Focus to infinity for stars - still blurry?

Moredhel · 16 Aug 2014 at 20:29

Hi,

I was attempting to get a shot of some stars, not with a subject or anything, just the star field, purely as an experiment and to get used to the kinds of settings needed.

I manually focused to infinity after setting both the body and lens to manual (Nikon D7100 using the Nikon AF-S DX 18-200mm f/3.5-5.6 ED VR 2 at 18mm f/3.5) but the shot has come out blurry. It looked nice enough through the camera at the time but got it home into lightroom and I'm quite disappointed in the result.

I figured stars were far enough away for infinity to be an ok assumption to make, was this incorrect? What should I have done differently?

The shot:

A 100% crop:

Moredhel · 16 Aug 2014 at 21:21

It's a 25 second exposure (I had read that any more than this risked star trails), I used my heavy old tripod in the hope that any wind did not interfere, it wasn't a windy night, and I used my IR release so that I didn't have to touch the camera to take the shot, I also made sure that myself and my two friends were not moving around near the camera in case our footfalls affected the shot.

This is one of several shots that all look the same, I had a similar from the same place looking over the city and that has the same kind of blur.

The lens has no blur issues in normal (automatic) usage. So I am putting this down to me somehow misunderstanding how to use manual focus and don't have good enough eyes to see it through the viewfinder.

Moredhel · 16 Aug 2014 at 21:25

Sh4rkie said:
And did you go to infinity on the lens or past it to the extreme of the focus range?

Hmm, looking at the lens it does have a bit of a chunk past the infinity symbol. Could that be the problem? Did I simply go too far?

Moredhel · 16 Aug 2014 at 21:34

D'oh, just had a play about here, and you're right, beyond inf takes it too far the other way. I was not aware that lenses had a bit at the end, I have never really played with manual focus before. Ahh well, shall just have to go trekking up the hill again some time.

Moredhel · 16 Aug 2014 at 23:33

I had not heard that it was necessary to turn off VR for this kind of shot. I had read (and it makes sense) that it had no benefit to have VR on when using a tripod, but not that it could impact on image quality. I'm very much chalking it up to what Sh4rkie said and my inexperience in manual focusing, not realising there was space the other side of infinity. I can reproduce it here at home now by going too far again.