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01/16/2005 01:13:14 PM · #1 |
I decided I wanted to take a few long exposures of the stars and was not very happy with the results. They all seem very overexposed and there appears to be a lot of noise. The camera was mounted on a tripod on the roof and there was VERY little ground light at all. Just wondering if I did something wrong or if there is something wrong with my camera...
First photo is resized only, second photo is 100% crop (90 min. exposure with sigma 15mm fisheye @ f16)
 
First photo is resized only, second photo is 100% crop (55 min. exposure with sigma 15mm fisheye @ f16)
 
First photo is resized only, second photo is 100% crop (156 min. exposure 28mm @ f16)
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01/16/2005 01:42:47 PM · #2 |
| Looks like sensor dust and hot pixels. No idea on the overexposure other than the obvious of you're not getting the right exposure settings. |
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01/16/2005 01:42:52 PM · #3 |
In a 90 minute exposure, ANY ambient light at all is going to ruin your photo. You might try doing a dark-frame subtraction for the noise, but 90 minutes seems excessive to me. I don't have any experience with exposures this long with a digital cam.
-J |
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01/16/2005 01:48:14 PM · #4 |
unless your tripod has a rotater that is aligned with earths rotation, you are going to long streaks. Especially with a 90 minute exposure. 90 minutes is entirely too long for stars, Ill have to look up the formula that gives you times for focal lengths. Excuse me, I have to scream as the eagles scored TD number 2
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01/16/2005 01:50:11 PM · #5 |
That's noise. That's one area where film beats digital, any exposure longer than a few minutes brings these hot pixels.
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01/16/2005 02:26:59 PM · #6 |
Thanks for the info... so this noise is normal and nothing I should worry about?
Jeremy... the reason for the photo WAS to get the long streaks. Is there another way to get the streaks without doing a 90min+ exposure? |
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01/16/2005 02:34:30 PM · #7 |
| A lot of that noise appears to be sensor dust. f/16 is WAY too much. The stars are essentially all on the same plane at infinity, so there's no reason to stop down. Open your lens up and point it at the North Star. The star trails will create concentric circles around Polaris. |
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01/16/2005 02:48:14 PM · #8 |
Couple questions:
- What was the ISO?
- Why f/16??
I assume you were going for star trails. I've found that it is much better to break these long exposures into shorter segments. Subtracting a dark frame from the segments will kill the hot pixels, and the averaging effect of combining the expusures will really reduce random noise.
It does look like ground lighting has crept in, and that's why everything is so washed out. The least little bit of ground illumination will really play havoc with exposures this long. Find the darkest site you can realistically travel to. The very best sites are at high altitude. Those of in the Midwest are at a bit of a disadvantage here.
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01/16/2005 03:08:10 PM · #9 |
When shooting trails with my 10D, I normally didn't exceed 25mins for a single exposure (the same goes for my 20D). If you want more rotation, do multiple exposures and combine in Photoshop or Astrostack. I have a TC80N3 intervelometer remote for my Canon which does everything for me. Set your focus to infinity, shoot iso100 and normally you want to use an aperature between f/4 and f/8. You can use NoiseNinja or NeatImage to filter out some of that noise, but I've made some decent sized prints and it doesn't bother me much at all.
The main reasons I keep my expsure time down are because of sensor noise and light polution. Light polution is what is making your sky so bright. If you are able to, travel away from city lights. Here's a useful map showing light polution in the USA //www.inquinamentoluminoso.it/worldatlas/pages/fig2.htm
Also, an interesting foreground is key if you are trying to make more captivating photos.
In case you're curious, here are a couple of the one's I've done (all with the 10d):
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01/16/2005 03:52:58 PM · #10 |
I've found it best to shoot at ISO 100 to capture stars, because so much less noise is present. In both of these images, my camera was set at ISO 100, and even then, it took a lot of cleaning up:
 . . . . . 
These were also relatively fast shutter speeds--under 30 seconds. Shooting for 90 minutes would be fun, but my camera can't do that (my film camera can, though). You might try a pinhole lens cap for such purposes.
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01/16/2005 04:28:57 PM · #11 |
I was shooting at ISO 100 and I choose f/16 as to not let in too much light. I'm actually located in Iraq on a military base and there is a rule about exterior lights on after dark (they are not suppose to be on). The only lights that could be illuminated are personal flash lights, possible car lights, or the red beacons on antennas.
Thanks for the advice... I'll give it another try with your suggestions. I'll try a larger aperature, shorter exposures, and combine the exposures.
Brian:
Those are alot like what I was trying to go far... those are great. How did you manage to illuminate the rock bluffs?
The photos I posted first were unedited... these are about the best I could come up with after a dark frame... that is, if I did it right.
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