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+--- Thread: Focus on stars (/showthread.php?tid=3744)
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Focus on stars - Guest - 08-11-2010
Hello everyone,
First thanks Klaus for this pleasant upgrade of PZ forum !
I've had lately an issue when shooting night sky with 2 tele primes. Actually the stars were blurred, just as if lenses cannot reach focus on infinite.
To give more details
- shutter was set on 30s (no moving star effect that happens when you have a several minutes exposure)
- manual focus on infinite (I double checked number of times)
- body on a tripod (and no wind)
- lenses were 135 at f2 to f2.8 and 100mm at f2.8
I tried to activate the focus micro adjustment available on my body, but I didn't achieve to get a sharp result. I got better results, but nothing perfect <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='
' />Any idea of what would be this phenomenon and how it could be corrected ?
Thanks !
Focus on stars - popo - 08-11-2010
You say manual focus - how so? Did you use the scale on the lens body? If so, that's usually not accurate enough to get pin sharp stars. I use magnified live view on a bright star when I want to do this. I'm not sure on a good method without live view.
Even at 30 seconds at those focal lengths I would expect some trailing if you're not using a tracking mount.
Focus on stars - Rainer - 08-12-2010
[quote name='popo' timestamp='1281565868' post='1694']
Even at 30 seconds at those focal lengths I would expect some trailing if you're not using a tracking mount.
[/quote]
Yes, that is definately the case ... a little depending on the region of the sky you are
pointing at, but generally .... yes.
With an 85mm you get blurred stars with exposuretimes longer than 4-8secs ... startrails
form from roughly 30sec onwards ... so, with an 135mm you need a shorter time
(something like 2secs or less) or you need to mount the camera on a telescope
and use the telescope as a tracking system.
Focus on stars - Guest - 08-12-2010
Well actually the starts were so blurry that it probably masked the trailing effect. Anyway I don't have the equipment required for tracking
Yes i use the scale on the lens, didn't even think to use live view. Thanks for the tip.
By the way, how were people focusing without live view ? I mean, during film era ?
Focus on stars - popo - 08-12-2010
[quote name='Etao' timestamp='1281632113' post='1720']
By the way, how were people focusing without live view ? I mean, during film era ?
[/quote]
I never did it myself so I can't be sure, but one technique I've come across is to use a Bahtinov Mask. This is a particular pattern you put in front of the lens like a filter, and it generates a diffraction pattern that changes as you go in or out of focus.
Focus on stars - miro - 08-12-2010
Focusing in film era was easy. We had 8x10 format.
On that time 35mm was the small-format with dummy dark viewfinder.
Nowadays they called superior Full format.
I use Live view often by my landscape shoots. In sunny day I use cloth – just nostalgia to old film days since the lens is on MF and camera on tripod. Almost no difference from 60 years ago.
Greetings,
Miro
Focus on stars - Guest - 08-12-2010
[quote name='Etao' timestamp='1281632113' post='1720']
By the way, how were people focusing without live view ? I mean, during film era ?
[/quote]
Back in the old days, the lenses usually had a hard stop at infinity (which was truly infinity). This is (at least for Nikon lenses) no longer the case - they can often focus beyond infinity.
Focus on stars - Rainer - 08-13-2010
[quote name='Etao' timestamp='1281632113' post='1720']
By the way, how were people focusing without live view ? I mean, during film era ?
[/quote]
Back in that time, all lenses had a distance scale ... so it was quite easy to focus
on something far away daytimes, and just take a note of what the distance scale
displays ... even at that time there were lenses that would focus past infinity ... typically
long tele lenses.
Later on (in the dark), you just set the lens to the value you noted daytimes and trusted
in this.
at least ... that's what I did.
Focus on stars - genotypewriter - 08-19-2010
[quote name='Etao' timestamp='1281632113' post='1720']
Well actually the starts were so blurry that it probably masked the trailing effect. [/quote]
You said you manual focused but did you actually move the switch on the lens from AF to MF? <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='
' />GTW
Focus on stars - Guest - 08-19-2010
yes of course I did =)
Thanks for the "historical" info above <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='
' />