“Give light, and the darkness will disappear of itself.”
What is Bracketing????
Exposure bracketing or Bracketing as it’s more commonly known is the process of taking multiple exposures of the same scene, which results in a series of images ranging from dark to bright. This is usually achieved by varying the Aperture, Shutter speed or ISO or even combinations of these parameters. Typically, the most common way of bracketing is to maintain the Aperture and ISO constant while varying the shutter speed. This is done to preserve the Depth of Field (DOF) across the shots. However, based on your needs or intent, you can bracket by varying the other two parameters as well. As you can see, the images here are a series of bracketed exposures and vary from dark to light. Next, we will delve into why we need to bracket exposures and when it is necessary.
Need for Bracketing : The Why and the When
The dynamic range ( The ratio between the brightest intensity and the darkest intensity) of the camera is very limited compared to that of the human eye. Additionally, humans use other visual cues to perceive more information in a scene. If we consider the dynamic range to be measured in f-stops, a typical camera has a dynamic range of about 8-10 f-stops, while the human eye has around 24 f-stops of dynamic range. So where am I going with all these stats? In a nutshell, a camera cannot accurately capture all the tonal variations in a scene if it is beyond a certain dynamic range. So, if the highlights in the scene are beyond the brightest brights that a camera can capture, they are rendered pure white. Similarly, if the shadows are beyond the darkest darks that a camera can capture, they are rendered pure black. This is called clipping. As a result, pixels in the image that are clipped lose their information and tonal values and are approximated are pure white or pure black, depending on whether they are from the highlights or shadows. More often than not, dramatic light situations often have a very very high dynamic range. So if you want to capture the scene as it appears to your eyes with all the tonal information intact, it’s almost impossible to do so in a single exposure of the scene. Camera meters are getting more and more intelligent each day, but the best lighting situations often confound the smartest of cameras. So what is the solution then? The answer is quite simply to bracket multiple exposures of the same scene to accurately record the highlights, shadows and the midtones. The best way to do this is to keep the Aperture and ISO constant, and vary the shutter speed. In effect, this keeps your image sharp ( assuming you focused well), keeps the noise at bay and gives you detail from end to end of the histogram. Remember, you don’t always need to bracket your shots. For example, if most of the tonal values in the scene are located in the midtones, and you don’t have extreme highlights or shadows, you could do with a single exposure of the scene. An easy way to check this is to look at your histogram after taking a shot. If both highlight and shadow areas are not clipped, then you don’t need to bracket. Bracketing helps when you can’t get all the information from the scene in a single shot. Additionally, you could also bracket for creative effects, which we’ll discuss in the following sections. The image below shows what the camera thought was the correct exposure for the scene, and the associated histogram. As you can see, the highlights are badly clipped. As a result, there is no detail in the sky areas.
Bracketing : How do I? How Many? What about software? Aaaaaaaaaah so many questions!
There are many questions that are asked about bracketing exposures. I’ll try to address some of them here.
- What you need : The most essential equipment that you’ll need is a sturdy tripod, one that doesn’t shake or vibrate when winds are blowing or if waves hit it ( err at a reasonable force). Most cameras these days have an auto-bracketing function so I don’t want to include that explicitly. However, check your camera manual on how to setup Auto Exposure Bracketing (AEB). In effect, what this does is that it makes your camera automatically capture multiple shots in a row (similar to burst mode), but at different exposure levels. In fact, you can even customize the spacing between exposure levels, i.e., +1/3 E.V or +1 E.V and so on. Some cameras even allow you to choose how many exposures you’d like to take, say 3,5,7, etc. I’d highly recommend reading your camera manual come to grips with this functionality atleast. A remote shutter release is very useful too. It ensures that you don’t shake the camera and can also help you take customized hand-calculated exposures if you don’t or can’t use the auto functionality in the camera. For example, if you are trying to bracket exposures at either during dusk or dawn, chances are, the exposure times would be north of 30 seconds. In such cases, you would need to use a remote release to manually bracket your exposures. To summarize you need : 1) A camera with AEB 2) A sturdy tripod 3) A remote release cable.
- How many bracketed shots do you need? : This is probably the most asked question about bracketing! Umm should I bracket three shots, or 5, or 7 or 9? Well sometimes, even two shots might be enough. There are two factors that should be considered here. First, the spacing between the bracketed shots. I’m referring to the E.V spacing. Typically, a smaller spacing between bracketed shots would result in a larger number of shots, while a larger E.V spacing would result in a smaller number of shots. There are of course exceptions to both cases. Not clear? Let’s assume that the scene in front of you has a dynamic range that can be covered in the -1 to +1 E.V range (But we don’t know this yet ). Well say that you decide to have 1 stop between your shots and you take 3 shots at -1, 0 and +1 E.V. Let’s say you repeat this exercise with a spacing of 1/3 E.V. You would then have shots at -1/3, 0 and 1/3 E.V. In the first case, you have covered the entire dynamic range of the scene in 3 shots, while in the second case, you are going to need more than 3 shots to get the entire tonal range of scene. Comprende? Second, the histogram. If three shots at -1, 0 and +1 E.V get you the complete range of shadows, midtones and highlights, then that’s all you need. You don’t need 7 more shots of the same thing! Sometimes, you may need just two shots, one for the highlights and one for the shadows. So how do you decide? First take a shot that the camera thinks is the correct exposure. More often than not, it’s wrong for a scene suffering from High Dynamic Range Syndrome. Now look at the histogram. Are the highlights clipped or are the shadows clipped. To know what’s clipped, just see which side of the histogram has spikes smashed against the histogram wall. If it is the left side, then it is the shadows. If it is the right, then it is the highlights. This tells you whether you need to bracket for the highlights, shadows or both. Next, choose the spacing between your shots. Typically you could start at a spacing of 1 E.V and then fine tune it based on the histograms that you get from the bracketing process. The end goal is to get the complete range of shadows, midtones and highlights in an image. As long as you do that, you’re good. For this image, I needed only 4 brackets to capture the complete tonal range. You can see them below. The brackets are -2, -1, 0 and 1 E.V in case you were wondering.
- Why don’t I use software to create multiple exposures later? : Post processing software has shown tremendous advances and the stuff you can do with such software these days is quite mind boggling. One very attractive tool that software gives you is the ability to make multiple exposures from a single RAW file. By adjusting the exposure slider, you can create any number of bracketed shots that you need. Another thing you could do is use a software based graduated neutral density filter to even out exposures. While these tools are no doubt useful, they only work well when you have the tonal information in an image intact. If you already have major clipping to begin with, you cannot recover that by any combination of slider yanking. So that’s why it pays to nail the exposure in the field and not be a lazy bum and rely on software to help you later. So the next time you think, I’ll do it in photoshop, think again. If you give a severely clipped image to photoshop, you will get a severely clipped processed image out! The histogram above really helps understand this. If I had just decided that I could use what the camera thought was the correct exposure and then use software to generate the other brackets, I would not have a sky to work with at all!
- Creative Bracketing : Sometimes, you can bracket to capture different aspects of a scene that you couldn’t catch in a single shot. For example, say that you take a shot and you love the way the clouds look. But the waves and water look bad. No problem, take another exposure when the waves look good and then blend them as they were bracketed shots! Photography is about condensing time and space into a 2D canvas. It’s definitely ok to capture different aspects of the scene in their best light and then render them as one entity. More often than not, that usually enables the viewer to experience the scene, rather than just see it. For the image below, I waited for the clouds to be lit up by the sun, so that they would glow and used one such image in my final bracketed series. I also picked a bracket for the flowers where they weren’t moving in the wind. This made blending easier.
- Some Final thoughts and Footnotes : While bracketing, it’s important to pay attention to movement. Anything that moves can mess up your blending later on. Not that you can’t do it, but it becomes a much more painstaking process. So be careful and aware when you take your bracketed shots to ensure that there’s as minimal movement as possible. Apart from exposure bracketing, there are other kinds of bracketing as well. For example, there’s something called Depth of Field (DOF) bracketing, where you take multiple shots of the same scene, but with different depths of field. You can then blend the sharpest areas in the bracketed images together to create an end to end sharp image. The possibilities are endless. So the next time you are confronted with a High Dynamic Range scene, you know what to do. Fire away those brackets!
Processing the Image : A quick time-lapse!
Below is a quick video timelapse of how I processed a bracketed series of shots. More detailed videos will come in the future. Stay tuned!
Comments, Questions or Suggestions? Feel free to drop a note below!