It wasnt the gain causing the soft focus but the beginnings of diffraction, like you said when you hit +12db you were at F5.6. I believe you said you also used sony ex cameras, theyre 1/2 " chips so they can go a little higher before they start to suffer diffraction. Also at +12db you will be gettting a lot of noise which can also appear to soften the image a little (depending on your coring settings). Your doing the right thing with trying another lens, lets hope its all due to something simple and easy to fix 