High dynamic range (HDR) photography is a technique that allows a photographer to take the same image at different exposures and then blend them together to produce one image that features the best exposed parts from each of the images. For a virtual tour, this is particularly useful, as most virtual tours rely on a 360 degree perspective. Often you will be shooting both away from and into a light source. Usually this would mean that you would have to compromise between the two and potentially have dark patches that are underexposed on one side and light patches that are overexposed on the other. Using HDR you can shoot at three or more exposure levels–low, medium, and high–and then blend them together to create the perfect panorama.
What are the disadvantages?
Although this technique is very useful it does have its disadvantages. First, processing time: using HDR imagery effectively triples the number of images you are dealing with. Unless you are using top end equipment (i5+ processors), this is going to greatly increase time spent during post-processing.
Second, ghosting: HDR images are three or more images taken one after the other. If you have movement in your images (for example, trees moving in the wind or someone slipping on a humorously placed banana), when you start piecing your images together, the differences in the images can cause a grey ghosting effect that can ruin your scene.
Finally, you will have to work with software that supports HDR imagery. Most top end software will work, but this is still a consideration, especially if you are used to software that is not compatible with HDR.
So should I use HDR to create my 360 virtual tours or not?
If you are considering using HDR, my advice is that you should first invest in a tripod and panohead. The ghosting caused by holding your camera by hand will cause you no end of trouble during post processing, and it can even make your scene un-stitchable.
If you are looking to sell the 360 virtual tours you produce, HDR is now an industry standard and an essential tool for creating professional, vibrant, and well-lit panoramas.
Technically, all you need to take HDR images is a camera that lets you manually change the exposure. You can then set the camera in a fixed position on a tripod and take several images at different exposures that you will use to create one HDR image. However, this process is time consuming, especially for a 360 virtual tour where you will need to take several images for each scene to stitch together later. When your virtual tour contains more than ten scenes, this can become a massive time sink. If you are serious about virtual tours, I highly recommend investing in a high-end DSLR camera that features exposure bracketing. Exposure bracketing is a feature on a camera that allows you to set the exposures at three or more levels (usually low, middle, and high). Once you press the shutter button, the camera takes the differently exposed images in quick succession. This has the added benefit of reducing ghosting due to the greatly shortened time between taking the images.
Taking HDR Images
To take HDR images for your virtual tour you need to have the camera set to manual exposure and manual white balance. Once I have my camera and tripod in place, I set my exposures by pointing the camera at the most over-exposed part of the panorama (the sun if outdoors, otherwise the brightest light source). I then adjust the exposure until the image is just on the dark side of clear. Once I have the base level set to my satisfaction I adjust exposure bracketing to at least +2 and -2 (some cameras are unable to stretch this far–you might have to take two bracketed images). You can test it by taking a sample picture; if the camera takes three images for each press of the shutter button, then exposure bracketing is set and you are ready. Now just take the images you would normally take for a panorama and, depending on how many you usually take (I vary between 6 and 12), you should now have a set of three exposures for each image–one light, one normal, and one dark. For extra stability, set your camera to a two second shutter delay; this will combat camera shake.
Now that you have your HDR panoramic images for your virtual tour, you are ready to stitch them together. This can be done in a number of ways. You can use your RAW images to create HDR images before stitching, you can stitch your images together using software that accommodates HDR and allow it to fuse your HDR images for you, or you can create separate panoramas at each exposure level and fuse them together afterward. Each of these methods has its merits, and I highly recommend experimenting with each to find which one suits you best.
Whichever way you choose, there is software out there to make your life a lot easier. Technically you can use high-end stitching software such as PTGui or Panoweaver to stitch the images, adjust the exposure, and output an HDR image. However, I have found that the HDR fusing mechanisms within stitching software are not as good as dedicated HDR software.
My personal HDR magic formula is to use PTGui to stitch the images together then output three separate panoramas at different exposures. I then use dedicated HDR software to fuse the images together into one beautifully exposed panorama ready for use in my virtual tour. There is a lot of software on the market, but for sheer professionalism of results, I have to recommend Photomatix Pro. Good luck and happy shooting.
About the Author
Charlie Tupman, head of photography for www.liontreevirtualtours.com has been producing panoramic photography for 360 virtual tours for several years.
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