A Gaussian beam has an intensity I(r,z), if the beam area at position Z is given by A(Z), then the beam gets focused by a lens of focal length f, what will the area of the beam be at the beam waist A(0) be? So I have been trying to figure this out for ages, I had to replicate an experiment in which a student simply assumed the beam focused like a cone, in which case you say the beam is 0 at the beam waist and the beam is "close" to the beam waist and choose/measure a distance. This seemed like pretty poor experimental work to me as you can make the intensity become infinitely large by "choosing/measuring" a value closer to the beam waist. In this scenario I have taken position z from the beam waist to be the focal distance f: A(f)/A(0) = (pi*w(f)^2)/(pi*w(0)^2) = (w(f)/(w(0))^2 = (f/x)^2 x is the distance from the beam waist that a person "chooses" or "measures". To see the above equation, I found it constructive to draw out a cone and put in the values. I read several Gaussian optics manuals and a better expression seems to be A(f)/A(0) = (f/ZR) Where ZR is the Rayleigh length and is given by ZR = pi*w(0)^2/gamma However the problem with this is that I don't know w(0). I read a limit for w(0)>/= 2*gamma/pi, however this seems to result is nonsensical answers. If anyone here knows or works with lasers, perhaps you could help explain it to me?