One of the things that makes night photography rewarding and fun is that there are really no absolutes of right or wrong when it comes to exposure.
Under “normal” lighting conditions, a well-exposed image is pretty easily defined. Clean highlights, no clipped shadows, a good histogram—there are plenty of ways to evaluate exposure, and it’s usually obvious when an exposure is “correct” or “incorrect.”
But at night, exposure is much more open to interpretation. Rather than a right or wrong exposure, the photographer has more leeway to interpret the scene to their own tastes or liking. Additionally, with regard to astro-landscape photography, exposure relates to more than simply an appropriate amount of light reaching the sensor. In particular, exposure length has a profound impact on the appearance of stars in the sky.
The solution is relatively straight-forward, though the reasons and the factors that play into it are many and intertwined. But let's explore! This is the first of a two-part blog post about all the decisions and considerations that go into determining the optimal exposure for creating astro-landscape images with star points.
Finding the (In)Correct Exposure
One of the most frustrating aspects of night photography is that it can be difficult to establish the “correct” exposure. Yes, I just contradicted myself, and there lies the rub. Night photography is not an exact science, and trying to make it so is an exercise in futility of the highest order. There are so many variables, some of which are beyond the photographer’s control.
Attempting to “get it right” is more than science, more than art, and more than an ambiguous combination of the two. Some nocturnal imagery is more about a feeling, an atmosphere or a mood, but other images are more grounded in technical considerations. Good night photography exposures can be like Supreme Court Justice Potter Stewart’s view on pornography—you know it when you see it!
Astro-landscape imagery is a type of night photography that requires a good deal of thought and many considerations about what might be the best exposure for any given situation. Usually, determination of a photographic exposure is led by one of the exposure variables—aperture, ISO or shutter speed. One would choose a small aperture to have large depth of field. For example, you might have a subject in the foreground and stars in the background that both need to be tack sharp. Alternatively, in the case of a portrait, a large aperture that yields a shallow depth of field would usually be the better choice. In the case of an image where you know in advance that you want to make a large print, a low ISO would take priority.
In the case of astro-landscape photography, finding a shutter speed that is fast enough to record the stars as points of light rather than showing them as trails is usually the critically important variable. (Unless, of course, you want to produce star trails, but that’s a whole other issue and technique. Here we’re just talking about reproducing the look of actually being under a starry sky.)
Additionally, the combination of extremely low light levels, short shutter speeds and the need for depth of field necessitate compromise. We need to consider all three exposure variables, and the challenge is to combine them in such a way that addresses both the technical limitations and the constraints of the image.
The stars are moving in space, but their distance from Earth makes that movement negligible in our photographs. Instead, it is the Earth’s rotation that causes long-exposure photographs of the night sky to show star “trails,” or lines of light in the sky that create a circular pattern centered over the polar axis.
Here in the Northern Hemisphere, stars appear to revolve around Polaris, or the North Star, which is centered approximately over the north polar axis. Stars in the northern sky form relatively small circles around the North Star, and stars in the eastern, western or southern skies form larger, longer star circles around Polaris.
For every image, there is a set of variables that affect how long an image may be exposed before the stars appear as trails, and managing those variables is key to a successful image. Accounting for those variables and weighing the pros and cons of compromising exposure, noise, depth of field and stellar movement can be a daunting task.
Many photographers attempt to scientifically calculate values for each of these variables and in turn come up with an exposure that is optimized for the conditions at hand—but this is tedious and, honestly, unnecessary. Having an understanding of the various factors that affect the appearance of stars in astro-landscape images is helpful, so let’s review them before coming up with a strategy to maintain sharp stars in your nocturnal landscape photographs.
More than anything else, the focal length of your lens determines the longest usable shutter speed. In general, the wider your lens, the longer you can expose without showing stellar movement. Some people use a formula such as the 400 Rule or 500 Rule to calculate shutter speed. These formulas can be helpful, but do not take variables other than focal length into account.
Camera orientation relative to the North Star is the next variable that needs to be factored into an exposure. Since it takes Earth 24 hours to make one complete rotation, a hypothetical 24-hour exposure with the camera oriented toward the north or south (depending on which hemisphere the photographer is shooting in) will record star trails that form a complete circle. Stars near the pole star will create small circles, while stars in the opposite part of the sky will create much larger circles in the same exposure.
Therefore, if a camera is directed due-north in the Northern Hemisphere, or due-south in the Southern Hemisphere, the resulting star trails created during a long exposure will be noticeably shorter than star trails in photographs where the camera is pointed away from the pole. This means that the longest usable shutter speed for “freezing” stars increases substantially as the camera’s orientation approaches north or south.
Sensor size, or camera format, also make a difference. Movements during an exposure cover a larger percentage of a small sensor relative to a larger one. This is similar in principal to a telephoto lens giving the impression of longer star trails due to a narrower angle of view than a wide angle lens. It takes less time for a star to transverse a smaller sensor than a larger one, or to cross the image plane of a telephoto image than a wide-angle one. The star trails are actually the same size in each photograph, but appear larger due to their size relative to the frame. Therefore, the smaller the sensor, the shorter the longest usable shutter speed.
Camera resolution also impacts apparent movement of stars, or anything else in an image. Higher-resolution sensors show more detail, and therefore amplify any flaws in an image as well as the image itself.
However, increased resolution’s impact on an image is tied to the last considerations in determining longest usable shutter speed: final image size, viewing medium and viewing distance.
If a photograph will be viewed as a small print, there is more tolerance for movement, because it won’t be noticeable to the viewer—and thus you can get away with a longer shutter speed than you could for a more highly magnified, larger print. The same is true if the image will be viewed only on a screen, as opposed to as a print.
Lastly, the more distant the image from the viewer, the less apparent any movement or other “flaws” will be. An extreme example of this is demonstrated by Apple’s use of iPhone photographs on giant billboards! A 12 megapixel file from an iPhone 7 must be magnified tremendously to achieve billboard scale, resulting a very low resolution image. But because billboards are viewed from a substantial distance, the images used can be extremely low resolution and still appear to be good quality. Typically, billboard images are printed at 40 to 50 dpi, as opposed to 300 dpi and higher for photographic prints.
This means that a low resolution image viewed from a great distance is very forgiving of subject, or for that matter, of camera movement. Conversely, a high resolution print that is meant to be viewed at close distance will reveal every possible detail (and flaw) contained in the photograph.
Putting It All Together
Despite the apparent complexity of this multitude of variables, a little experience is all that it takes to effectively determine the longest usable shutter speed for astro-landscape photographs. By keeping these variables in mind while photographing, making critical observations and appropriate adjustments, sharp star images are well within reach.
In the second part of this article, I’ll compare the use of the 400 and 500 Rules with different focal length lenses to help you further refine your technique.