

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You are correct. My comment only applies for plano-convex lenses where one side is flat and the other is convex. I wasn’t considering that someone could be using a positive meniscus lens where one side is convex and the other is concave.