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05-12-2009, 07:51 PM   #1
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Taking pictures of Stars on K200D

Hi all!

I recently purchased myself a k200d, and I am new to dSLR's in general.

I was trying to take a quick test-shot of some stars, but I couldn't get it to focus. The result was this:




I set the focus to manual, and to infinite. Other settings were:

Exposure: 30 seconds
Focal Length: 50mm
ISO: 800
F: F/5.6
Spot Metering

Any hints?

Cheers,
Rage

05-12-2009, 08:17 PM   #2
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I'd imagine your lens has focused past infinity - you might need to back off a little.

Also, 30 seconds might be long enough that you'll be starting to get star trails rather than single points, although that would depend on your focal length.

Last edited by Marc Sabatella; 05-13-2009 at 09:20 AM.
05-12-2009, 08:39 PM   #3
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The stars in your image are defocussed. They are not suffering from trailing that you get from a too long exposure for the focal length. Many lenses focus past infinity when racked out to the "infinity" stop so your lens could be doing this. You will have to find a way to judge focus better in your viewfinder.
05-13-2009, 03:03 AM   #4
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The kit lens will focus past infinity, like it did in your picture. I am assuming you used the kit lens.
Either pre-focus on something bright, practice manual focusing in daylight to get the feel for the lens and the infinity focus point vs. where lens hits the stop or mark the lens barrel.
Pre-focusing works the best for me,.

Cheers. Mike.

05-13-2009, 03:15 AM   #5
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I found a post on another forum that gives the maximum exposure time for various focal lengths and declinations to avoid trailing. For 50mm the maximum exposure lies between 20 and 75 seconds and it's recommended to use only 1/3 of the maximum time to get sharp pictures.

QuoteOriginally posted by Guide to the maximum exposure time to avoid star trails:
Hi,
Seem like quite a number of people like to "shoot stars". Below are a guide to the maximum exposure time allowed in order to avoid star trails when using a tripod. To get perfectly sharp stars, use only 1/3 of the maximum exposure time.

When aiming at celestial equator (Declination 0 degree)... basically overhead in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 55s
24mm: 40s
28mm: 35s
35mm: 30s
50mm: 20s
100mm: 10s
135mm: 7.5s
200mm: 5.0s
300mm: 3.3s
400mm: 2.5s
<snip>
05-13-2009, 03:16 AM   #6
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A cheap (but good!) manual focus lens (you can get an old Pentax-M 50/2 or the equivalent Chinon for peanuts) will help here since not only is it easier to focus manually with the large, damped focus ring; also your camera has focus confirmation with manual focus -- i.e. if you have the AF switch set to "AF" when using a manual lens it will beep when actually in focus and refuse to take a picture unless it is. It will help you confirm that infinity is actually infinity.
05-13-2009, 12:56 PM   #7
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QuoteOriginally posted by KMB Quote
I found a post on another forum that gives the maximum exposure time for various focal lengths and declinations to avoid trailing. For 50mm the maximum exposure lies between 20 and 75 seconds and it's recommended to use only 1/3 of the maximum time to get sharp pictures.
The times in this table are quite long. We used as a rule of thumb 15 secs for a 50mm lens as the acceptable upper limit - this would also be, what I find in amateur astronomy books usually. With this time, the stars just started to develop a small, just discernible tail at about 20 degs off the celestial equator.

The times can be simply multiplied with the result of dividing the 50mm fl through the fl used for the image.

Ben

05-13-2009, 01:02 PM   #8
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QuoteOriginally posted by Rage Quote
Hi all!

I recently purchased myself a k200d, and I am new to dSLR's in general.

I was trying to take a quick test-shot of some stars, but I couldn't get it to focus.

I set the focus to manual, and to infinite. Other settings were:

Exposure: 30 seconds
Focal Length: 50mm
ISO: 800
F: F/5.6
Spot Metering

Any hints?

Cheers,
Rage
The clear misfocus is one of the typical problem with astro photos. Focusing is tricky and very often I am just a tad off, which is enough to ruin the image. Here it is more than "just a tad". I would suggest to really look through the viewfinder for focusing, the distance marks on the lens are useless for this.

Also, adding a good viewfinder loupe helps tremendously. I use the Olympus Varimagnifinder, but ofcourse the Pentax ones do work just as good. The added advantage is, that these are right angle finders, which make focusing much easier, if the camera is very low or in an awkward position mounted (to a telescope for example).

On the K20 Liveview would be of help, if it could display fainter stars, which it can't...

Ben
05-16-2009, 09:22 AM   #9
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Thanks for the help everyone! I'll be trying again once the moon retreats..
05-16-2009, 09:25 AM   #10
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05-16-2009, 05:59 PM   #11
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QuoteOriginally posted by Erik Quote
A cheap (but good!) manual focus lens (you can get an old Pentax-M 50/2 or the equivalent Chinon for peanuts) will help here since not only is it easier to focus manually with the large, damped focus ring.
I've used manual focus lenses for star photos since the 70's. It's not the damped focus ring that makes them easy to use, it's the fact that you can just turn the ring towards infinity until it stops - and there you are, perfectly focused on the stars.

That's not true on the newer autofocus lenses - but I'd expect that aligning the infinity mark (sideways "8") to the focusing index should do the same thing.
05-17-2009, 01:18 PM   #12
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My early 1980s M* 300/4 focuses past infinity at the upper stop so you can't just say all manual focus lenses you can just "twist and forget". The selling point for focusing past infinity at that time with * lenses was that it allowed you to adjust infinity focus for different lens barrel temperatures (tended to focus past the stop when it got hot), and it allowed you to do infrared infinity focus.

Jack
05-17-2009, 07:52 PM   #13
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QuoteOriginally posted by jbinpg Quote
My early 1980s M* 300/4 focuses past infinity at the upper stop so you can't just say all manual focus lenses you can just "twist and forget".
I'd still guess that aligning the infinity mark with the focusing index would work just fine for shooting stars with this lens.
05-18-2009, 02:23 AM   #14
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QuoteOriginally posted by Sean Nelson Quote
I'd still guess that aligning the infinity mark with the focusing index would work just fine for shooting stars with this lens.
No, not with longer focal length lenses. Temperature can have a very real effect on the lens barrell and the collimation of the lens elements inside are dependend on the temperature, too. For example all mirror lenses (I know off) can be focused beyond infinity as the high magnification the secondary mirror applies, makes them very sensible to dimensional variations induced by temperature shifts.

With a lens, that can be focused beyond infinity I would never rely on the printed or engraved infinity mark, because you can simply not repeat a position - your reading will be not exacting enough. On photographic telescopes you either have a 1:10 gear for fine focusing or a micrometric scale. Focus is probably the single most sensible factor for astrophotography and barrell markings are simply not precise enough - and you can hardly ever turn the lens barrell to exactly the same position as well.

Ben
05-18-2009, 01:09 PM   #15
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QuoteOriginally posted by Ben_Edict Quote
For example all mirror lenses (I know off) can be focused beyond infinity as the high magnification the secondary mirror applies, makes them very sensible to dimensional variations induced by temperature shifts.
Mirror lenses are a bit of a special case because light travels the length of the barrel three times, thus exaggerating any thermal expansion effects.

I've never had any issues shooting stars using the hard infinity stop on manual (non-mirror) lenses up to 200mm over a range of temperatures. If nothing else, in the absence of something bright enough to focus on manually (such as the moon, for example) the infinity mark at least provides a place to start in trying to determine what the correct focus should be.
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