Hi all, new to the site.
I've just purchased a used (from London Camera Exchange) Nikon 20mm f/1.8G lens to use to start some nightscapes/astro shots with my D810. I took it out for the first time last night, and while I got some shots I'm pleased with, I'm disturbed by the fact that focus seemed to keep moving slightly - enough to take stars out of focus - between shots.
The temperature was stable at 16-17 degrees (Celsius) I had the lens and camera on a tripod and both (!) in manual focus mode. I would focus on a bright star using Live View mode zoomed in all the way. I'd be quite confident that I got stars to be as pin-sharp as possible after a few moments going back and forth. Then I'd take a shot (usually a 13-second exposure using the 2-second delay), often check it, and proceed to take a few more. It seemed to me that after a couple shots, I was suddenly getting stars across the field to be a bit out of focus, as in the attached example photo.
Nothing, I am certain, was bumping the focus ring between exposures. (Reviews praised the focus ring, except for its slight bit of play before it 'catches', but I don't think it's particularly precise or solid feeling at all.) And it certainly isn't too long an exposure or the beginning of star trails. (I used 13 seconds for all shots last night, and a good handful came out sharp; but a good number were lost to this problem, as well.)
Is it possible there's something 'loose' and, say, the mirror snapping up and back down a few times has caused the lens to lose focus just enough for stars to start to blur?