Focus distance question

[childof76]

This might be a stupid question but I figure it would be a good newbie question.

If I have a lens that says it's focus is 50.8mm does that mean the ideal focus point would be 50.8mm from the front of the mirror to the work piece?

Would that mean a lens with a focus of 100mm would let the work piece be 100mm away?

Is that the only value difference between the two focus lengths or is there a power/size/difference?

Basically I'm looking for the crash course on focal differences for laser cutters so I can decided what to do about my optics.

[Tech_Marco]

What you said is correct. 50.8mm is the distance between the lens surface to the focus point where the power is at peak. Your assumption is good for engraving but not on cutting especially for thick object like 1/2" pine wood. You may want to focus the to the center of the wood to minimize the effect of the incline slop happened to the cutting surface.

It's the reason why 75mm or 100mm lens is better for cutting while 50.8mm or 38mm is better for engraving. Remember that a light beam coming out from the focus lens is "cross over" at the focus point. Imagine that it is a 'scissor' shape or two long isometric triangles with one upside down and on top of another. The center is the focus point and there is most hottest spot then diverse out.

Marco