Exposure Bracketing from the ground up.
Exposure Bracketing, or simply ‘bracketing’, is a technique used to extend the dynamic range of a camera. The approach is rooted in the general photographic philosophy which suggests that if your camera can’t capture all the detail you wish for in one single frame, then why not take multiple photos and combine them into one when you arrive back in front of the PC?
Exposure bracketing, along with all other multi-frame techniques, such as panorama stitching, focus stacking and even perspective blending, is concerned with fusing together the information present in multiple images to create a single data-rich output file. Exposure bracketing, in particular, incorporates two or more photos of precisely the same scene, only with each individual image captured using a unique exposure value or EV.
The rationale behind the multiple-exposures method is that each photo with its unique EV recovers some or other detail in the scene that its bracketed kin do not. A popular set of photos used in just such an undertaking includes one image that is 1‑stop underexposed (-1 EV), one that’s correctly exposed (0 EV) and a third that's 1‑stop overexposed (+1 EV). By extracting the highlight detail from the underexposed photo, the shadow detail from the overexposed photo and the mid-tones from the correctly exposed image, editing software such as Adobe Lightroom can produce a final composite image that has the potential to feature significantly more detail than any one of the input images on its own.
With that said, the ability to extract increased detail from a set of bracketed exposures is not a given. To realise any advantage from the system, the scene before you must have a dynamic range that exceeds the native capability of the camera. If, for some reason, you chose to employ exposure bracketing while gazing at a low‑contrast scene, one with a low dynamic range, there is very little to be gained, except, perhaps, more time behind the computer screen if that's your thing.
A tell-tale sign that you have reached your camera’s dynamic-range limit is when exposing for the highlights results in the shadows becoming black or, alternatively, when exposing for the shadows causes the highlights to blow-out. In both cases, the camera is simply incapable of simultaneously capturing the highlight and shadow detail currently contained within the scene.
Of course, if you are more geek than artist, then the camera’s histogram can provide precisely the same evidence too. If all the histogram data does not fit within the bounds of the diagram itself then, again, you have reached the very same tipping point.
The important thing to take away from these ramblings, though, is not how you come to realise that you have exceeded the capabilities of your camera, but rather to recognise that the outcome of any such scenario is always the same. Faced with an excessively high dynamic range, one has only two choices: either settle on an artistic solution or pursue some sort of dynamic-range-extending tactic such as exposure bracketing.
Luckily, of all the tactics out there (and there are a few), exposure bracketing is arguably the most accessible, even if it’s not necessarily the easiest to follow through to completion. To effectively execute exposure bracketing, all one needs is the camera itself, as well as some way of keeping it perfectly still between shots. Indeed, most (if not all) cameras today are equipped with an Automatic Exposure Bracketing (AEB) facility that does a lot of the heavy lifting for you.
A typical AEB system requires just two user inputs, how many photos you wish to take (which is routinely three but not limited to that number) and how big an exposure increment you require between each image. Armed with this information, and cognisant of the camera’s current shooting mode, the AEB will calculate the various exposure settings required to produce the complete set of exposures.
Additionally, if you have your camera set to one of its continuous-shooting modes, those modes that see a burst of rapid-fire shots taken for just one press of the shutter button, the AEB will help out even further. One press of the shutter button and the system will spit out an express consignment of bracketed images.
To illustrate how the AEB system operates, a camera configured to take three photos separated by one third of a stop will typically result in the device rattling off images with the following EV values: -1/3, 0 and +1/3. By contrast, programming the camera to take five photos, each differing by a 1-stop increment, will produce a more sustained spray of fire and result in the following EV values: -2, -1, 0, 1 and 2.
In the interest of telling the whole story, I have to point out that there are a few other more nuanced settings available with most mainstream AEB implementations, such as the ability to dial in an exposure offset as well as the option to tailor the firing sequences. To be frank, though, these settings don’t represent meaningful adjustments when you consider that you have the option to rapidly capture a raft of photos each with a unique EV.
Instead, if you are in search of ultra-fine exposure-bracketing control, I’d suggest that manual exposure bracketing is probably a better bet. As the name suggests, manual exposure bracketing is based on precisely the same principle as the AEB, except that instead of some arbitrary piece of silicon deciding upon the exposure setting, the photographer dials in those settings into the camera manually between each shot. Manual bracketing offers near-infinite exposure control and the option to shoot only as many photos as you need.
It goes without saying, though, that there is one quite obvious flaw with the manual approach, the almost inevitable bump of the camera between frames. For people of inescapable clumsiness, like myself, unintended recomposing is a consequential concern, especially when presented with fleeting photographic opportunities that don’t permit a ‘second chance’. However, in the more deliberate arena of landscape photography, there is no knock in the world that can’t be corrected with a fresh set of photos. For this reason, and in spite of my wayward limbs, when conditions permit it, manual exposure bracketing is my preferred mode of attack.
In conclusion, if everything that I have written here appears less than groundbreaking and not unduly complicated, that’s because it is. In practice, the fieldwork involved with exposure bracketing is little more than an exercise in redundancy; taking multiple photos of the same thing. Instead, the true complexity associated with exposure bracketing only reveals itself when you get back to your desk. Having three different exposures for the same scene might sound like a boon, a great way to safeguard against getting the exposure wrong in the first place, but when you’re sitting with three exposures on your computer screen, what then? The number of options available to you is enormous and they vary wildly in terms of difficulty and effectiveness, but that’s a matter for another day.
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