Lens convex side up or down?

Which way do people orient their lenses (plano-convex or meniscus)? According to Russ Sadler, correct is convex up, however most Chinese machines ship with lenses installed upside down (convex down). I broke mine before I could see which way it was installed. What do people here do and why?

Russ did a number of tests, as well as provided the light ray tracing which shows that the focus is with the flat side down, convex side away from the work. With a meniscus lens, both sides are curved, the concave side is pointing at the work.

If you’re able to find the right video and the right segment, you’ll see that there isn’t a true focus with the flat side upwards.

Convex side up.

The Russ Video I saw explained that it did not matter which side was up or down, both ways work fine. But the focal length varies depending on which side is up. To have the stated focal length the flat side should be down, but flat side up is also fine, you just need to figure the focal length for flat side up.


This is the image I recall, perhaps from a different Russ video. One desires the smallest spot size for the most power, the image on the right represents that situation. Other sources on the 'net confirm this orientation.

This is the image Russ finally came up with. WIth the flat side up the area of focus was a bit “longer” which makes it more suitable for cutting through thicker materials, since the beam is focused over a longer distance. The downside is that the beam diameter at the focal point is slightly larger by a few percent. So having the flat side down is better for engraving.

It is easy to test for yourself, just flip the lens over and find the new focal point. Then measure the focused beam diameter and how well it cuts.

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