I have an older lens (Goerz Dogmar 16.5cm) that I'd like to figure out the filter thread for (and a couple of others as well.) I have a set of digital calipers so I can measure the diameter of the inside of the thread. Is that all there is to it, or do I need to account for the depth of the thread? If so is there a common depth that I can just add to the measurement? Sorry for the really simple question, but a search didn't find the actual filter size, or how to do the measurement reliably. Its around 40/41mm, and I don't have anything existing in that range to try screwing on.