I use the 400mm 3.5 all the time. Check the filter in the removeable cartridge and make sure it is perfectly clean and not
fogged. Don't use lens tissue it makes fibers, prefer use a pure piece of chamois and a huff of breath is good enough.
Next with a flashlight shined through the front, check the recessed rear element for dirt or residue. If it's dirty this is a trick to clean.
Use a long stereo-cleaner q tip and a little bit of alcohol or even vodka is good on the q tip and in two steps, immediate,
right after each other: First gently swab the rear element with the q tip, then with a second one, cut a very small piece of
cotton under shirt, about an inch square, fold it over the tip in a ball and tape it around the wooden shaft, this will be to
quickly dry and polish the rear element. Practice first on something else to get the hang of it, like the back of a shot glass
in a paper towel tube. As an alternate, you could also cut and secure a small piece of chamois around the end of a pencil eraser. Just fold it around and tape secure to the pencil shaft.