I am new to enlarging 4x5. I have a grain focuser left over from my 35mm enlarging days which seemed to work fine way back when. I now have a 4x5 enlarger, and, when projecting a 16x20 image, in no way can I check the edges or corners- the view in the scope goes from a small "ufo" saucer shaped image, to no image at all at the edges. I guess my old cheap grain focuser is not good for this. I am now using a 135mm lens. Is this the fault of my focuser, my lens, or what? Is there a feature in a grain focuser I should look for to be able to check the edges of my print?