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Recently I’ve been doing lots of stuff with HDR photography. In case you don’t know what HDR is, here’s the Wikipedia definition - “high dynamic range imaging (HDRI) is a set of techniques that allows a greater dynamic range of exposures (the range of values between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows.”

Basically, it involves combining a number of exposures into one final image, especially useful in cases where you have light and dark parts in a scene which are impossible to capture in one single exposure. The best way to explain this is by example.

Here are three photos of the same thing taken in quick succession with different exposures (using the bracketing feature on my dad’s Nikon D80):

Airplane at YYZ - exposure 1

Airplane at YYZ - exposure 2

Airplane at YYZ - exposure 3

The first image is taken with the same aperture and shutter speed as would result from using the “Auto” setting on the camera (0.00 EV, 1/125s shutter speed, f/5.6 aperture). The second image is taken at -2.00 EV, 1/500s shutter speed, and f/5.6 aperture. The final image is +2.00EV, 1/30s, f/5.6. This is done using “exposure bracketing”, which is basically taking a number of exposure with varying exposure values (EVs). There are two ways of changing the exposure: changing the shutter speed (amount of time during which the shutter is open and light is being absorbed by the image sensor) and changing the aperture (an opening of variable size through which light passes and hits the sensor. You can see that the shutter speed is changing with each exposure in this example, while the aperture is staying the same. This is because changing the aperture can produce undesirable effects. One such effect is the changing of the depth of field (the part of the image which will appear in focus), and with different parts of the image being in focus it will be impossible to blend the exposures into one final image when producing the HDR image.

Looking at the first image, you can probably notice several problems. While the nose of the plane appears to have a good exposure - not too dark, not too light, the upper portion of the sky is over-exposed. Here, clipping occurs, which means that colour information is lost. The parts of the sky that are completely white are overexposed, and there is no software that will be able to restore what the colour details that existed in the original scene. You can try to open the image in Photoshop and move the white point around, but that will only result in a shade of grey - you will not get the same details that you see in the image with a lower exposure value.

The next exposure has quite the opposite problem - while the sky has been captured much better, the bottom half of the photo is under-exposed. Shadows clipping doesn’t occur as easily as highlights clipping, however if you try to restore the dark portions of the image you might be somewhat successful, but chances are it will turn out very grainy and will look bad.

The final exposure depicts the area under the plane (and the plane itself) very well, but the sky is completely gone. Without using HDR imaging, a photographer would have to choose something to sacrifice - the highlights or the shadows. Alternatively one could go with the first exposure and keep the midtones, but lose some of the shadows and some of the highlights.

The purpose of HDR is to avoid losing any details in the photo. It works by taking those three (could be done with more or fewer) exposures, and combining them into one image. There are numerous software applications which work with HDR. My favourite one is Photomatix, however Photoshop is also capable of this (I use CS3). Here is the result from Photoshop:

HDR Airplane at YYZ - HDR with Photoshop

Personally, I don’t like it. Maybe I’m just not great with Photoshop’s HDR features, but to me that image looks really flat and the colours are somewhat lacking. Here is my result from Photomatix:

HDR Airplane at YYZ - HDR with Photomatix

In the rendered HDR image you can see both the sky and the dark details at the bottom. I realize it does not look realistic, and very much edited/photoshopped, but it is very rare for an HDR image to look convincingly lifelike. The purpose is to capture a large amount of light information, which you can’t do by regular means - and that’s what is done. There are many aspects of the resulting image which Photomatix lets you control when combining exposures into HDR, and it takes quite a while to get it to look the way you want to. As a general rule, I always post-process all of my HDR photos in Photoshop after Photomatix, as I don’t often like the result produced by Photomatix - but only very minor adjustments.

There are many things you can do with HDR - and not all so drastically different from original exposures but much more subtle. It is also not always necessary to use multiple exposures for HDR - it is possible to create an HDR image from one single RAW file (however it will not be as effective). In fact, my most “popular” photo as suggested by Flickr of the cow in the Alps is a single-exposure HDR - but maybe more on that later.

[Edit]: How did I just post this without mentioning what kind of plane it is?! It’s an Air Transat Airbus A310-300. K, that’s better.