Sunday, 19 July 2009

HDR photo experiments

While in Hong Kong, we took a bus tour of Hong Kong Island. This took us up Victoria Peak. I had left my camera bag with the filters in the bus when we got on the tramway to go to the top. With the difference in brightness between the sky and the foreground being quite large, I decided to try taking three images and different exposure values. I set the camera's automatic exposure bracketing to ±2 stops and to aperture priority (Av). The camera (Canon EOS 300D with an EF-S18-55mm f/3.5-5.6 lens.) was hand held as tripod wasn't in the luggage that we had with us in Hong Kong.

An HDR image (high definition range) allows for a greater range of variance in the exposure level giving an image that is closer to what the human eye would recognise. One of the references that I had read suggested locking the shutter speed and allowing the aperture to adjust. This would ensure that each image had a different depth of field. A fixed aperture as I have used in these sequences retains the same depth of field.

The first two sets of photos also go together to form a panorama although I wasn't consistent with the focal length. The three shots for the first set have and ISO of 100, a focal length of 41 mm and aperture of f/11. The shutter speeds are 1/100 sec, 1/400 sec, and 1/25 sec. The combined image is the last in the sequence.

The three shots for the second set have and ISO of 100, a focal length of 34 mm and aperture of f/11. The shutter speeds are 1/100 sec, 1/400 sec, and 1/25 sec. The combined image is the last in the sequence.

I completed the building of the individual HDR images using Photoshop CS4. I made a slight adjustment to the brightness for each image and then stitched them to form the following panorama. The left image obviously had more light that the right hand image. Photoshop automatically adjusted the images for the difference in focal length.

A third set of images took the scene from a slightly different angle. Part of the problem in taking panoramas hand held is that the camera doesn't rotate around the nodal point of the lens. This can cause objects closer to the camera to appear to move in relation to objects in the background. In this case, I moved quite significantly before taking the third set of images.

The three shots for the third set have and ISO of 100, a focal length of 39 mm and aperture of f/11. The shutter speeds are 1/100 sec, 1/400 sec, and 1/25 sec. The combined image is the last in the sequence.

It looks like I should have taken more photos with a smaller gap in the exposure value differences. One suggestion is to use a difference of 1 stop between images and to take five or more images. Further experimentation will occur once I have all of my photography equipment together again.

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