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In this course, photographer Ben Long describes the concepts and techniques behind high dynamic range (HDR) photography, a technique used to create images that have a wider range between the lightest and darkest areas of a scene than a digital camera can typically capture. The course begins with some background on dynamic range, on how camera sensors detect shadows, and on the kinds of subjects that benefit from HDR. Ben then describes and demonstrates several methods of generating HDR images, starting with single-shot HDR, which relies on masking to subtly enhance the dynamic range of a shot. Next, the course covers multi-exposure HDR, which involves shooting several photos of a scene, each at a different exposure, and then combining them using software tools. Ben demonstrates how to use Photoshop and the popular Photomatix software to process HDR images whose appearance ranges from subtle to surreal.
When people talk about HDR photography, they're usually referring to a process wherein you shoot multiple frames in a particular way and then merge them together using special HD or processing software. Learning that process is going to be the bulk of this course. Here is how it works. You find a scene that you like that has more dynamic range than your camera can capture. You frame your shot and you take a picture according to your camera's normal metering. This picture is probably going to have overexposed highlights and possibly shadows that are too dark. Next, you take a second picture with the exact same framing, but this time underexposed, usually by a full stop. This underexposed frame will probably bring those overexposed highlights back into the range of your camera. those areas will now have detail in them. Finally, you take a third frame, this one overexposed usually by one stop. This image will have wildly overexposed highlights, but all of its shadow detail should be much more visible. So we've got three frames. If you want you can shoot more and continue the bracketing outward for example. You could shoot a fourth frame exposed two stops under and a fifth exposure two stops over. Typically, I find that I only need three and a one stop bracket over and under is all that I need to get a good final result. Now let's think about what we've got here. We have three images. We have one with clear detail on its mid-tones, we have a second with very good detail on its highlight areas, and a third with very good detail in its shadows. In other words, this set of three images covers a much broader dynamic range than any one single shot. Now I'm going to take a second to talk about that term High Dynamic Range, because honestly it's a little misleading. Your camera like your eyes has a dynamic range that it can capture in a single shot. We can't do anything to expand that. We can only hope that image sensor technology improves and when that happens the giant multinational corporation that made you camera will feel so bad that you have older technology that they will just give you a new one for free. And as long as we're hoping, let's just go all the way. More importantly than your camera though, and I'm assuming here that you're ultimately going to print your image, a piece of paper has a dynamic range. The darkest tone that it can hold depends on the black ink that you use and how that ink sits on that particular type of paper. The lightest tone is dependent on the color of the paper itself, because white in a print is just a spot on the page with no ink on it. There's nothing we can do to expand the dynamic range of a piece of paper and paper is always going to have a smaller dynamic range than your camera or your monitor. We can buy better paper, which will have a bigger dynamic range, but it will still be a smaller dynamic range than our cameras. When we talk about the HDR process, we're not actually talking about a process of expanding dynamic range. That's simply not possible for any chosen device. Instead, we're talking about a process of compressing dynamic range. HDR software starts by taking that bracketed set of images that you took and layering them on top of each other, getting them perfectly registered. Then it intelligently combines them through a process called tone mapping. The result is a final image that has a broader sampling of tones than you could capture with a single shot. The overall dynamic range though remains the same, the overall dynamic range of your output media. After I print an HDR image, the darkest thing on this piece of paper is no darker than the darkest thing that can be printed from a normal single shot image. Similarly the brightest thing is no brighter than what I can get from a normal single shot. And everything that I'm saying here also applies to your monitor. When you create an HDR image, your monitor cannot suddenly display a darker or brighter tone that it could before. What tone mapping does is it goes to the underexposed image and it looks up the brightest tones and it puts those into a new image. Then it does to the overexposed image and grabs the darkest tones, which are actually much lighter than the same areas of the underexposed image, and it puts those into our final image. In other words it looks through all three images to cherry-pick tones that are well exposed. It remaps a selection of tones from all of three images to create a single final image that can show detail throughout the entire tonal range. That's a broad overview of the process and for it all to work, you have to take special care when shooting. Also your HDR software will provide you with a lot of control of that tone mapping step. Obviously, this process depends heavily on good post-production, using very specialized tools, but once you've done this all a few times, you should find that overall it's fairly simple procedure.
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