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:

Ben Cole · 16 Aug 2014 at 21:09

How long was the exposure?

How solid was the mounting?

Sh4rkie · 16 Aug 2014 at 21:21

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

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.

SDK^ · 16 Aug 2014 at 21:22

As above; your exposure was probably too slow - stars move very fast

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?

Sh4rkie · 16 Aug 2014 at 21:31

Moredhel said:
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?

Yes. Focusing goes past infinity on lens to protect the AF system, so when using it manually make sure you don't go past it otherwise oof shots of varying degrees.

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.

danoliver1 · 16 Aug 2014 at 21:34

Try manually focusing using liveview.
Might be worth a read. http://www.davemorrowphotography.com/p/tutorial-shooting-night-sky.html

PU7E · 16 Aug 2014 at 22:10

A trick I use is to get AF to spot focus on something a few miles away, can be a farm light or something similar, this will put the lens to infinity for you as the distance is outside the focal range of the lens so it'll just focus on infinity, once it has focus switch it to manual, point at your target for the night and start shooting

.

When I get a new lens I usually go outside and work out where infinity is and mark it on the lens. Not every lens is made the same and the marked infinity is sometimes off slightly, so I'd go out and take a series of test shots remembering what setting each test shot is on and then mark it for later use when I'm lucky enough to be far away from any form of light.

Oh how I miss a focus window on my XF lenses

Sh4rkie · 16 Aug 2014 at 22:24

Moredhel said:
D'oh, just had a play about here, and you're right,

I know, hehe

alle · 16 Aug 2014 at 22:55

...Would forgetting to switch off the VR on your lens (when on the tripod) not be helping?

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.

Sh4rkie · 17 Aug 2014 at 00:05

The general line is having VR / IS on when on a tripod then it will introduce movement trying to correct what isn't there e.g. motion. So it's VR / IS off on a tripod. Some IS systems detect tripod use and don't have the issue.

Amp34 · 17 Aug 2014 at 01:32

Moredhel said:
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?

Most do, best bet is to use live view and focus that way.

NickK · 17 Aug 2014 at 13:18

Zoom in - then focus, then zoom out.

Welcome to the wonderful world of shooting at night

Also as the lens cools the focus will shift. So best go out there for an hour with the gear and then start shooting.

Read up on FHWM focusing - it's possible to attach the DLR to a computer and use that to assist in focusing.

D.P. · 17 Aug 2014 at 13:32

Just recap/re confirm what other says:

Switch VR off, for almost all lenses this will make for soft images on a tripod.
The focus can go past infinity, so you need to use autofocus to focus on something far away. Doesn't need to be too far because the DOF will still stretch to infinity. Use a DoF calculator to verify.
Lenses will change focus with temperature (whiy they go past infinity), so easiest when lens reach same amient temperature.
Obviously you need to keep exposures fairly short, I think 15s is about the limit.
You need a good tripod, no a tripod that slowly creeps over the exposure time.

NickK · 17 Aug 2014 at 13:52

One additional point to note - depending on the quality of the lens.

You may not a slight difference in focusing of colours (chromatic aberration) and possibly sharpest on-axis then off-axis being not as sharp.

Lastly - take 10-20 shots and "stack" them.

There's two bits of terminology here (often used in the wrong context):

* Image registration - aligning the images so that the stars perfectly match location between images.

* Image stacking - mathematically adding the image signal (in a variety of ways) onto of each other to increase the final signal and reduce noise.

There's several applications out there - some free ones too. Deep Sky Stacker, Registax etc.

englishpremier · 17 Aug 2014 at 13:56

The body lens should be smart enough to switch off VR when a remote is used.

I know Sony and Pentax bodies do this automatically.

Rather than focusing to infinity you could always try hyperfocal techniques instead. I presume you're using a high-iso and wide aperture, but it should still work just fine.

K.C. Leblanc · 19 Aug 2014 at 23:23

englishpremier said:
The body lens should be smart enough to switch off VR when a remote is used.

I know Sony and Pentax bodies do this automatically.

Rather than focusing to infinity you could always try hyperfocal techniques instead. I presume you're using a high-iso and wide aperture, but it should still work just fine.

On Nikon the VR is in the lens not the body it probably needs switching off manually.