Sharp Stars Vs. Blob Stars

I got both, but I'm not sure how.

http://i67.photobucket.com/albums/h2...d4morecopy.jpg
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http://i67.photobucket.com/albums/h2...3/th_stars.jpg
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The first image is a 64 second exposure and the stars are sharp (but the image is low quality for hosting). The second image is a 30 second exposure of a different section of sky. The 64 second exposure is the best and sharpest star picture I've taken yet but I don't know why.

Sometime between the two images I adjusted my focus to make sure it was at infinity; I think I pushed it past infinity.

I want to be prepared to take sharp star pictures outside of the light polluted suburbs when I travel. Please help! :)

I can't see the images since I'm at work, but do you know what aperture (f-stop) you used for both? It has nothing to do with the DOF like how we usually talk about aperture, but more to do with exposure and being blown out to a certain degree.

The sharp picture was f2.8 and the blob picture was f2.0.

Now that I can see the shots, it isn't what I thought. I think you're right about the focus, some lenses can be focused past infinity. I had one that did this and read the reasoning behind it although I'm not 100% clear on how all of that goes together (I just shoot, they design the lenses :)). If you can focus on something like a tree or a sign in the distance, then you're all set for this.

So when the tree was in my picture it probably helped the overall focus?

I tried more 30 second shots last night. I refocused the lens to infinity and shot at f2.8 and it was noticably sharper than what the lens was left at and f2.8.

The next clear night I have I'll try varying both focus and aperture.

Are you using autofocus? That's going to be tough in a situation like this, which means that your results might not be consistent. If this is the case, I'm surprised the camera can "see" the tree, but no doubt it's helping.

If you're using an SLR or any camera that you can focus manually, do that and focus on something that's a good distance away - whatever you have available. Obviously the stars are farther away than that, but focusing on something 50' away should give you pretty good results. And, I'd argue that if the tree, sign or anything else is in the shot, it's more important for that to be in focus than the stars which have no detail anyway. You also might try experimenting with a smaller aperture like f5.6 too.

I hit the autofocus and the lens went right to infinity and that's when I put it on manual focus.

The tree was probably 20-30 feet from me. I could always go into the street or to the park where I shot my panorama to get something farther away than an object in my back yard and I'll definitely try a smaller aperture and maybe a higher ISO than 200.

Maybe the tree was clearly visible because I flashed it with my point-and-shoot?

The camera I have is a Rebel XTi and the lens is a Sigma EX 30mm.

I did a few pictures tonight with f5.4 at ISO 400 and the stars are now very sharp but the trails seem more pronounced. Is that because of the ISO setting?

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Originally Posted by Master Shake

I did a few pictures tonight with f5.4 at ISO 400 and the stars are now very sharp but the trails seem more pronounced. Is that because of the ISO setting?

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The longer the exposure the more sky will move. The Telescope have a polar mount and they track a bright star with a spotting scope so they do not get star trials. The stuff they take photos of end up with 30 minute or longer exposures for most objects of interest.

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Originally Posted by freygr

The longer the exposure the more sky will move. The Telescope have a polar mount and they track a bright star with a spotting scope so they do not get star trials. The stuff they take photos of end up with 30 minute or longer exposures for most objects of interest.

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Ok, I thought we got this all cleared up with Copernicus.:mad2:

The sky does not move. The earth rotates on it's axis at 1037.5646 miles per hour, or 17.2927 miles per minute, thus making it appear that the heavens move. :D (sorry, I just couldn't resist.)

Which will still have an effect on the sharpness of objects photographed in the heavens. An equatorial mount is a must for an exposure of any length. Nice little tutorial on Astrophotography:
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

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Originally Posted by gryphonslair99

Ok, I thought we got this all cleared up with Copernicus.:mad2:

The sky does not move. The earth rotates on it's axis at 1037.5646 miles per hour, or 17.2927 miles per minute, thus making it appear that the heavens move. :D (sorry, I just couldn't resist.)

Which will still have an effect on the sharpness of objects photographed in the heavens. An equatorial mount is a must for an exposure of any length. Nice little tutorial on Astrophotography:
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

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Heathen! Renounce the devil's science! We all know the earth is 6000 years old, flat, stationary, and there are dragons where the oceans meet the edge of the world.

:P

Just off the top of my head (and without much to back it up besides a vague memory of something I think I once learned), I thought a larger aperture (10, etc) would yield a sharper edge, just because you're forcing the light to come through a narrow opening and travel a more restricted and direct path to the sensor.

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Originally Posted by gryphonslair99

Ok, I thought we got this all cleared up with Copernicus.:mad2:

The sky does not move. The earth rotates on it's axis at 1037.5646 miles per hour, or 17.2927 miles per minute, thus making it appear that the heavens move. :D (sorry, I just couldn't resist.)

Which will still have an effect on the sharpness of objects photographed in the heavens. An equatorial mount is a must for an exposure of any length. Nice little tutorial on Astrophotography:
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

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Now Now - Have you not heard of the theory of relativity. To the observer the sky moves. Yes the earth is flat:D

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Originally Posted by Master Shake

I did a few pictures tonight with f5.4 at ISO 400 and the stars are now very sharp but the trails seem more pronounced. Is that because of the ISO setting?

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You probably mean f5.6? f5.6 at ISO400 = F4 at ISO200 which is of course less exposure (by one or two stops) than you had before. Was this shot in the same area (in other words, was one shot in the city and one in a rural area)? Same focal length for both shots? Hard to tell without an example.

The earth is indeed flat, at least here in Northern Illinois - until you you ride your bike in an event called the "Ups and Downs". Then you realize it's decidedly not flat. :)

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Originally Posted by another view

You probably mean f5.6? f5.6 at ISO400 = F4 at ISO200 which is of course less exposure (by one or two stops) than you had before. Was this shot in the same area (in other words, was one shot in the city and one in a rural area)? Same focal length for both shots? Hard to tell without an example.

The earth is indeed flat, at least here in Northern Illinois - until you you ride your bike in an event called the "Ups and Downs". Then you realize it's decidedly not flat. :)

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I never understood exactly how flat Northern IL was until I drove through Iowa and Nebraska... and thought 'don't they call these states flat?? This is hilly!'