Landscape Astrophotography 101
A Basic Tutorial
April 17, 2013
As part of this three-part series, I will tackle how I was able to capture the above image of the Salt River Canyon Jail. If you want to remain in the dark on this and just want to revel in its majesty, read no further. But if you want to know how you can do it too, read on. In Landscape Astrophotography 101, I will go over the basic gear and settings I use when I'm shooting. The above image was actually one single click of the camera and some flashlight waving. I did not do a single bit of post-processing, so you can get this too without ever opening up Photoshop, Lightroom, or any other photo editing application. From there, we'll go into Landscape Astrophotography 102 where I'll give you a brief glimpse into why I don't always use single shot images for prints that I intend to sell and how I actually ended up with the final image of the Salt River Canyon Jail. Finally, in Landscape Astrophotography 103, I'll go over some tricks and lessons that I've learned, and some of the extraneous gear that helps me. So without further ado, let's get to it.
Oh, and a couple of quick notes. This primer assumes you know the basics tenets of exposure (aperture, shutter speed, ISO), how to shoot in manual mode, and that you're using a DSLR. If you're already scratching your head, you may still be able to get some good information out of it, but it will probably come more naturally to you if you come in with this knowledge ahead of time.
LET'S GET STARTED
First off, it's imperative that you find the darkest sky possible. This is really a topic for a whole separate post, but thanks to our growing cities and poor light management, our dark skies are diminishing. International Dark Sky Week just wrapped up and hopefully it helped bring some awareness to the public about the growing ill effects of inefficient and poor lighting. I know we just purchased some IDA (International Dark Sky Alliance) approved lights for outside our house, so we're starting to do our part. If you're east of the Mississippi, GOOD LUCK finding a good sky. I live on the very outskirts of Phoenix where the highway pretty much ends, and I still drove 90 minutes past two small mining towns, and then 3000 feet down into a canyon, at 2 in the morning, to find the sky for the photo above. It ain't easy anymore.
How to find the Milky Way... well that's another post as well. But for now, if it's dark out and the moon has set, look somewhere east to southwest. This time of year, it will start to rise close to midnight, but it will be rising earlier and earlier as the summer continues on. There are a lot of software tools that you can use, and I'll cover those some other time as well.
Anyways, when first starting out with night shooting, it's definitely best to get the hang of capturing a good night sky first and then worrying about the details later. Because it's dark out and you don't have any big light sources to light up the sky you're at the mercy of little points of light billions and billions of miles away. Let's just say that you need to work with some extreme settings that you might not have known you had on your DSLR (like I said, we're going to talk expressly about DSLR cameras, but some higher end point and shoots might be capable of this too) or have been too scared to try. When you hit the shutter button on your camera, you're immediately confronted with three different factors/components that will affect your shot: 1) aperture, 2) ISO, and 3) shutter speed.
Our goal is to get as much light as possible on the sensor when you open the shutter, so how do we do that? The aperture is a mechanical component that opens up or closes down the size of the hole that the light enters through. Let's just open it up as wide as possible. The ISO is the digital compensation of the camera, similar to the sensitivity of film in the film days. Let's crank up the ISO sensitivity as high as we can (test shots will determine how high we can go before it looks bad; it is also camera dependent). And the shutter speed determines how long the the shutter is open and how long you're exposing the sensor to the light. This gets a little tricky when shooting the stars since they're actually moving and we don't necessarily want the motion to show (unless you're doing star trails). So we keep it open as long as possible before that motion comes into play.
Now let's talk specifics. Some of this is camera dependent and some is lens dependent.
First the aperture - we said wide open. What does that mean? Every lens has a maximum aperture, or f-stop. It's the letter-number combination that's printed on the lens, "f/1.4," f/2.8," "f/4," etc. I don't want to go into specifics here, you can find out more about lenses elsewhere, but for our purposes f/4 is acceptable, f/2.8 is better, and f/1.4 is best (although with a tradeoff - which I might remember to mention in Landscape Astrophotography 103... remind me if I don't). I have 3 lenses that I rotate through for this kind of shooting: Canon 16-35mm f/2.8 II, Canon 50mm f/1.4, and Samyang 14mm f/2.8. My preferred lens is actually the Samyang and amazingly it's also very affordable. The Canon 16-35mm is the most expensive, but it is also a zoom lens so it is convenient for composing with a foreground where you are limited with your positioning and need to zoom in for better effect. The Canon 50mm is the cheapest, but it's also not a wide angle lens, so it's more of a special purpose lens when I really want some amazingly detailed stars and very little (or none) foreground. The Canon lenses have an issue with coma, which I'll mention in Landscape Astrophotography 103. It's not a big deal until you want to make large prints and then you'll notice it. And then you'll never UNnotice it. Be warned.
The ISO is a setting that is very camera dependent and is also a very technical subject that I choose not to tackle in earnest. But I'll give you my perspective and thought process. The technology that is getting poured into the newer cameras is amazing and the low-light to no-light performance just keeps getting better and better. Full frame cameras and more expensive pro models are definitely going to have the best performance, but even entry level cameras can do an adequate job. When we're talking ISO, we'd actually always want to have as small of an ISO as possible because you lose detail and add noise to your image when you bump it up. But we also want to use it to our advantage since it is making the sensor more sensitive to light and is thus allowing us to capture a lot more of it. So our goal is to follow the law of diminishing returns, go as high as you can until the effects are no longer adding any value to our image.
In my short career, I've had two camera bodies. I learned on a Canon T2i (the newest model in this series is a T5i) and through a lot of practice I determined that I couldn't shoot much higher than ISO 1600 before the noise got too bad. For comparison, I now shoot with a Canon 5D Mk III and a lot of my star shots I use ISO 6400 (3200 looks great as well, I sometimes go back and forth, or shoot both and then decide later which is best when I'm in front of a big computer screen). On the ISO scale, that is 4X better than ISO 1600 - going from 1600 to 3200 is twice as much light, and going from 3200 to 6400 is twice as much light, so therefore going from 1600 to 6400, that is 4X as much light. Which is more than enough to bring the dimmest of stars out to play. So for me, upgrading to the 5D Mk III was more than worth it.
Ah, now we're getting to a very subjective setting. This is where you need to decide whether you want your stars to be points or streaks of light through the sky. Stars are points of light, roughly the size of a pixel on your sensor. They are also moving through the sky. So if you want to maintain the single-points-of-light look in your image, you need to stop your exposure before the star moves on to the next pixel. Or maybe you can handle two pixels. It all depends on the size of the image, personal preference, etc, and is definitely something you can do trial and error with. But if you want some settings to get started with, the often cited Rule of 600 is a good start. There's a lot of astronomy techno-mumbo-jumbo associated with this rule, but just know that this is fairly good estimate for determining the longest you can leave the shutter open for before you start to see noticeable movement in the stars. I, and others, like to use 500 instead, but that may just be because we can handle the higher ISOs that a lot of beginners can't. Anyways, the rule is.... take 600 and divide that by the effective* focal length you're shooting at. If you're shooting with a 14mm wide angle lens, that would be 600/14=42 seconds. If you're using the "nifty-fifty" 50mm, that drops to 12 seconds. The stars move a lot faster when you're zoomed in! So play around with it, take some test shots and figure out what you like. If you're going for star trails, you might want to consider keeping the shutter open for a couple of hours, or just take a series of 1-2 minute shots and then combine them in Photoshop - but that's for a later post.
*NOTE: when talking about"effective focal length" vs. "focal length," you need to take into account the size of the sensor of your camera. Almost all entry level DSLRs are considered "crop-sensor" cameras because the sensor is smaller than the ones on a "full-frame" camera. The effective focal length ends up being somewhere between 1.3x and 1.6x the real focal length. So if your camera is a 1.6x crop sensor camera (like the Canon entry level line of cameras), then a 14mm is really equivalent to 1.6x14mm = 22.4mm. A lot of what the lens sees is being thrown away by the camera because the sensor is too small. Anyways, use this focal length when applying the Rule of 600/500.
Alright, now that we've covered the basics, let's grab our camera, a tripod, dial in the settings, focus on infinity*, and take a couple of shots. Suggestions would be to set the aperture to the lowest number, ISO to 1600 or 3200, and the shutter speed to 30 seconds (or Bulb, if you have an intervelometer).
*NOTE: focusing on infinity is one of the hardest things to do if you're new to this. You can't use autofocus because the camera won't pick up the stars since they're so faint. You need to turn off autofocus, and figure out where on the lens "infinity" is and then manually rotate the focusing ring to that spot. This is best figured out at daytime. However, if the moon is out or you can find Jupiter or another bright star and your camera has live view, pull up live view, zoom in to 10x, find that bright spot (it might be fuzzy or hard to find), and rotate the focus ring until it becomes as sharp of a point as possible. That's basically infinity.
Figure out what you like about your test shot and what you don't like. If it's too noisy, bring the ISO down a little. If not, keep bumping it up. Are the stars trailing too much? Bring the shutter speed down a little. Maybe 30 seconds is too long, maybe it's too short. At this point you're probably best off keeping the aperture open as wide as possible, so I'd recommend leaving it at f/2.8 or f/1.4 for now. In Landscape Astrophotography 102, I'll give you some reasons why you'd want to change it to get something like the image on the left. But for now, happy shooting. If you have any questions, leave them in the comments or send me an email. I'd love to know if this has helped!