Landscape Astrophotography 102
Multiple Exposure Technique
May 12, 2013
One of my favorite images is from my late night jaunt to the bottom of Salt River Canyon. The Salt River, which provides a good deal of water to the Phoenix metropolitan area, begins its journey high in the mountains of northern and eastern Arizona and winds its way through a series of canyons. There is one particular section where state route AZ-60 descends down into the canyon and crosses the river at the bottom before quickly rising back to the upper parts of the canyon. It's a gorgeous and scenic drive and definitely worth doing.
At the bottom of the canyon there is an old abandoned gas station and "jail house." Information is pretty scarce, but it looks relatively legit. However, skeptic that I am, I'm more likely to argue that it was built at the same time as the gas station for tourist purposes. Even if this was the case, the abandonment aspects of the site still give it a centuries old feel to it.
SINGLE EXPOSURE VS. MULTIPLE EXPOSURES
If you read Landscape Astrophotography 101, you'll be familiar with the basics of proper exposure for getting the Milky Way or a star field in your photos, but there is a big gotcha with this and I'll explain in a second. If you aren't too concerned with printing your images and just want to share them on Facebook, Google+, or other social networks, this section might not apply to you. But it's good information to know if you want to take the next step.
So I take a picture and I can see the Milky Way in all its glorious-ness, now you're saying that it's not all it's cracked up to be... what's the big deal? If you only care about sharing online or viewing on your computer, there's absolutely nothing wrong and it's no doubt spectacular - who else do you know that's taken a picture like that, right? But if you want to print it out at 20x30 or something large like that, when you're standing closely in front of it, you'll start to notice some small problems. Zoom in and take a close look at your foreground. You'll notice that it's really grainy and it might have a lot of red/green/blue dots all over the place. It's not very sharp and you can't see much detail. The reason for this is predominantly in the settings required for capturing the Milky Way. Since we didn't want to see star trails, we had to do two things: limit the exposure to 30 seconds or so, and then amp up the ISO to get a good exposure. The ramifications to that high ISO is all this noise we're talking about. If you look at this image to the right, you'll see the difference between the two settings. In the ISO 400 example, the wood grains are much more detailed. The question now is how do we take advantage of the high ISO capabilities without having it affect the rest of our image? The answer is multiple exposures.
There are some that will argue that multiple exposures ruins the photography aspect of this and makes it fake, but I'm not into taking that bait. Opinions are all well and good and I'm not trying to claim that this is what it looks like in real life. All I want is to make a good image that is pleasing to someone. If at the end of the day someone else says that they like it, I'm fine with the critics having issues with the technique. Anyways, here's the final image of the jail (click the image to purchase).
Pretty cool difference from the earlier one, right? It requires some extra planning, extra software, and a lot of extra time in post-processing, but when it finally comes time to printing it, the effort you put in will be well worth it. Let's take a look at how this was put together.
1) First off, this was made of four different exposures (fast forward to the image below). It's much easier to work piece-meal when you're doing light painting and work on different parts of your final image one at a time. The first thing I always do is take care of the sky first, especially if the sky is clear and dawn is approaching. You don't want to run into the problem of running out of darkness, so it's best to take care of this first. If the Milky Way isn't in the perfect place, you can always come back later if you have time.
2) Next, start with light painting the outside of the foreground. In this instance, I had a flashlight with CTO gel taped to the front of it since I wanted the light to be orange rather than the blueish look of the LED light. The biggest key here is to lower the ISO to something much smaller than the 3200-6400 you probably used for the sky. It doesn't matter if your exposure goes into the minutes because we aren't going to use the sky for this part of the image. All we care about is how the front of the jail looks. So I set the camera up to start taking 1-2 minute exposures, took about ten steps to the left just out of frame, and started waving the flashlight on the jail. It took a couple of attempts to get it just right, but once I liked it I moved on. The reason I moved to the side and didn't just stand next to the camera is because of the shadows. If you stand right behind the camera and wave the flashlight around, you aren't going to get any shadows, they'll all be lined up with the back of the jail and you won't see them in the image. Shadows typically add intrigue and complexity to an image, so if you stand to one side and lightpaint the jail, you'll still see the shadows on the opposite side of the jail.
3) I kept the same exposure, maybe lowered the time a bit (but definitely kept the low ISO), went behind the jail and started waving the flashlight in the window sill on the left a handful of times. Again, this was probably a multi-step process, but once the window sill looked good, I moved on.
4) Finally, I wanted to light the rest of the interior. I knew that if I put a light source inside and pointed it at the ground, the resulting glow would approximate that of a candlelight which is the feel I wanted since I was trying to replicate the kind of lighting typical of an 1800's-1900's jailhouse. I also tried a few different exposures with the flashlight on different power settings, but once it looked good, I moved on.Here's a look at the different exposures that went into it.
As you can tell, the sky is different in all of them, but in the final image I only used the sky from the upper left. To combine all of these into one final image requires a program like Photoshop where you can use layers to blend different images together. You can probably find a much better tutorial online about how to do this than I could possibly put together, so I would suggest turning to the power of Google and looking up "multiple exposure blending" to get some good examples.
That about sums up this post on post-processing tips for landscape photography images. Come back in the future for more tips that I will share.
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