Hi, Peter. Sorry so late in replying: from what you say above,this is a great help ,as I can get a good field of view, but will not give me the critical focus to make a pin sharp image. Am using double Plossls at moment, which I think will have to be precisely positioned as regards erector focal lengths, and ocular focal length, which I think, works out at about 0.5". Thanks sincerely for your time and effort. Much appreciated, Alan.