Welcome to Part 3 of my guide to Basic Backyard Astrophotography. If you missed Part 1 or Part 2, please click the links and we will see you back on this page in a few minutes. In this segment, we get down to brass tacks on how to capture astrophotos of the night sky. As you are about to see, astrophotography is not overly complicated, but there are things you need to keep in mind when trying to capture stellar images. Part 4 will discuss basic post-processing techniques for the photos we are about to capture.
Capture
-
Set up your camera on a tripod and point it at something interesting in the sky… or just a bunch of stars!
-
This is where you can decide how much and what, if any, foreground to include in your shot. If you live in a scenic area, you might want to include the local mountain range or a landmark in the frame. The top of a tree line usually won’t add much to the shot, unless the trees are particularly photogenic. Some astrophotographers will say that you need something in the foreground to give the shot a sense of location, but I am not necessarily in agreement there because I find the stars beautiful all by themselves, and I don’t feel the need to prove that I am standing in the epic scenery of my backyard.
-
Turn off your High ISO noise reduction and any in-camera noise reduction systems (there are rare exceptions… and there might be more exceptions as camera sensor tech evolves, but let’s KISS (Keep it Simple, Stupid) here again, OK?.
-
Camera resolution should be set to raw capture.
-
White Balance can be set to Auto. Again, some astrophotographers have preset WB settings, but we just said we are shooting in raw format, so we will be playing with the WB later. One exception: It might be beneficial to use a WB preset for panoramic images, but panoramic starscapes are outside of the scope of this article.
-
Focus your camera. This can be tricky, and here are some tips/options.
-
Autofocus: Point your camera at the moon or a bright planet or star. Sometimes the autofocus will lock on to that target and you are good to go. Then, switch your camera to manual focus and don’t touch the focus ring!
-
Manual focus: Again, point the camera at something bright in the sky. Again, the moon or a bright planet might fill the bill nicely. Adjust the focus manually until you get a sharp image. Many autofocus cameras have focus indicators that will help you know when you have focus.
-
Focus Peaking: Mirrorless and DSLR Live View might have focus peaking to assist. Be sure to zoom in to 100% to ensure sharp focus because the “peaking” might show over a narrow range of focus.
-
Hyperfocal focusing: If your lens has the correct markings on it, you can employ hyperfocal focusing.
-
Exposure
The conversation about exposure for astrophotography can get muddy and complicated. This is another area where photographers get intimidated and walk away from the idea of shooting the stars. There is so much we can discuss regarding exposure—signal-to-noise ratio, downstream noise, high ISO noise, exposure noise, dynamic range, etc. But you really need not worry about that stuff unless you want to spend more time reading than photographing. Here, our mission is to make photographs, not get lost in the noise (no pun intended). KISS.
Unfortunately, there is no universal exposure formula for a night-sky photograph because we are all shooting in different locations, under different atmospheric conditions, and with different cameras and lenses. Therefore, we need to discuss each setting as a base for creating a good exposure. In astrophotography, there is a technique known as Expose to the Right (ETTR). What this means is that we want to, if possible, overexpose the astro image without blowing out large portions of the sky. The “Right” segment of that acronym refers to the camera’s histogram, and how we want the information to fall on the right side of the histogram. Be sure to have your overexposure “blinkies” on so you can make sure that you are not exposing too far to the right and blowing out segments of the sky.
The goal of your exposure is to maximize the amount of light coming into the camera while minding physics and the technological limits of the gear. Let’s look at each side of the exposure triangle regarding astrophotography.
Aperture If you are shooting a “kit” zoom lens, you likely will want to open your aperture to its widest setting (the lowest f/number). The advantage of this is that you are letting in the maximum amount of light, but the disadvantage is that lenses, especially kit lenses, are rarely at their peak performance regarding things like sharpness, coma, and vignetting when shot with their aperture diaphragms wide open. If you have a super-wide-aperture lens such as an f/1.4 or f/1.8, it might be beneficial to stop down to say, f/2, to get a bit more optical performance while not keeping out too much light. Note that this will also be dependent on your ISO and shutter speed. Your maximum aperture is a physical limit to light-gathering.
Speaking of the super-wide-aperture lenses, regardless of brand loyalty, almost all camera brands make inexpensive large-aperture 50mm lenses (and sometimes 35mm lenses) at or around f/1.8, which are usually incredibly reasonable and represent photography’s absolute best value in a lens. The drawback of these lenses for astrophotography is their longer focal lengths (we discuss why that is, soon). However, keep an eye out for inexpensive wide-angle lenses with apertures around f/2 or f/2.8. They are out there, as well.
Shutter Speed News Flash: The Earth is round and the Earth is rotating—more than 1,000 miles per hour at the equator. This means that we are faced with the option of star trails or star points in our photos. For all but abstracts and star-trail images, we need to freeze the action in the sky overhead. Many photographers use the “Rule of 600” or “Rule of 500” (or now the “Rule of 200”—see the notes below) when figuring out how long the shutter can stay open without the stars showing the Earth’s movement. The formula is: 600/focal length (35mm equivalent) = slowest shutter speed. Example: A 35mm lens on an APS-C (1.5x) sensor has a 50mm equivalent. 600/50mm=12 seconds. So, a shutter speed longer than 12 seconds means the stars will start to streak through the frame as we are moving. To “freeze” the stars, your focal length and the speed of the rotation of the Earth give you a shutter speed limit.
Note 1 The Rule of 500 gives an even shorter shutter speed if you want to be extra conservative about avoiding star trails. And, for DX/APS-C shooters, you can use the Rule of 400—using the actual focal length of the lens—if you don’t want to do the crop sensor conversion.
Note 2 The Rule of 600 is a general rule set up for easy math. Know that as you look at stars farther from the north (or south) celestial poles, the faster they appear to move. I recommend to vary your shutter speed to ensure you get pinpoint stars instead of trailing stars, but use the Rule of 600 as your base calculation.
Note 3 Now that you did all that math in your head, your friends who are looking for a specific street address on an exoplanet orbiting a distant star, blasting away at the heavens with 40+ megapixels of resolution, are noticing star trails at Rule of 500 shutter speeds. If you have a high-resolution camera, and you like pixel peeping, you might be forced to try out the “Rule of 250” or “Rule of 200.” Using our trusty 50mm-equivalent lens, you are limited to exposures of 4-5s. Thank goodness High ISO performance is getting better all the time!
Don’t get me wrong, I have zero objection to star-trail photos, but when we think of awesome shots of the Milky Way or other features in the night sky, freezing motion is the key. If you want to do star trails, knock yourself out and let the shutter stay open for multiple minutes. Star trails can make for beautiful abstract photos, or super-long star-trail pics can give you those epic long-trail images with the stars circulating around a pole star.
ISO Generally, we want to photograph at the lowest native ISO setting the camera allows. You may throw that idea out for your astrophotos. We all know that higher ISO means more digital noise, but that is not always the case, depending on your camera. Because there are physical limits to our aperture and shutter speed, ISO is where we really adjust the bottom-line exposure to achieve our overall exposure goals. ISO performance varies from camera to camera, and it is also affected by atmospheric conditions, but what I have found is that I can get clean ETTR images all the way up to one stop below the camera’s maximum ISO. If you can shoot at lower ISOs and still maintain a fast-enough shutter speed and the aperture you want, do it! But you will want to experiment with your own camera each night to get the best frame by varying the ISO in multiple shots of a specific scene.
Exposure Setting in a Practical Application
I know all that information may have felt like a bit of a firehose, but now let’s try to simplify the whole thing by giving a real-world example of how to set your exposure:
Gear DSLR camera with an APS-C (1.5x) sensor and a kit 18-55mm f/3.5-5.6 lens at 24mm.
Aperture Set to f/3.5—the widest possible aperture at that focal length.
Shutter speed Let’s apply the Rule of 600 for the 24mm focal length to figure out our maximum shutter speed. 600/24x1.5 = 16.67 seconds. Dial-in a shutter speed of 16 (or 15) seconds or use “bulb” mode and a stopwatch to time your exposure (even more fun!).
ISO Take a shot at a high ISO close to your camera’s maximum ISO setting and see where your exposure lives on your histogram. If you have flashing “blinkies” indicating blown-out highlights, dial the camera to a lower ISO. If the image is dark or medium-dark, you can dial-in a higher ISO, if your camera has higher ISO available.
Note The image on the back of your LCD will be pretty darn ugly when you use the ETTR technique. In fact, it is difficult to see if you have achieved proper focus or gotten the composition you want. You can see some detail, so, if possible, zoom into the frame at a bright star or planet to try and verify focus. Then, shoot at various ISO settings to cover your noise-production bases. Also, review the image to verify framing. Of course, now that you have a very bright image on your LCD, a downside of the ETTR technique is that post-processing will be absolutely required to make the image beautiful. The upside of ETTR is that you are maximizing your camera to, in effect, collect as much light as possible.
The sky, although dynamic, is a relatively static subject for photos, so take several shots at different apertures, shutter speeds, and ISOs if you can. Varying atmospheric conditions and other factors can make a change of settings advantageous.
Finishing the ETTR topic, here is something to whet your appetite: As I just mentioned, an ETTR starscape on the camera’s LCD is just a very bright and almost featureless image. Because of this, even though shooting digitally, we get to enjoy a photographic experience like what we had in the days of film, when we had to wait for the film to be developed. Here, with ETTR images, we really don’t know what we have captured until we get home, open the image on the computer, and move some sliders in our post-processing software. It is awesome to see the hidden stars and night sky come to life on your home monitor.
Speaking of post-processing, Part 4 is my basic guide to making your astrophotos come alive!
Exposure FAQs
You may now have some questions about exposure settings. Here are some common ones.
Question “I want my lens to be set at the aperture that gives me the sharpest possible image. Why would I shoot at a wide aperture where I get softness at the edge of the frame?
Answer The goal is to maximize the amount of light reaching the sensor. At small apertures, light eventually makes it through the tiny hole (think of a pinhole camera), but we want to get as much light to the sensor as quickly as possible, hence, a larger aperture is better. Also, as great as sharpness is when photographing anything, remember we are photographing stars that are incredibly far away through a thick atmosphere—sharpness is going to suffer simply because we are Earthbound.
Question “You just said you want to get more light to the sensor. Wouldn’t a longer shutter speed allow more light to get into the camera?”
Answer Yes, a longer shutter speed lets more light in, but we may wish to avoid creating star trails or getting blur caused by the Earth’s rotation. Therefore, unless we are trying to do star trails, we use shorter shutter speeds based on the focal length of the lens, using the Rule of 600/500/400 as a basis for determining the maximum allowable shutter speed. Also, the longer the shutter is open, the longer the sensor is energized and the more heat it produces. That heat produces digital noise—bad.
Question “Why wouldn’t I shoot at a lower ISO to avoid noise?”
Answer The simple reply is that we want to maximize the amount of light and signal reaching the sensor. In the example above, a 12-second image of the night sky at f/3.5 is going to be pretty dark. When we turn up the ISO, we allow the camera to capture more of the stars above. Yes, we add a bit of noise, but it turns out that we usually get less noise by overexposing in the camera and correcting in post-processing than if we underexpose at capture and then have to increase exposure in post.
Question “I don’t mind star trails and I want to maximize sharpness. Why wouldn’t I shoot a super-long exposure (letting more light in) at a “sweet spot” aperture like f/8 and a low ISO?”
Answer There is nothing wrong with that approach. However, you might find that you get a very noisy photo due to the long shutter speed that allows the camera’s sensor to heat up and generate digital noise in the frame. If you are doing star-trail photos, shoot at different shutter speeds to experiment and find at which point the sensor’s noise performance starts to degrade. This setting will not be a constant, because ambient heat and humidity both play a role in this heating.
