April 3, 2011

Ask Dave, April 3rd - Star Trails

Ask Dave, April 3rd
'Ask Dave' week eight… I get tons of questions each week, I respond as quickly as possible, and this is week number eight of answering them here too. If you have questions you can email me at Learn@DaveShumway.com.

From too many folks to name...
How do you photograph star trails?


How to photograph star trails...
First you need a clear night in a place where you can see stars; but I assume you knew that ;)

Typically I suggest using lunar charts to shoot around a new moon, and or when the moon will be underfoot at night (set before you start and rise after you finish). It is dark enough about one to two hours after sunset and before sunrise; but there is often lots of air traffic until 11pm, so if you are near flight routes you might want to wait until after 11pm. (most agree that 1:00a-4:30a has virtually no air traffic)

Two ways to think about composing the star trails in your shot are:
  • To place Polaris (the North star) in your frame, where all of the other stars with appear to circle around it (assuming you are in the northern hemisphere). I normally place Polaris in my shot when shooting wide angle shots.
  • To keep Polaris (the North star) out of your frame, this will create arcs, and not complete circles. Any composition facing north will yield arcs in the northern hemisphere. Typically I do not shoot south in the northern hemisphere because the "arcs" become more like straight lines and are not very nice. When shooting more telephoto shots I avoid placing Polaris in the frame, and use the "arcs" in my composition. (When I say; "North"/"South" I mean north/south of a theoretical line between East and West.)
How to you find Polaris? Easy, first you need an iPhone or other smart phone with a star finder app, then you look it up :) (Star Walk rocks) Or you can do it the old fashioned way; find the Big Dipper (Ursa Major) follow in a line up from the outside edge of the pan to the first big star (that is it). When you are scouting use your compass and assume Polaris is North :) Remember that the further North you are, the higher in the sky Polaris will be.

Some other star notes:
  • Stars move about 15° each hour, so if you can expose for 6 hours you would have stars going 1/4 of the way around Polaris (6 hours is hard to do in summer months, and batteries die too quickly in winter months to get a 6 hour exposure and a 6 hour noise reduction exposure).
  • The higher your elevation the more stars you are likely to see.
  • The best star viewing is 25-150 miles away from towns/cites (the more people the further you should be).
  • The weather needs to allow views of the stars for the entire expose, if clouds break your view you will have breaks in the trails, also clouds tend to be collectors of light pollution. (if clouds break up your exposure it could look like this)
  • Avoid moon light, as it will cut your maximum exposure time dramatically. A 1/4 moon opposite your field of view will limit you to about a 30min exposure before the land looks like it is sunlit.
  • You will be forced to use "B" aka. "Bulb" mode.
  • RAW is the only way to go.
  • The further away from town (light pollution) the better the star trails will look. (typically)
  • Looking away from town (light pollution) is normally a good idea.
  • Remove camera straps and anything else that will blow in the wind.
  • Turn off your cameras/lens' image stabilization.
  • Normally it is a great idea to turn on your cameras long exposure noise reduction.
  • Shooting lenses wide open can show more vignetting and other aberrations.
  • It is easier to start experimenting with wider angle lenses 17-35mm.
An issue in photographing star trails is dealing with focus in the dark. NO, you should not just turn your camera to infinity, that is a very common thought; but not a correct one (most of the time). What you really want to do is focus your lens at its hyperfocal point for the aperture value you are going to be shooting at. Wide angles are much easier, for example, my 17mm on a full frame camera at f/4.0 focused at 8 feet will, for a normal print size, makes everything from 4 feet through infinity appear to be in focus. Alternatively at 300mm at f/22 requires focus at 450 feet to have everything from 225 feet through infinity appear to be in focus, for a normal print. (You can always focus further away than the lenses hyperfocal point and everything from half of that distance through infinity will appear in focus, for a normal print). I normally set up the camera before the sun sets, so I can set everything with plenty of daylight to aid in focusing.

Things you need/should have:
  • A tripod and remote shutter release are required.
    • The remote must be lockable or an Intervalometer
    • Intervalometer remotes can be pre programed allowing you to sleep in peace without worry about sleeping through the alarm to tell you to release your locking remote. (a computer or similar device can also work as an Intervalometer).
  • A bubble level will make your life much easier.
  • Live view on your camera will help you get the focus set properly.
  • Mirror lockup is not a must. (with exposures this long it really does not matter)
  • A headlamp, hat, gloves, warm boots/coat/pants, snacks, basically all the things you need to spend time outside in the "cold" night.
  • I am assuming you have a camera and lens that you can control, and know how to use. If you do not know how to use your camera in the warm, well light, indoors, then you will not be able to use it in the cold, dark, outdoors.
Now for getting the exposure correct…
  • I typically do some quick head math; but it is not hard math, so don't worry:
    • 4hr - 2hr - 1hr - 30min - 15min - 8min - 4min - 2min - .1min - 30sec
    • ISO50 - ISO100 - ISO200 - ISO400 - ISO800 - ISO1600 - ISO3200 - ISO6400 - ISO12,800 - ISO25,600
    • Those are all one stop apart.
  • If I do a 30sec exposure at ISO25,600 it will be just about the same as a 4hr exposure at ISO50. Do you see what I did?
  • Think about everything in stops, so you can then play around to find the exposure you need without having to do crazy long exposures, as tests.
  • This is an example of my high ISO test to get a proper exposure, and the two hour 'real' exposure is the shot at the top of this post.
  • Your aperture should be set based on the needed depth of field (or hyperfocal) settings and how many stars you want to see, if you can't test the full spread you need with ISO you can also move the lens in full stop increments.
  • Remember a 4 hour exposure to an 8 hour exposure is only one stop, so if you miss by an hour or two you might only miss by a half stop (or so)… not the end of the world with digital files.
  • Honestly if you are going to miss, you should miss to the side of overexposure, one full stop is easy to save in post; but one stop under will not work as well for most situations/compositions.
  • I really like two to three hour exposures (with wide angles); both because of how my camera handles it, and the resulting 30-45° arch that the circling stars make around Polaris. When I shoot more telephoto images (not at Polaris) I really like the one hour realm.
  • I like to use flashlights to paint light in on my foreground subjects; but using foreground elements as simple silhouettes can be very cool too.
  • Once you have this mastered try to shoot a foreground subject that is light by strobes, and then turn your focus to the stars and let the 1+ hour exposure continue to bring in your star trails. Note… It helps to cover your lens (blacked out) when changing your focus (trust me).
I am sure that I missed something, so as you have questions, just ask.

I believe in shooting the star trails in a single exposure; but you can also do lots of individual exposures and put them together with Achim Schaller's freeware StarTrails program, or Chris Schur's Star Trail Action inside of Photoshop to make a single image that is the equivalent of a single long exposure. One big plus is that you can avoid the issues that you run into with long exposure noise reduction; but some agencies, competitions, magazines, and art buyers do not accept multiple exposure composites, so the choice is yours. Most folks keep their short exposures under 10min; but well over the point that the stars actually blur/move.

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