lens focus distance

[geekinesis]

Hi
Where do you measure the focal length of a lens from. Is it the front of the lens or the mid point inside the lens.
Also should the lens be a certain way up?

[Tweakie]

According to the text books, focal length is measured from the mid point inside the lens but to be practical an external surface has to be chosen. As far as laser heads are concerned once the best point of focus has been obtained then measure from any convenient metal surface.
From my physics days (long ago) the best power from a plano-convex lens was from the shortest depth of field which would be curved surface towards laser and flat surface towards work.
With the curved surface towards the work the usable depth of field is greater but the available cutting power is less.
On my system it is curved side up but looking at the cheap Chinese machines they are usually fitted curved side down.

Nothing is ever conclusive.

Tweakie.

[geekinesis]

Well I turned it over - flat side down and I appear to be cutting 50 percent faster at the same power. Mainly I am able to get a much tighter focus so the power is more concentrated.
It counerintuitive and possibly due to other factors as well, but maybe there is a difference. Also is there a slightly reflective surface on the convex half compared to the flat half. Would this make a big difference to the beam?

[Tweakie]

Both sides of the lens should be AR coated, at least mine are.

Tweakie.