HDRI in Photoshop

This is done using High Dynamic Range Imaging (HDRI), which uses 32-bit floating point data to describe a far wider colour space, enabling virtually the entire range of tones in a scene to be recorded. While some digital SLRs can capture a wider tonal range in RAW mode than is usually displayed in the finished JPEG image, none can yet record in full 32-bit HDRI, so to produce images of this type a little Photoshop magic is used.

Editions of Adobe Photoshop from CS2 onward (currently CS2 and CS3, but I’m sure there’ll be more) have an option to merge a series of digital images into a single 32-bit HDRI image, incorporating image data from several pictures into one. The process is simple and fully automated, but in order to use it you need to take a series of photos in a particular way. They need to incorporate exposure data from both ends of the dynamic range you wish to capture, as well as a number of shots in between. Although the effect can be achieved with just three shots, the best results are obtained using six or seven.

For this series of example shots, I fixed the camera on a good solid tripod, so that all the pictures would be from precisely the same angle and position. I took an average light reading from the scene as a whole, and then set the camera to manual exposure mode and adjusted the exposure to three stops slower than the metered value, in this case 0.4 seconds at f/8.0, and took the first shot. I then took six more shots increasing the shutter speed each time at one-stop intervals, so that I covered a range from three stops below the recommended exposure to three stops above. There are one or two digital cameras that have a plus-or-minus three-stop range of exposure compensation, but most only have plus-or-minus two stops, so you’ll need to have the option of manual exposure for this.

Next comes the automated step. Simply open all seven images in Adobe Photoshop CS2 or CS3, open the File menu, mouse down to Automate and select Merge to HDR… from the sub-menu. You can do this directly from Adobe Bridge file browser in the Tools > Photoshop menu. If you are using large DSLR files, the merging process can take a long time. The processing speed will depend largely on the power of your computer. If you have a modern dual-core processor and several gigabytes of RAM it will only take a couple of minutes, but those with older, slower machines may be in for a bit of a wait.

When the process is finally finished you’ll see a screen like the one below, showing you the results of the process. You can save and preview the HDR image as a full 32-bit file, but bear in mind that it will be a very big file. Some of my 32-bit HDR files made from images from my 10-megapixel DSLR are around 114MB in size!

The preview in Photoshop includes an exposure slider so you can see the full range of exposure available.

While later versions of Photoshop can process 16-bit and 32-bit images, of course your computer and printer can only display 8-bit (per channel), so in order to display the HDR image we’ll need to reduce it down to a manageable size. If you open the Image menu, under Mode you’ll find several options.

If you select 8 or 16 bit options, you’ll be presented with a choice of conversion options. For images of this sort, with a wide latitude of brightness, the best option is Equalize Histogram, since this will balance the extremes of brightness and shadow across the available colour space.

The result is an image that includes the shadow detail from the over-exposed frames with the highlight detail from the under-exposed ones, all in one nicely balanced image, with a full range of tones from black to white. It’s not quite a totally accurate depiction of the real scene, but it’s the best available with current display technology and it does look nice, which is after all the main point of the exercise.

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