Photo Stacking and Long Exposures – Part 1: Introduction
Have you ever seen a star trail photo — a long exposure where the stars make long streaks of light — and wondered how to do it? Have you tried, and ended up with a badly exposed photo full of noise and not many star trails? Photo stacking could be the answer! This article is the first in a three-part series about a long-exposure technique called photo stacking. Photo stacking is a clever method of combining multiple short exposures together to create a single image. The result looks like a long exposure, but has some significant advantages over just leaving your shutter open for an hour.
This installment will cover the general ideas behind of photo stacking and long exposures, with an emphasis on the ingredients which you'll need to get started with photo stacking. I'll discuss the equipment you'll need — both camera and computer — to be able to stack photos. In future installments, I'll talk about planning and execution of a long exposure shot, and the post-processing required to stack photos.
Digital Photography and Long Exposures
First, let's take a look at the basics: what is a long exposure, and why is taking one different than taking any other photo? In this article, a long exposure will mean anything from a couple of minutes to several hours. The unusual and stunning effects which a long exposure can achieve have always attracted me, as they are something that the human eye can't see on its own.
Right now, you might be asking why we need special techniques to take longer exposures. Why not just point the camera at a subject, tape down the shutter, and leave it for an hour? This is the usual solution for film cameras, where can work well. Things aren't quite so simple with digital cameras. The single biggest problem is that digital cameras' sensors overheat during long use, leading to amp noise — colored clouds which form throughout your image and obscure it. Some cameras are better off than others, but all will eventually develop amp noise. Another problem is that digital cameras require electricity, and batteries can run out before your exposure is over. Film doesn't have either of these problems — but if you're using a digital camera, photo stacking can address each of these issues.
What is photo stacking?
Photo stacking is, simply, a technique for creating a single long exposure by combining many shorter exposures of the same subject. It gives (more or less) the same effect as a single long exposure, without many of the problems. The key ingredient of photo stacking is a set of many photos of the same subject, with exactly the same composition and exposure. These photos are then “stacked” on top of each other, letting the brightest parts of each image show through (as if lit from behind). The result is very similar to a single long exposure. Photo stacking is best used when you want to take a long exposure, but can't — whether for a lack of battery life, too much sensor noise, difficulty getting the right exposure, or many other reasons.
One of the most common uses for photo stacking is star trail photos, which show the motion of stars through the night sky. These photos can be extremely dramatic, showing a side of the world which is invisible to the unaided eye. If you want to make star trail photos, then photo stacking is your friend! While this article is aimed at those who want to take star trail photos, the techniques I describe could be adapted to most long-exposure situations.
Before we continue, I'll spare a word for HDR (High Dynamic Range) photos. This too is a method of combining many photos. However, HDR images aim at combining several photos with different exposures, so that the “properly exposed” parts of each photo are used in the final photo. Photo stacking, on the other hand, uses many photos with the same exposure. The purpose of photo stacking usually is to emphasize motion in a fixed composition, as opposed to improving the exposure.
What equipment do I need to start stacking?
In order to take a series of stackable photos, you only need two main pieces of hardware: a tripod, and a camera. Depending on your camera (or your patience), you may also need an intervalometer.
- Tripod: These are a subject that could take up a dozen blog posts, so I won't get into details — except this: a tripod is essential to keep your camera steady, in the exact same position, as you take many exposures. Many photographers will argue about the details of ball heads and carbon steel legs, but consider this: I've successfully used the same cheap, broken, wimpy tripod for all of my star trail photos. In a pinch, a big rock or a tree stump will work fine, if it's in the right location!
- Camera: Any digital camera (with a lens of your choice) will work, but it will be much easier to use a DSLR which is capable of taking multiple photos, without needing you to babysit it. Some cameras include a built-in function allowing them to take photos at specified intervals. If yours doesn't, you'll need some sort of external intervalometer.
- Intervalometer: An intervalometer is a piece of hardware which triggers your camera to take a photo at predefined intervals. These are ideal for photo stacking, since they can produce a sequence of identically exposed photos. Some cameras have an intervalometer feature of their own. If not, you'll need an external one. There are many options, involving varying levels of expense and effort. One option is a remote release (such as a cable release or infrared remote) which includes a timing function. Many releases do not have this functionality, and those which do are usually more expensive. Two good examples are the Nikon MC36 or the Canon TC 80N3. Note that most wireless remotes do not have intervalometer functions. Similarly, the Pclix is a third party intervalometer which can be adapted to many popular DSLRs. If you're handy with a soldering iron, there are many “homebrew” options for intervalometers. One example is available at Instructibles. Some people have even programmed graphing calculators to act as intervalometers — search around and you'll find quite a few examples.
My preferred intervalometer, however, is my laptop computer. It is possible to use a laptop to control your camera in a very flexible way. For this, you'll need two things: an appropriate USB cable, and software which will control the camera. The cable depends on your camera, but most cameras have a mini- or micro-USB connection which allows you to extract images. A cable should have been included with your camera — if not, or if you've lost it, they're identical to many similar cables used for many other peripherals. If the cable fits, use it!
The software is trickier. There are a variety of options. Most camera manufacturers have some sort of software which will control your camera, although it may cost a bit. My preferred choice is gphoto2, which is completely free and works with almost every digital camera known to man. The main limitation of gphoto2 is that it only works on Linux or Mac OS X — there is (currently) no Windows version. You will also require the knowledge to properly install the program, however, this is generally doable with your preferred package manager. I won't go into any more detail here; if you're interested, you can read some more in my article about installing and operating gphoto2.
What software do I need to stack photos?
Once you have captured a series of photos, the key tool for creating a stacked photo is stacking software. This is specialized software which creates a composite image in which the brightest parts of each individual image show through. There is a lot of stacking software available. Part 3 in this series will focus more on this aspect, but for now I'll mention a number of available options:
- Startrails: This is the software which I use. While it is Windows-only and fairly rudimentary, it also does an excellent job of combining exposures. It can also create movies from the exposures.
- CombineZ: Actually a set of several different programs (CombineZM and CombineZP), this is another popular option. The ZM version can create movies, and ZP is aimed at creating still images.
- Photoshop or Gimp: While these involve more effort on your part than the other software, Photoshop and Gimp both have the tools necessary to stack photos by hand, giving you extra control over the result (at the cost of much more effort).
- Registax: A commonly used stacking program which I have not tested.
There are many others — search for “photo stacking software” to find more examples.
The next part of this series will cover the planning and execution involved in actually taking star trail photos. We'll start with with scouting a location and go all the way through actually taking the photos. Until then, it's time to start daydreaming about your favorite landscape location — and how it could become a star trail location instead!