Rangitoto and HDR

Today we went to Rangitoto Island in Auckland bay. The island is only about 600 years old, having being created by an active volcano. The island is gradually being taken over by vegetation, but there are still big patches of barren volcanic rocks to be seen. We walked to the summit , a fairly simple 45 minute walk, although we took a little longer because I was taking a few pictures on the way. The track to the summit was pretty easy going – I was expecting it to be rough, like walking on the uneven volcanic rocks, but it was smooth just like most tourist walking paths.

At the top there is a boarded walkway that runs around the crater, again I was expecting a crater full of barren volcanic rock, but it was filled with lush green vegetation. The views from the top were pretty good, and on a truly clear day would be spectacular, but we had a bit of haze and cloud.

After wandering round the summit, we went to find the lava caves. The path to the lava caves was a little more tricky and felt like we were actually exploring a little. We arrived at the first cave area and I liked one of the trees that was catching the light outside one of the depressions in the rock (not really a proper cave). Unfortunately the sunlight hitting the tree was too bright relative to the darkness of the ‘cave’ for the camera to be able to handle that range of brightness. I could have used flash, but the foreground was already lit and I didn’t really have a powerful enough flash unit, just the on-camera one, to properly illuminate the scene. Fortunately though I did have my tripod and so I set up the picture for a HDR (High Dynamic Range) photograph.

The idea behind HDR is that you capture multiple exposures (individual pictures) at different shutter speeds (i.e. you let in different amounts of light). By taking a range of exposures the aim is to capture the full range of detail in the brightest parts of the image as well as full detail in the shadows. In practice it’s a fairly simple process, put the camera on a tripod, compose the scene, then take exposures gradually decreasing the shutter speed each time. It’s the shutter speed that has to change, because changing the aperture will affect the depth of field (which refers to how much of the image is in focus).

To construct the HDR images 8 pictures were taken at shutter speeds evenly spaced between 1/400 of a second to 30 seconds. The 1/400 of a second captures the brightest parts of the scene, whereas the 30 second exposure captures detail in the very darkest parts. First, here is one of the exposures, at 1/100 of a second and this would probably the one I would choose to make if I was taking just a single image. It shows the interesting shape of the tree, but the detail of the ‘cave’ is lost in the shadows. It is essentially a compromise.

There is still a sense of the cave due to the shadows and the tree feels like it’s leading you into the cave. A picture like this leaves a lot to the imagination, because you don’t really know what’s hidden in the shadows.

HDR on the other hand, gives the opportunity to open up the shadows and exposure all of the detail that is hidden in the above picture. I have used the Luminance HDR program, which is freely available, and it provides a number of the early ‘research’ algorithms for performing HDR. There are other commercial programs available that provide algorithms that produce more ‘photographic’ results, but Luminance is still fun to experiment with and does produce some pretty good images.

The following picture has been generated using an HDR algorithm. There are many ways the image data can be compressed and some HDR algorithms will look more ‘realistic’ than others, but this is one way that 8 images can be ‘merged’ into a single image.

So, what do you think? Do you prefer the mystery of the ‘normal’ image, or the greater detail of the HDR image?

Shortly after taking these pictures we found a proper cave and were able to walk through for about 30 metres in near darkness over the uneven floor of the cave. I had to take my rucksack off at one point because the ceiling got so low. By the time we emerged again, it really did feel like a mini adventure and for me was a highlight of our walk on the island. After that we headed back to the start and took the ferry back home.

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