calculating divergence while adjusting the beam size

[syrah]

When adjusting the beam size with lenses do you have to measure the beam size at two points to calculate the divergence ( http://www.pseudonomen.com/lasers/ca...alculator.html ) or can you calculate the new divergence value for a changed beam size if you already know what the divergence value is at the same distance for X beam size?

[syrah]

bump .

[laserist]

Assuming adequate optics and alignment you can assume that doubling the beam diameter will cut the divergence in half, but it's based on those pesky assumptions.