HDR and Expose to the Right

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Today’s Question: Your advice [in Tuesday’s webinar presentation] to “expose to the right” on the histogram made sense, but it triggered a question immediately regarding HDR [high dynamic range imaging]. I understand your point about noise being a major reason for the advice to expose to the right. But does HDR override that advice given that the series of separate HDR frames include a wide range of exposures? I assume it does, and for the most part, I don’t see the noise problem you illustrated today.

Tim’s Quick Answer: With a good approach to capturing the original frames for your final HDR exposure, you will indeed achieve the basic benefit of “expose to the right”. This issue is less critical with HDR captures, but that is in part because of the blending of multiple exposures.

More Detail: My two key recommendations for capturing the individual frames for an HDR image address the benefits of the “expose to the right” approach, which can help maximize detail and minimize noise in the final image.

First, I recommend that for an HDR sequence you start with an exposure that is as bright as possible without losing highlight detail. In other words, your first exposure is an “expose to the right” capture for the overall scene. Of course, if HDR techniques are required this exposure will be lacking shadow detail.

My second recommendation relates to how many exposures to capture. In short, I want to capture enough images that the final image (the brightest of the sequence) shows the far left end of the histogram display at about the midpoint of the histogram display. In other words, areas of true black in the scene would be rendered closer to middle gray in the final capture.

By having this extended range of information that generally covers the full tonal range of the scene, you’ll ensure that you have good information (light) to blend together. That, in turn, will help ensure minimal noise for your final result.

Again, this isn’t a major concern for HDR imaging under normal circumstances, but by taking the approach outlined above you’ll be sure to gain the benefits of the “expose to the right” approach for your HDR captures.