Long Exposure Noise

I recently tried to shoot star trails with my 40D but got a lot of noise. I shot at ISO100, f/16 and 45 minutes, long exposure noise reduction off. I did shoot in RAW, but I was so frustrated I deleted the photo before trying noise reduction in LR. I've heard that digis can't do long exposures like film. My questions are, is there any way to do a very long exposure (30 minutes+) without ridiculous noise? and what is the longest exposure I can get a good photo with on my 40D.

<p>I have done long exposures with digital and had quite good luck. Depending on the light level, you might have been underexposing, which can add a lot of noise. I would try that same exposure at f/8 or f/5.6 and see what that gets you.</p>

<p>Incidentally, for long exposures I usually start with trial shots at ISO 1600 and with the lens wide open. You can do some test exposures at 30 or 60 seconds to get an idea what the picture will look like. Once you're happy with the composition and exposure, drop the ISO and aperture and raise the exposure time. Then you only have to wait through one 45-minute exposure.</p>

<p>Yep, use the dark frame mode (long exposure noise reduction on). Just realize the dark frame used to remove noise takes exactly the same length of time as the original exposure. So your star trails would take 90 minutes total to complete but would have looked great.</p>

<p>If you were mainly shooting the sky, shoot wide open or stopped down just enough to tighten up corners. F16 is too much.</p>

<p>One problem is that the sensor can heat up during long exposures and that leads to excess noise. You can actually buy modified cameras converted to use active sensor cooling for long exposure work, but it adds $1000 or more to the cost of the camera.</p>

<p>Dark frame subtraction makes things better, but not perfect. Some noise is random and (random-random) doesn't equal zero. You don't have to use the camera to do the subtraction on every shot. You can just take a shot with the lens cap on (for the same duration as your exposures), then do the subtraction later in software.</p>

<p>You must turn the long-exposure noise reduction setting on. Sorry.</p>

<p>Dan</p>

<p>As Puppy said, there's no point whatsoever in shooting star trails at f16. Everything is at infinity do there's no depth of field issue. Shoot with your aperture wide open. This will greatly reduce exposure time and therefore greatly reduce noise.</p>

<p>For example, if you took the same shot at f2.8 using ISO 400 your exposure time of 45 minutes would be reduced to just 1 minute 40 seconds! That not only saves you lots of time but will give far less visible noise.</p>

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<p>You can just take a shot with the lens cap on (for the same duration as your exposures), then do the subtraction later in software.</p>

 

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<p>Bob, or someone, could you please elaborate on this a bit more?</p>

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<p>Bob, or someone, could you please elaborate on this a bit more?</p>

 

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<p>When I use software to do this, I'll take the dark frame, put it in a layer on top of the photo and set the blending mode to 'difference', which usually does a decent job. I do find however, that I'm usually please with the work the camera does on its own and only use the manual dark frame subtraction if I've combined a number of shots.</p>

<p>I'm not sure what Bob's method is, but this is what I've found to work the best.</p>

<p>Star trails? Nothing beats stacking for those. Shoot continuous 30s frames and stack them later. There are various tutorials for this on the web.</p>

<p>I didn't carefully read your original post before I responded earlier. In my first response I just wrote "turn the long-exposure noise reduction setting on." This is correct - without that you will get "hot pixels" and a generally noisy shot.</p>

<p>I failed to notice your aperture and ISO settings. There are quite a few reasons to not stop down to f/16. They include:</p>

<ul>

<li>On your cropped sensor camera you will at least slightly reduce overall sharpness of start trails and subject through diffraction blur at this small aperture.</li>

<li>Star trails will tend to be rather dim since effectively the only two factors controlling their exposure are ISO and aperture.</li>

<li>Your exposure time will necessarily be extremely long - though perhaps that is what you wanted?</li>

</ul>

<p>At ISO 100, f/8 is typically a good aperture for star trails. Depending upon what you are going to end up doing with the photos you might try ISO 200 at f/5.6 if you want to shorter the exposure a bit.</p>

<p>Although I haven't done it myself (I have never exposed longer than about 15 minutes at night) I know of many folks who use and favor the exposure stacking method for the very long exposures. I know folks who have made star trail shots of several hours duration using this approach. I wish I had a link to some of the programs that are available to help you with this, but Google is your friend.</p>

<p>Dan</p>

<p>Thanks everyone. I originally used f/16 because I wanted a long exposure to get longer trails, but now I realize that I can still have a long exposure with the aperture opened. I'll use the long exp. NR next time too. Thanks for all the tips and help.</p>

<p>nathan g-on dark frame. normnally if the shutter speed gets long enough you should be using dark frame subtraction to make less noisy imsages. the problem with this is the dark frame is as long as the each original shot. so when you a series of image, that a group of 30sec imazges, each is going to be followed by its own 30sec dark frame. the effect ofter stackimng the images it a dashed curve across the sky, with lots of long dark dashes(the dark frames). a way around this is to shoot with NO DARK FRAMES. and simply shoot after you are done a single eposure with the lens cap on. then in photoshop simply subtract this one dark exposure from each of your good images. now you have dark frame subtraction for each image. you have taken 1 dark frame and not a series of them. this also cuts your field time way down.</p>

<p>there is a nother problem you must consider. and that is the length of the curve in your final image. you mentioned 30minutes. but that is only 1/48 of the day. 1/2hr / 24hrs=1/48 OR 1/48 of a circle. OR 48/360=.13 of a full circle. that is not much. yrs ago when i shot some skytrails i used film and it was a lot easier. you simply went somewhere away from city lights and left the camera on bulb for 4-6hrs and you got a very nice startrail. with digital becasue of heat built up noise you are limited to so mnay seconds and have to make a series of exposures to get the same thing. my suggestion is to forget digital and find a film slr with the same lens mount that your digital slr does and shoot film. maybe you still have a older film body somewhere. i have 3 film slrs bodies in the basement. remewber you do not even a working meter since the exposure is on bulb. try-<br>

<a href="http://www.danheller.com/star-trails.html">http://www.danheller.com/star-trails.html</a><br>

<a href="http://www.astropix.com/">http://www.astropix.com/</a><br>

<a href="http://www.astropix.com/HTML/J_DIGIT/TOC_DIG.HTM">http://www.astropix.com/HTML/J_DIGIT/TOC_DIG.HTM</a></p>

 

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<p>For example, if you took the same shot at f2.8 using ISO 400 your exposure time of 45 minutes would be reduced to just 1 minute 40 seconds! That not only saves you lots of time but will give far less visible noise.</p>

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<p>Problem 1: The earth doesn't turn very far in a minute and forty seconds. The OP's star trails would look like star whiskers.</p>

<p>Problem 2: Because the stars "move" in relation to the camera's position, they don't cast light on the same spot on the sensor for long. Therefore, you can't apply reciprocity law to alter the time of exposure. That would make sense only if the light source were not moving steadily.</p>

<p>For star trails, the exposure value is the same whether you're tracking the stars for five minutes or five hours. The aperture only changes the width of the trailing lines. At f/2.8 or f/4 the lines will be full and prominent. At f/16 they're be thin and stringy.</p>

 

<p>Here is my photo using multiple shot method.</p><div></div>

<p>Dan South: You make 3 excellent and very valid points, a couple of which I never even considered. I agree with you entirely.</p>

<p>I, personally, dislike the look of most stacked startrail images, they often don't come close to the look of film star trails, which I think is the standard to match for this kind of photography. Also, in bigger prints, the gaps are definitely noticeable.</p>

<p>If you do a 30 min exposure, then a 30min dark frame, it will certainly help but noise at 30 minutes will be a problem regardless so i recommend using something like Noise Ninja. It does wonderfully on these kinds of images, leaving the star trails sharp and reducing noise on the background. Using noise reduction in the camera does help a lot but does make for a long exposure. Stacking multiple images is fine but the little gaps between images really does ruin the trail as far as I am concerned. Star trails is one area where film has it all over todays cameras. There is no way I know of to truly do a nice, clean 4 hour exposure with digital cameras like one might with a film camera. Anyone who wants to specialize in star trails might do well to use film. If you can stand the gaps created by stacking it is a great technique. In fact it's really the only technique for a 4-6 hour exposure.</p>

<p>You can do various things to reduce it, but you'll never avoid it.<br>

Look into software such as Neat Image (my fav) or Noise Ninja. They're simple to use, and go a long way to recover from nuisance of noise where it's not wanted.</p>

<p>Due to the response curves of film and digital sensors being different, you will not be able to replicate extremely long (or short) exposures of one format with the other. Each will have its unique look.<br>

<br /> Actually, another advantage of manual film cameras is that some do not require a battery to take the multi-hour exposure.</p>

<p>Attached photo is a stack of digital exposures.</p><div></div>

<p>A film exposure of 7 hours duration.</p>

<p>On my site: http://homepage.mac.com/tarashnat/astrophoto/MF-00319-10.html</p><div></div>

<p>Using a wide enough lens and a short enough gap between exposures and the wider your aperture is open minimizes the size of the gap and the gaps may actually just become just "notches" in the trails or not noticeable.</p>

<p>Slightly defocusing the image may also blur out the gaps. I haven't tried this yet.</p>

<p>@Taras:<br /> I find that 25-30sec. exposure is the ultimate compromise between visible trail and a "stick". So, I just have continuous shooting (just lock down my shutter release cable)<br>

Also, good point on focusing as well. I focus to infinity, then pull back just a tad. In my experience, that produces the sharpest image.</p>