Wednesday, May 25, 2011
Photography has always involved multiple steps, with exposure being only one part of the equation. In the days of film, the darkroom enabled additional influence to be applied during development, and then again during printing. Unless you were using a Polaroid camera, there was no such thing as "straight out of the camera." Maybe you trusted someone in a drug store to do your developing and printing for you, but that wasn't an optimal situation and that certainly doesn't mean that no adjustments were made. For the most part, the entire concept of "straight out of the camera" is a myth that is best set aside as soon and as thoroughly as possible.
Today with digital cameras your darkroom is on a computer, implemented in software. Milky Way shots are a great example of images that you won't get the most out of until you get in the habit of spending 5 minutes in your digital darkroom to complete the photographic process.
If you find a dark place outdoors to shoot and you can make out stars and the lighter, more dense band of the Milky Way, a little postprocessing can get you a lot further. As with my previous blog post you need to shoot on a tripod, using manual aperture and manual focus. Having your long exposure noise reduction turned off is not critical since we're dealing with single exposures for Milky Way shots.
Shoot with your widest focal length lens to minimize star movement in the field of view, have it opened to its widest aperture to minimize exposure time. You may still have little enough light that you shoot at the longest exposure time (generally 30 seconds) or you may need to shoot in "bulb" mode for a longer time in order to get enough light. Always shoot in RAW mode so you have far more adjustment capability in post-processing software.
Shoot near the date of a new moon, so there is as little light pollution as possible. The last thing you need to know, probably the most important thing during planning your shot, is how to predict when the most intricate, dense, bright center of the Milky Way is in the night sky! In Summer the sun is up roughly 2/3 of every day, but the Milky Way crosses the sky in the night. The center of the Milky Way is towards the constellation Sagittarius. You can look up the dates when Sagittarius is high in the sky, and that's when the Milky Way is most intense: http://homepage.ntlworld.com/mjpowell/Astro/Sgr/Find-Sagittarius.htm
In simple terms, it highest around midnight around July 22, two hours later per month earlier (2am in May), two hours earlier per month later (10pm in August). So really your best shooting will be on days near the new moon dates, and preferably within 7 weeks or so of July 22.
So lets assume you go out on the right night, shoot south towards Sagittarius, capture a RAW file with some stars showing, and maybe you can barely make out the bright stripe of the Milky Way and its slightly more dense center.
Well, if you were in a darkroom... how do you lighten the Milky Way while keeping the background sky dark? The simple answer is dodge and burn... selectively darken some areas while lightening others!
In Adobe Lightroom (download a 30 day trial if you don't have it already, AFTER you collect some images to process) use the paintbrush tool (under the Develop module) to select and lighten the area around the Milky Way. Use the paintbrush tool to darken the sky everywhere else (this makes both the Milky Way and the stars pop).
to do even better, you can also increase contrast while performing these functions, further darkening background light levels, including noise. Adjust exposure and brightness so the fainter stars in the Milky Way get brighter while background and noise gets dark. You can increase saturation slightly on the Milky Way, but increasing contrast has that effect already, so you might not need to.
Don't worry too much about how much noise your camera produces... after you adjust contrast and brightness, just crank up the noise reduction. After all, there isn't generally much detail to lose by doing that. In Lightroom for dark, noisy photos I try to max out noise reduction at 25 or maybe 30, but lately for night skies I've been going into the 60s.
Lightroom can also selectively adjust saturation and brightness of individual colors. If you shoot too close to sunset and "blue hour", or during a too-bright moon that is too full (and creating blue night sky), it can be handy to darken a blueish background sky to help separate that from the stars (just did that on a star trails shot). On the other hand for dark new moon skies, like you should have if you're planning ahead for Milky Way shooting, there is little or no light scattering turning the sky blue. A lot of the Milky Way stars have a slight blue tint though, so selectively raising brightness of blue can help separate them from dark background and any noise.
It's a balancing act between white balance, adjusting individual colors, and tweaking the brightness and especially contrast of the area the Milky Way covers, but you can find a reasonable compromise pretty quickly. Once you do fairly well adjusting one shot, Lightroom enables you to copy your develop settings and apply them to additional photos.
There are other subtle tricks of course, but I save them for workshops. If I continue making good progress towards completing my book, I'd like to offer some night workshops this Fall.
You'll need to capture images covering 30 to 60 minutes or more. A sturdy tripod is essential. You can trigger exposures manually, but that's tedious and not fun to do for an hour at night. Ideally you'll have a remote trigger that can lock down to fire off sequential shots, or even better, an intervalometer which takes shots at certain intervals which you define.
You'll need to focus your lens on an object at infinity, and leave the focus mode set to manual so it won't hunt for focus in the dark. The focus ring of your lens probably continues past that point to compensate for differences as temperatures fluctuate, so note where this is, or if your lens has continuous focus ring with no visible scale, focus your camera during the day, switch it into manual, and carefully tape the focus ring with removable painter's masking tape.
You'll want to take sequential shots close together so the star trails to be continuous, so turn off your camera's long exposure noise reduction.
You'll also want a consistent exposure, so put your camera in manual exposure mode.
Cameras have different sensitivity to light, and the moon phase and local light pollution can affect your exposure, so you'll need to run some tests to determine what settings to use for your individual shots.
With no moon in perfectly dark skies I use f/2.8 at ISO 6400 for 30 seconds. Don't have f/2.8 or ISO 6400? No problem, lengthen your exposure time. If you have f/4.0 and ISO 1600, you'll double the time for the aperture and then another 4X for the two stop loss in sensitivity to 1600, so your exposure time could be 4 minutes. Your actual time will often be less due to some ambient light from the moon or light pollution. Take lighter and darker shots to ensure that you've identified an exposure which isn't too bright or too dark.
Once you have the basic exposure figured out, take that exposure over and over until you reach 30 to 45 minutes or more total. Leave as little time as possible between shots.
For this example I used about 80 shots of 30 seconds each, covering 40 minutes. I used the free StarStaX software (www.StarStaX.net) in "lighten" mode, where the lightest pixels are kept as images are merged, creating the trails as the stars move from frame to frame.
Unfortunately in Yosemite Valley the cars driving by also create lasting light trails. To remove them, there's also a "darken" mode which keeps the darkest pixels, eliminating the car headlights.
This also eliminates the star trails, but by using layers in Photoshop and blending that center portion of the darkened sequence into the lightened star trails sequence to get a star trails image without the car lights.
Go give it a try!