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 Blurry Stars 
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Joined: Wed Dec 03, 2008 1:25 pm
Posts: 89
Location: Kansas, USA
Post Blurry Stars
I made a timelapse last night with my Canon 50mm f/1.8 focused to infinity and set at f/1.8.
I was very sad to discover that my images were REALLY blurry.
Examples from that night-
f/1.8 15seconds ISO800 below
Image
f/1.8 15seconds ISO800 below
Image

So tonight I went in my front yard and focused it to infinity and did a comparison between f/1.8 and 5.6 to see if it was just the aperture.

I found that the house across the street was in focus when the focus was set to infinity with f/1.8, does that mean that the narrow depth of field at f/1.8 will make things that are significantly farther than it (like stars) out of focus? Even if it is focused at 'infinity'?

Examples-
100% crop of f/1.8 below
Image
100% crop of f/5.6 below
Image

I don't think the lens is messed up because it is still sharp in those last two pictures. I did a final test by shooting the only star I could see, but I don't think it is fair comparing those pictures because I think passing clouds probably messed with the clarity of them.

If you missed my question, it was right before the house pictures.
Also, I've been thinking of getting the 50mm f/1.4 - Would its infinity focus at f/1.4 be better than my 1.8 because it is better quality? Look at this image taken with the 50 1.4 wide open, how much of the good IQ is from the 1DsMKIII and from the lens?- http://www.flickr.com/photos/mikejonesphoto/2613270589
Thanks,
-matt

P.S.
Does anyone know what is causing the green flare in the house pictures? I know it is a lens flare of some sort from the lights on the house, but is it because of a poor quality UV filter or is it just the lens?

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Sun Dec 07, 2008 8:27 pm
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Joined: Tue Aug 26, 2008 3:30 am
Posts: 824
Location: Sydney, Australia
Post Re: Blurry Stars
Re: your lens flare - if you look closely, you'll see that the flare pattern matches an repositioned "upside-down reversed" image of the very over-exposed xmas lights in your frame.
The green cast comes from a combination of the lens coating colour and internal optical reflections inside the lens barrel.

Re: your soft focus, at wide open the focus point is critical, some one else on this site suggested focusing on a fairly distant bright steet light or similair point source to lock in a sharp focus. (I assume you had the auto focus disabled)
Sometimes you have to set your lens focus by eye - the infinity datum mark on the lens may not be true - could be just a little off one way or another.


Sun Dec 07, 2008 9:09 pm
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Joined: Tue Jul 08, 2008 12:36 pm
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Location: Antarctica/California/New Zealand
Post Re: Blurry Stars
I had a lot of trouble initially sorting out the focus on stars. What I have found works best overall is:
-Definitely no filters.
-Distant light source for initial auto-focus, then switch to manual.

Some lenses definitely get softer approaching wide open, but most are pretty good up to f2.

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Sun Dec 07, 2008 9:38 pm
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Joined: Wed Dec 03, 2008 1:25 pm
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Location: Kansas, USA
Post Re: Blurry Stars
Thanks for the input everyone. I guess I'll bring a really bright flashlight next time and set it on a tripod far away and let it auto-focus and check that.

Anyone think it is worth it to stop it down a little and use a longer shutter speed?

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Sun Dec 07, 2008 9:52 pm
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Joined: Sat Dec 06, 2008 9:35 am
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Location: Huntsville, TX
Post Re: Blurry Stars
Hi Matt... I think I can help you here. I have the same lens, 50mm f/1.8 II, and I use it for piggybacking on my telescope mount for semi-widefield astrophotography.

First thing: The focus is not at infinity and the focus is VERY tricky to get right the first few times... it's very sensitive. I didn't catch which camera you are using but if it has a live view feature I've found that is the best way to focus stars... be sure to enlarge a bright star around the center of your field to focus on. It's fairly easy once you get used to the ultra-sensitive focus of the 50mm.

Second: Don't use f/1.8 for star fields... I use around f/3.2 or 3.5 depending on the atmospheric conditions, lighting, moon... etc. For demonstration of this:

Image
Notice the distortion along the top and corners. This was taken wide-open at f/1.8.

Image
This was taken at f/3.2... notice that the stars remain fairly spherical around the edges. I probably could have used another stop here.

The f/1.8 trumps the f/1.4 in my opinion. Yes, it is optically inferior but the price point on the f/1.8 makes it the obvious better buy. Since you already have the f/1.8 I'd advise against buying the f/1.4... at least until the cheap one breaks!

A quick note about ISO settings for astrophotography... you used ISO800. During the colder seasons that's probably just fine but during the warmer months you'll want to drop that down to 640 or 400. These DSLRs aren't cooled so noise buildup becomes a real problem. I was used to CCD cameras that were cooled to -50C and it took me some time to become adjusted to an unregulated camera. I found lowering the ISO settings to be the solution to a noisy night sky... aside from hours of post processing for a stacked image!

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Mon Dec 08, 2008 10:38 am
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Joined: Wed Dec 03, 2008 1:25 pm
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Location: Kansas, USA
Post Re: Blurry Stars
Thanks Brian! By the way, those pictures are beautiful! I'm still trying to figure out image stacking, but it looks like I can only use one light image because I wasn't using any sort of tracking. But at least I can try to figure out how to subtract my darks.

I'm using Deep Sky Stacker right now, and their site said they computed ISO800 to be the best for the my camera, but I can see how if it were warmer that would not work very well for such long periods of time.

After researching the 1.4 more last night I decided with what you said- that I already have a good enough lens. I have a Rebel XT right now, and I am contemplating selling the body and kit lens and use the money to make buying a 40D less painful. The liveview- especially when tethered to a laptop- would have helped SO much last night, as well as with a lot of macro photography I'm trying to get into.

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Mon Dec 08, 2008 2:57 pm
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Joined: Tue Jul 08, 2008 12:36 pm
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Location: Antarctica/California/New Zealand
Post Re: Blurry Stars
One other thing to also be aware of, some cameras and lens combinations are much better at focusing.
My 28mm sigma 1.8 will easily lock focus directly on stars on my 5D, but my more expensive Canon 24mm 1.4L will not.
I can sometimes get the 24mm to lock on to Venus or Sirius.

For the length of exposure it really requires a bit of trial and error with the camera lens combo you have. The XT is not that good beyond 800 ISO, I probably would try to limit it to 400 ISO personally.
The 5D I will happily push out to 1600 ISO to reduce the exposure time to reduce other artifacts.

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Mon Dec 08, 2008 9:22 pm
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Joined: Sat Dec 06, 2008 9:35 am
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Location: Huntsville, TX
Post Re: Blurry Stars
Hi Matt. I'm glad you liked my pictures.

Tracking is fairly essential for any long exposure astrophotography, well obviously. However, there is still plenty to shoot if you don't have a mount... namely star trails, the public loves 'em. The other thing I like to do is attend star parties and film the people at night. Set an exposure of around 30s to a couple of minutes and you capture an interesting scene...

As for stacking... I mainly use photoshop now. I used a program called IRIS for quite some time but ditched it as my green channel was being spliced. It's a wonderfully technical program that you can run entirely from a command line interface. It would be worth your time to become familiar with it as it'll give you a better understanding about how to stack in other programs. The best part: It's free and comes with a huge tutorial base.

When you image you'll want to capture dark frames along with your light. This is rather simple to do: just put a cover on the lens and shoot a few (3 or more) exposures of the same length as your lights. These are called dark frames and are essentially a map of your camera's noise. I'm not sure how DSS uses darks but I'm sure it has some automated setting somewhere but what you do is subtract the master dark from your sublights.

BIAS frames aren't much of a concern at the moment but if you get into more advanced astrophotography it'll become necessary. BIAS frames are just a readout of the native noise in the camera's sensor... an exposure taken at or near 0 seconds.

As for trading in your camera for the 40D... I love my 40D and would have a hard time parting from it. Wonderful camera and couples great with shooting stars. The liveview function is almost required and is certainly a huge help for focusing. If you can I would agree that getting the 40D would be a good idea. Especially now that the 40D and the 5DII are out, prices drop on the lower-end models.

Here are some great references for processing astrophotos with photoshop:

http://www.amazon.com/Photoshop-Astrono ... 326&sr=8-1

http://www.amazon.com/Photoshop-Astroph ... 326&sr=8-2

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Sat Dec 13, 2008 10:31 am
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