Night photography: Faking long exposures for fun and profit
This image was constructed from nearly 700 separate exposures, instead of a single long exposure. On a digital SLR, long exposures tend to introduce large amounts of noise. Faking a long exposure by composing many different shorter exposures together represents an alternative that can work quite well. It also gives a lot of flexibility in tweaking the result: If a plane flies in front of the camera, you can simply remove the frames that were being taken at the time.I’ve recently picked up a power supply for my Canon DSLR camera, with the intention of doing some nifty long term exposures and time lapse with the tethered shooting feature of the camera.
So, I decided to try to capture some star trails over the city, from our 14th floor apartment balcony.
I set up the timer to take one 30-second exposure, the longest I could use for tethered shooting, every 30 seconds (actually, every 35 seconds, as a few seconds were required to transfer the image). There needed to be as small a gap as possible between exposures, as otherwise gaps would appear in the star trails. For this, one needs to turn off the camera’s automatic noise removal facility, which takes a “dark frame” exposure of the same length as the primary exposure. The dark frame can be used to remove the noise, but at the cost of doubling the time to take the image.
This noise removal process can be done manually, however: Take one or two dark frames, then subtract them from the images later. In ImageMagick, this looks like:
convert -compose minus -composite dark-frame.jpg photo.jpg photo-output.jpg
To start, I first took a dark frame exposure with the lens cap on. Apparently, one should do this at the end too, as the noise signature changes over time, but I didn’t bother and it worked out fine.
I turned the timer on, and went to bed.
At the end of the process, I had a large number of photos to put together. This can be done by overlaying each image, with a ‘lighten’ compose operation.
I wrote a script which uses the ImageMagick toolkit to subtract the noise pattern from the dark frame from each photo, then compose each photo together. Here’s the script:
To use it, drop to the Terminal and type something like:
./compose.sh -noise dark-frame.jpg exposures/*.JPG
This assembles all the images and outputs a single jpeg.
Voila!
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13 Responses to “Night photography: Faking long exposures for fun and profit”
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Olá Mike, this is getting to be a regular habit (addiction?) checking out the blog when I should be working… none the less, a couple of ideas i thought of when i saw this 1) i didnt quite follow the justification for the 30 s gap between exposures to avoid gaps in the star trails, it would seem that a longer gap between shots would increase the gaps in the trails…. 2) since you have that 30s gap in between the shots, is it worth take a second low exposure shot in to balance out the saturation you get from the city lights. Yes, this would add HDR into your shot, but what I assume is the Carlton Exhibition Building and Rathdowne/Elgin/Lygon maybe? st are both quite overexposed whereas the skyscrapers come out nice, even for a 30s exposure…. 3) i hadnt come across the noise issue in Dig cameras, it would be interesting to see the historgrams or anything else you have that shows the problem.. do you have anything? 4) sweet photo again!
take it easy…
p.s. the weather here is looking awesome, still only 1 or 2 degress most days, but crystal clear skies, looks like summers gonna be a good one, get over here!
Ah, procrastination…Tis a harsh mistress.
Ah yes, I see there was a bit of ambiguity there. Nope, no gaps – by 'there needed to be a minimal gap', I meant as small a gap as possible. Obviously bigger time gaps, means bigger trail gaps. 30 sec exposure every 30 secs means a constant exposure (as opposed to a 30 sec exposure every 1 min, meaning 30 secs of idle time)
Because it's made of many different shots, I actually have quite a bit of control over exposure.. That's fairly deliberate; I did do some tweaking, but I was happy with it. I guess taste is subjective!
I'm not entirely convinced about the noise issue yet either, not having tried it yet – I just read about it in a few places over some forums (I can't remember where, I'm afraid). Next thing I'm trying is just leaving it on, although it means getting up :P
Looking forward to seeing Sweden, is gonna be great!
ahh, makes a bit more sense now.. i suppose i actually should have figured that out based on the fact you used 700 shots… :)
it would be interesting to see the effects of noise.
Actually, have you read John Wyndhams "Day of the Triffids"? This is very reminiscent of the meteor shower that starts the book.
Again I am blown away, wow! It is an inspiration, reminds me I need to think out side of the box more! I have several good vantage points around Atlanta to do something similar, how much time did it take?
T.
Cheers, Terry! That was basically all night, from sundown to sunup
Oh, I don't know if I could do all night, us old geezers need our beauty sleep! LOL! Still, the work is inspiring, I have been grinding along now for quite a few years and it's nice to see fresh work. :)
T.
Amazing!.. It doesn't look like its taken on separate exposures..
Is it bad for the Camera to take so many photos with such a long exposure?
Not at all – why would it be? Unless you were aiming the lens into the sun, I guess..
really cool picture!
how did you get ure camera to take pics all night long? i got a canon eos 40d and i'm just starting to get into dslr photography…
You'll need a laptop, a copy of the software that came with the camera (specifically, the EOS utility), and an AC adapter for the camera. Then, you set the EOS utility to timer mode, and it'll capture an image at regular intervals.
Oh, okay :) Got it…
Thank you!
I use to set my camera on “continuous shooting” and shuter speed at 30s and then lock my remote control.
And thanks for the script, it’s useful.
//Fredrik