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08/18/2006 08:33:25 PM · #1 |
I am going to finally be getting a DSLR soon (probably the D30) and one of the things I have been looking forward to is being able to do very long exposures. For the purposes of this conversation, I mean above 30 seconds up to maybe 5 hours.
In talking with my brother  trnqlty he seemed to suggest that you have problems taking long exposures because the sensor heats up and causes an inaccurate color hue on a photograph. He noticed the problem on this one minute exposure.
Editing has taken care of it, but the original shows a purple color cast in one corner of the photo. He had thought is was flare, but after talking to someone else about it, he concluded that in may have been sensor heat. I don't know.
ANYWAY, the question is am I going to be able to take loooong exposures, say 3 hours for a star streak shot in Big Bend National park. (i.e. no lights for miles and miles). Are there any inherent limitations to shooting with digital for this type of shot?
Message edited by author 2006-08-18 20:34:09. |
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08/18/2006 08:40:53 PM · #2 |
actually the benefit of long exposures on digital is that you can get a more accurate hue than film will give you (film has reciprocity failure in both exposure and color reproduction at exposures of a few minutes or more)
your problems come in with noise, and most of all battery life. Dont expect to get a 5 hr exposure on a digital slr, your battery will be dead well before that. A battery grip will double the time, but i wouldnt hope to get more than an hour really. The sensor staying on really drains the battery.
Ive done a couple exposures on my d70 for 5-20 min, and noise becomes a bit of an issue, but nothin i really mind. the 30d will be better with noise as well.
That purple haze was sensor heat, its happened on my exposures too. that's not lens flare. |
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08/18/2006 08:45:11 PM · #3 |
No limitations that you can't get around. The sensor heating thing is a reality. It can be compensated for by using dark frame subtraction. For star trail shots, this complicates things slightly. normally you'd do the dark frame after each exposure, using the same time of exposure. That's problematic when your exposure length is measured in hours.
Sensor heating is much less problematic with exposures in the minutes.
A good way to approach the star trail shot with digital is to break up the exposure into many shorter exposures. For example if you're after a 3-hour exposure, try 30 six-minute exposures, or 15 12-minute exposures. You'll need to fire the next exposure immediately after the preceeding one to avoid gaps. At the beginning and end, shoot a couple dark frames with the same settings, and average them all. Use the averaged frame as your dark frame.
One huge benefit of breaking up the exposure is that you'll average out random noise, and the final image will be smooooth. Overall, the long-exposure results possible with a DSLR are stunning.
Edit:
Here is a 6-minute exposure with the 5D. Click on the "O" below the image to view the largest size. That size is still reduced to less than 1/4 of the original pixel count :-)
It's also a single exposure from an area with moderate light pollution. It could be much better if I'd used a more sophisticated workflow (stacking four exposures, using dithering to avoid color artifacts, actually shooting & subtracting a dark frame, etc.). In fact, it's just a test image to gauge the performance of the Canon 24/1.4L lens on the 5D. Needless to say, I kept the lens.
Message edited by author 2006-08-18 20:58:11. |
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08/18/2006 08:49:16 PM · #4 |
| awhile back ago, someone asked the same question and had the same problem. For the problem, everyone concluded that it was light creeping inside the camera somewhere, because they where near there porch with their lights on from i think like 20ft or less (i don't remember). as the for the noise, i don't remember the mabo-jumbo tech stuff there where saying, but what i do remember is that noise is visible on any camera no matter at what iso, Bear_Music also gave us a lecture on how film has a similar problem at a certain time. I can't find the thread, since i'm not good at searching for threads, i'm sure someone could find it. |
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08/18/2006 09:22:35 PM · #5 |
Some Nikons get purple corners (one or two, always the same) at long exposures. Sometimes I have it and sometimes I don't. Depends on how dark the background is. Even at -15 celcius I had it on 30s exposures.
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08/19/2006 09:12:59 AM · #6 |
I read a year or so back in a technical article about CCD's vs CMOS that CCD's build up a fair bit more heat than CMOS. That's the primary advantage of CMOS.
The 30D is therefore a pretty good choice and will probably outperform your brother's D70.
Having said that, I like kirbic's suggestion. There is image stacking software available for free (I think)... I think it was from sourceforge.net can't remember right now and it's bedtime. Sorry.
Worth it to do the Dark frame subtraction thing... I would guess that it works quite well...
As a point of interest, the 30D (and reportedly the 5D) apparently has a sensor map applied as part of the sensor cleaning cycle.
As your camera gets older, occasionally some pixels will get stuck or go hot. This seems to show problems a lot more in long exposures. However, the benefit of having sensor mapping in the firmware is that it will map these out and you will get a lot more consistent performance in the long term.
This hasn't been officially confirmed by Canon, but has been independently confirmed in the forums on DPReview.com.
For this kind of thing, I wonder if it might be worthwhile to have a battery grip? |
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08/19/2006 09:16:05 AM · #7 |
Originally posted by eschelar:
Having said that, I like kirbic's suggestion. There is image stacking software available for free (I think)... I think it was from sourceforge.net can't remember right now and it's bedtime. Sorry. |
Registax has worked well for me in the past. |
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08/19/2006 01:29:05 PM · #8 |
Thank you, yes, that was the name of the program.
Hey, I didn't go to bed yet!
Stupid me spending four hours researching and an hour posting my ramblings on printers... at least I didn't screw up and accidentally delete the whole thing just before posting though... :) |
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08/19/2006 02:14:54 PM · #9 |
Originally posted by noodleboy:
awhile back ago, someone asked the same question and had the same problem. For the problem, everyone concluded that it was light creeping inside the camera somewhere, because they where near there porch with their lights on from i think like 20ft or less (i don't remember). as the for the noise, i don't remember the mabo-jumbo tech stuff there where saying, but what i do remember is that noise is visible on any camera no matter at what iso, Bear_Music also gave us a lecture on how film has a similar problem at a certain time. I can't find the thread, since i'm not good at searching for threads, i'm sure someone could find it. |
I remember the thread and I had inquired whether that individual had covered the eyepiece, they had not and that was 'part' of the problem. They had some light leak coming through the eyepiece which is not completely closed off from the sensor and you will get backlighting. That is why they provide the eyepiece cover for long night exposures. Hope this helps!
Message edited by author 2006-08-19 14:16:51. |
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