I have that one too, from amazon. Mine came calibrated, but you put fresh RO/DI water on the lens and I believe it should read zero. Just test it once you get it, they give you instructions on how to calibrate it. The only complaint I have with it is that the part you look through untwists to focus it, like a binocular eyepiece, and once you focus it and extend the eyepiece, it will not fit in the case without screwing it back in, no big deal, but its just a bit of a pain if you're in a hurry.