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Author Topic: Night Sky - focus or exposure time problem  (Read 4966 times)

stever

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Night Sky - focus or exposure time problem
« on: July 25, 2016, 04:35:47 pm »

i'm in the mountains for a few more days with clear sky and late moonrise.  Have read a variety of recommendations for simple wide angle shots.  Not happy with my results & am having a hard time separating issues of star motion, focus, and noise.

shooting 5d3 with 35 f2 (new model), 16-35 f4.  read a recommendation that 8 sec at f2 ISO 6400 should work.  this seems to give more circular stars than f4 30 sec with 16-35.  lots of noise (or are they faint stars?).

have been trying to focus using liveview magnified on a bright star & can't tell if the stars are focus or motion blurred at f2 - 8 sec (probably motion blurred at f4 30 sec)

have also read that it's possible to pano stitch sky shots - don't understand why this should work

any advice on technique, development technique, and resources appreciated.  i'm not trying for perfection, just some decent shots.

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stever

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Re: Night Sky - focus or exposure time problem
« Reply #1 on: July 25, 2016, 04:38:19 pm »

PS - have long exposure noise reduction turned on.  is this correct?
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Stephen Girimont

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Re: Night Sky - focus or exposure time problem
« Reply #2 on: July 25, 2016, 08:01:26 pm »

Motion blur should result in elongated stars (star trails). Focus blur would result in blobby stars (not pinpoints).

Where are you located? How humid is it there? Is your camera lens in air conditioning during the day? Could it be fogging up at night? That would also result in blobby stars and a distinct lack of dimmer stars.

Paul2660

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Re: Night Sky - focus or exposure time problem
« Reply #3 on: July 28, 2016, 12:10:08 am »

Any exposure longer than around 17 sec will show trails, so if you are working the Milky way, which it sound like you are, I would work with a ISO combination that allows you to get a nice noise ratio.  Faster lenses, from Canon tend to show coma towards the corners, but this is very obvious (butterfly wings). 

For focus, it's not really a huge deal, as you should be able to focus on a stationary object before dark (finding infinity) and then use that setting later on at night.  This is what I have done for years now, Nikon, Canon, Fuji, Pentax, and P1.  You can also go out and focus on the moon (I realize this is not going to help in your current situation), mark that spot on your lens for use later as if you can get the moon in focus the stars will be also.

Your blur on longer exposures, as already mentioned, will be trails. 

Using Live View also works and Canon's is much better than most for night work.  The trick is to figure out a magnification level in Live View that works as over mag, will make it hard to get the image sharp and under mag, is even more frustration.  I would also try to pick a planet like Venus as it's always out low in the sky, and use that for your target, or a brighter star, making sure it's in the center of your frame.

Also check out Adam Woodworth's site as he has a lot of good tips. http://www.adamwoodworth.com/

Yes, you can pan for the Milky way, I would have the camera in the vertical orientation when doing this.  Can't really do this with star trail work, at least I have never figured out a way.

Paul C

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Paul Caldwell
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stever

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Re: Night Sky - focus or exposure time problem
« Reply #5 on: July 28, 2016, 10:22:52 am »

thanks. Fortunately i was able to do some further testing

- seems more reliable to preset infinity focus, even with the image magnified in liveview it's hard to focus on a relatively bright star without an additional magnifier

- maximum exposure is related to focal length, and the relationship i found online of 500/focal length appears seriously optimistic for the 5D3 resolution.  i found 200/focal length to give round stars at 1:1 viewing.

the 35 f2 and 16-35 f4 are marginal for aperture and i'll have to wait until i get home to the large monitor and make some prints to evaluate the tradeoff between noise and elongated stars.  was able to borrow a Rokinon 14 f2.8 which turns out to be a marginal improvement over the 16-35 as the extreme vignetting is very hard to correct and maintain color consistency and is noisier than the 16-35 when corrected.

found that long exposure noise reduction appears to make no significant improvement

next i'll try some pans with the 35 at 6 seconds
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Paul2660

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Re: Night Sky - focus or exposure time problem
« Reply #6 on: August 08, 2016, 11:04:13 pm »

The Rokinon should correct very well in LR for the vigneting you can use the Samyang profile as they the same lens.

Paul C
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Paul Caldwell
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