Hello! Recently I've been checking my enlargers alignment. What I observed was that all four corners had the same focus level and it was different from that of the center. Then, by focusing on the center of the image, I checked the corners and observe as I stop the lens down. Here, I found something interesting. I found the different lenses behave differently. For example: My Rodagon 80mm f/5.6 had blurred corners and they became sharp when I step down to f/8-11 My El-Nikkor 50mm f/4 had also blurred corners but they became sharp at about f/11 Unlikely, the El-Nikkor 50mm f/2.8 had very sharp corners even wide open at f/2.8. Finally, one Schneider Componar 105mm f/4.5 had to be stopped down to f/16 to have acceptable sharp corners. I know that the El-Nikkor 50 and the Rodagon 80 are both 6 element lenses so high quality is expected. Also, Nikkor 50/4 has 4 elements and the Componar has 3. Now, my questions are: Is it normal that the Rodagon has not sharp corners like Nikkor 50/2.8? Could that mean a default on my lens? Also, I noticed a difference in the detail I could see between the Nikkor 50/2.8 and the Rodagon. The negative was the same. Is that ok? Please note that all the observations were made with a grain focuser, but I plan to do some tests on paper too. Also, the Nikkors are in mint condition, the 2.8 is never used, according to the seller. The 50/4 makes a sound when I shake it, could that be an element that is loose? The Rodagon is used, but it is very good condition, its optics are clean and clear. Could you help me identify what is the version of the lenses I have? Is my Nikkor 50/2.8 N or not? It hasn't the same design as the Nikkor 50/4. And the Rodagon has a rubber aperture ring and a red strip. (I can provide pictures and serials numbers if they can help) Thank you for your repsonses and your attention, George P.S.: Is there any site that I can see the technical specifications of enlarging lenses? From what I searched I found a Rodenstock's pdf file that has some information about its newer lenses. Can these specifications apply to my Rodagon?