Monday, December 1, 2008

Photographing Star Trails


Joshua Tree Star Trails

I have always been fascinated with Star Trails. It was only a few months ago that I shot my first star trail at Sequoia National Park. This past week, I decided to head out to Joshua Tree National Park, CA to practice this technique. I arrived at Joshua Tree the morning of Thanksgiving (yeah, yeah, yeah!) and after setting up camp, I headed out to the park around 2:30 pm to scout locations for the evening shoot. I must have spent around two hours driving through the park and came upon a few locations that had great potential for the shoot. Was I a happy camper or what? What could possibly go wrong? I had my locations marked, my camera was raring to go, it was a beautiful new moon night with clear skies- the Gods were with me! Or so it seemed.

Death Valley Star Trails

After eating dinner, I headed back to the park around 9 pm. As I drove through the park, I realized how very different it looked at night and since I had not clearly marked the exact locations that I had scouted earlier that morning, I did not know where to stop my car or position my camera. It was pitch dark and after an hour of driving aimlessly, I gave up and returned to my campsite. Lesson learned. The next morning I headed out to the park- this time though I noted the exact locations that I wanted to use for my shoots (I even noted down the GPS coordinates- okay maybe I went a little overboard). My OCD paid off as I was able to find the locations I wanted and got some decent shots.

Joshua Tree Star Trails

Here is my recipe for shooting star trails:

  • Plan on shooting star trails when the moon has set or during a new moon (you can get the moon phases from sites like Sunrise Sunset). You don't want any moonlight in the sky as that will wash out the star trails and will lead to an overexposed image (considering the length of time we intend to keep the shutter open to capture decent star trails)
  • I always try and shoot after 11 pm or 12 am to avoid interference from planes flying overhead. Post 12 am the frequency of the flights drops and you won't get too much interference in your shot from the navigation lights of the 'birds'
  • Do your homework earlier in the day and mark the spots where you think you want to shoot. I like to shoot star trails with the North Star in the frame. Since the earth's axis points N/S, the north star will appear static while all the other stars will appear to revolve around it (due to the rotation of the earth). That is why we get these awesome circular patterns whenever we have the North Star (Polaris) in the frame
  • If using a digital camera, lower ambient temperature, will result in less noisy images that don't suffer too much from hot pixels. Some photographers suggest that one take multiple shots of short durations (say 40 shots of 30 seconds duration) and then stack them up into one shot of 20 minutes duration. This technique will lead to lower noise. Alternatively, one can shoot a long exposure of say 20 to 40 minutes (or longer) and then use dark frame subtraction to eradicate hot pixels (i.e. take the shot and then immediately take another shot of the same duration, but with the lens cap covering the lens. The idea is that any hot pixels that showed up in the first shot, will now be captured in this second shot too. Using software, one can then map these hot pixels out from the original photograph). Most modern digital SLRs can do this for you (look for a feature called LENR- Long Exposure Noise Reduction). Remember that by enabling this feature, the camera will take a second shot of the same duration
  • Set the camera on a tripod
    Death Valley Star Trails
  • Set the focus to infinity. Sometimes the infinity mark on the lens barrel will not correspond to true infinity, resulting in out of focus shots. There are a couple of ways to get around this. You could focus on a distant object during daytime (you will not be able to focus in the dark) and mark the position of the infinity symbol on the lens. Alternatively you could just focus on a distant object during daytime and then not touch the lens till you shoot the stars in the evening (I prefer the former method as my camera is free to shoot other things. Besides it is very likely that the focus ring might get disturbed before the shoot). Remember to switch the camera to manual focus
  • Keep the ISO at 100 to 200 (I shoot at my camera's base ISO of 200)
  • Set the shutter speed to the 'bulb' mode. You can buy cheap remotes or wired triggers for your camera from ebay (Phottix is a good brand that has served me well). This way you can release the shutter with the remote and keep it open for the duration you want.
  • Set the aperture to say F2.8 or F4. In my experience on a completely dark night, you should get a decently exposed shot using a shutter speed of 20 to 25 minutes and an aperture of F4. Experiment from there and see what works for you. Needless to say, you will get better star trails if you expose for a longer duration
  • If your camera has a viewfinder eyepiece cap, slip that on (to prevent any light leakage- from your torch or a passing vehicle)
    Sentinel Dome Star Trails

I normally will take a quick exposure for around 3 minutes with the aperture at F4 or F5.6 (or whatever aperture you want) and with my ISO at around 3200, just to check if the focus is accurate (you can then zoom in the picture using the LCD to check if it looks sharp). This is also a good way to determine your exposure. Let's say you set the aperture to F4, the ISO to 3200 and shoot for 3 minutes. Check to see if the focus is accurate and also to see if the exposure is correct (the image will be very noisy, but you will get a good idea of the exposure). If it looks underexposed, then shoot again, but this time for 4 minutes. If the exposure is now accurate, then you can determine the correct shutter speed for your base ISO. Let's say your base ISO is 200 (remember that the lower the ISO, the less the noise in long exposures)- the difference between ISO 200 and ISO 3200 is 4 stops (400, 800, 1600, 3200). So if you now set the ISO to 200 for a relatively noise free image, then you must compensate by adjusting the shutter speed 4 stops. If the initial shutter speed (at ISO 3200) was 3 minutes, then you should set the shutter speed to 48 minutes (6m, 12m, 24m, 48m) to get the same exposure at ISO 200.


Joshua Tree Star Trails

That's it! You are all set. Take the photograph and enjoy the results.

12 comments:

  1. Thanks. This is interesting and informative. Would the sky be very over exposed if the shutter speed was an hour or more?

    M

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  2. Hey Matt- it depends how much longer you keep the shutter open. Having said that, don't forget that you would need to keep the shutter open a lot more minutes to overexpose by just one stop. You can increase the aperture to counter it, should you wish.

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  3. How about battery life? How did you get on with this?
    Do you use a grip or not?

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  4. Hi Kev- I find that I can comfortably shoot two 50 minute exposures (at an ambient temperature of 32F) with my D700 battery. I have a spare battery with me- so while one is shooting the other one is charging using the battery charger (powered through my in-car inverter- the model I use is 'Xantrax' which also has a USB port to charge USB devices like MP3 players).

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  5. Hi Nikhil Thanks for the tips. Could you pl throw some light on how to use dark frame subtraction to eradicate hot pixels (which software do you suggest)? Can my D80 be able to pull this up with LENR enabled?

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  6. Hey Aniket. Yeah the LENR feature of the D80 will do the dark frame subtraction for you. Keep in mind though that the camera will take a second exposure of exactly the same duration as the first one. So if you expose for 40 minutes, then with the LENR on, the second exposure will also be 40 minutes long. I actually don't use LENR at all- instead I map out the hot pixels by cloning them out in Photoshop or Capture NX (or any photo editing software of that you might choose to use). I would love to see your results whenever you do take some pics.

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  7. Hi Nikhil. Thanks a million. I definitely will share the pics and get your valuable advice. Happy snapping.

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  8. Hello Thanks for sharing. I also plan to go to Joshua Tree for star shooting.

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  9. very cool site and amazing pictures. I stumbled across it while researching a Nikon D700 as I'm considering upgrading from my D80 and your work has me inspired.

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  10. Your photography is breathtaking. Glad I "bumped" into you. Love your people section too.

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  11. I am glad to find your impressive way of writing the post On Northstar Campers. Now it become easy for me to understand and implement the concept. Thanks for sharing the post.

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