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11/27/2006 02:10:58 PM · #1 |
I went camping this weekend and I thought I would try to shoot some star trails. I made two attempts. One exposure about twenty minutes and the one below was about thirty minutes. What is the fog in the lower left? It appears in both shots. Is it some kind of electrical interference from within the camera?
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11/27/2006 02:25:02 PM · #2 |
| This is standard with such long exposures in the D70 (I imagine other cameras have similar problems). The heat from the camera starts to register on the sensor and causes this. |
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11/27/2006 05:02:08 PM · #3 |
Originally posted by eqsite:
This is standard with such long exposures in the D70 (I imagine other cameras have similar problems). The heat from the camera starts to register on the sensor and causes this. |
If you use the Noise Reduction option in the shooting menu then the purple will disapear - but the exposure time will be twice as long, so make sure you are happy with your composition before hitting the shutter on a half hour exposure.
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11/27/2006 05:05:57 PM · #4 |
Originally posted by dr_timbo:
If you use the Noise Reduction option in the shooting menu then the purple will disapear - but the exposure time will be twice as long, so make sure you are happy with your composition before hitting the shutter on a half hour exposure. |
Are you sure about this? Because I could swear that I've tried that and still had the purple -- but it has been a while so I could easily be mistaken.
Edit to add: It really doesn't make sense to me that this would work, because the noise reduction is really just a dark frame subtraction, and the dark frame should get the same purple haze from the heat as the live image did.
Edit again: Now that I re-read my first edit, maybe you are right. If the dark frame gets the same purple, it should subtract out, right? I'm just talking to myself here and getting myself confused :(
Message edited by author 2006-11-27 17:09:11. |
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11/27/2006 05:11:18 PM · #5 |
had the same problem in my D50 with the long exposures.
also tried to get the star rails, with an exposure of about 2 hours.
but then the noise reduction will take 2 hours as well? pretty long time, is the time for that process the same in other cams (for example canon)?
edit for spelling
Message edited by author 2006-11-27 17:11:49. |
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11/27/2006 05:39:22 PM · #6 |
Thats interesting as I had several 15min+ exposures and never had this problem. I would have guesses a lens flare. I haven't done any long exposures(more then 30 sec) with my D200 since I have no form of cable release. Using the Noise reduction doesn't double the actual exposure time. The time the shutter is open is the same, it just works on the file to get rid of hot pixels and such for the same amont of time as the exposure. Although if you shoot in raw somehow the hot pixels goes away when converted in CS2. Here is an example without noise Reduction and shot in jpeg and raw. The top photo is the raw file.
 This shot was around 15 or 20 min. |
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11/27/2006 05:59:22 PM · #7 |
It has something to do with the CCD sensor in the d70, I think. I did some pretty long ones without noise reduction on my XT, and didn't have that problem ar all.
This one was a 1hr 45min exposure, with NR.
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11/28/2006 12:55:56 AM · #8 |
Yes, that is a problem with the D70.
Noise reduction helps. What it does is take a black frame exposure (record data on the sensor without opening the shutter) for the same length of time as the exposure just taken. This black frame then contains only the sensor noise, which is then subtracted from the image. The removes the noise.
The same thing can be done by taking a set of black frames on your own for the same time periods as the exposures you use. Then subtract the appropriate black frame from the image in PS. It won't be exact as the sensor noise does change a bit, but it will be fairly close without having to double the exposure time for each exposure.
David
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11/28/2006 11:04:47 AM · #9 |
Originally posted by David.C:
Yes, that is a problem with the D70.
Noise reduction helps. What it does is take a black frame exposure (record data on the sensor without opening the shutter) for the same length of time as the exposure just taken. This black frame then contains only the sensor noise, which is then subtracted from the image. The removes the noise.
David |
Thanks for evryone's input. This explains a lot. Now I have a clearer understanding of how the noise reduction works. |
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