Ingelborough

Camera	Sony DSC-F828
Exposure Mode	auto bracket (±1 EV)
Focal Length	14mm
Aperture	f/7.1
Exposure	1/250 sec.
ISO Speed	64
Flash	off

The automatic settings on most digital cameras are great when the scene you are shooting is evenly lit. Unfortunately we don't have to try too hard to find interesting things to photograph that are unlikely to be evenly lit -- landscapes are an obvious example. Good landscapes (usually) consist of a mixture of interesting land and interesting sky which can often require widely different exposure values in order to capture all the detail. Let only a small amount of light onto the sensor (or in days of old film) and the sky will be full of detail while the land is mostly black. On the other hand if we allow enough light to fall onto the sensor to capture details in the landscape the sky gets blown out, often to the point where there is no detail left at all, just a bright white sky. I've been disappointed more times than I care to remember by landscapes ruined in this way.

One of the features on my main camera is autobracketing. When enabled the camera takes three photos in succession each with slightly different exposure settings. I haven't used this setting in a while but back in 2006 when on holiday in the Yorkshire Dales I experimented with it a little. The autobracketing doesn't work in the fully automatic mode so I switched to setting the shutter speed by hand. I would point at the scene, adjust the shutter speed until I thought the display matched as well as possible the actual light conditions, and then press the button to take the photo and the camera would take three photos in sequence; the settings I'd set and one photo, 1 exposure value, either side leaving me with three slightly differently exposed photos. My hope was that one of the photos would have roughly the right exposure settings. Unfortunately this was never the case. I'd either get black land or white sky but never a good compromise. These photos languished in the bottom of the digital 'photo draw' until this week when I decided to revisit them and see if anything could be salvaged.



So my starting point was these three autobracketed images of the sun setting behind Ingelborough. As you can see the left hand image contains plenty of detail in the sky but the land is mostly shadow. Things are a little more balanced in the middle image which was the photo I actually set the camera to take. The right hand image contains the most detail on the land but the sky is almost completely blown out. What I really need is a way of combining information from the three images to create a single correctly exposed image. Just as with last weeks 3D image lets go back to the dawn of photography for my first attempt.

In 1857 Gustave Le Grey exhibited a set of seascape photographs taken in the summer of 1856 and the spring of 1857. Seascapes, like landscapes, are difficult to expose correctly. Most early photographers exposed for the sea which resulted in the sky being completely blown out. Le Grey, however, exhibited seascapes that were perfectly exposed. He had of course 'cheated'. The images were actually printed from two different negatives, one exposed for the sea and one for the sky. The image on the left is my digital recreation of this technique. In Paint Shop Pro (PSP) I opened the darkest image, showing the detail in the sky, and then added the lightest image, showing details on the land, as a second layer. I then used the smart edge freehand selection tool to select the sky on the top layer (with some feathering to hide the join) and then deleted it allowing the detailed sky to show through. I'm reasonable happy with the result although it should be noted that this approach will only really work where there is a clear divide between two exposure levels. For example, light shining through trees might benefit from the combination of two exposure settings but it would be impossible to manually merge two such images (unless you really fancy selecting the gaps between every leaf and trunk). Also the approach is really limited to just using two images which doesn't take advantage of the three bracketed images I originally took. So lets move from the Victorian era to the present day for an approach that will combine all three of the bracketed photos.

HDR, or High Dynamic Range, is a technique that allows you to blend multiple photos with different exposures resulting in an image with more details in the shadows and highlights then is possible with either the camera on it's own or the exposure blending technique I discussed above. Since it's introduction in the mid to late 1990's producing HDR images has usually required dedicated and/or expensive software. The explosion in HDR photos really started when Adobe added support to Photoshop, but at over £600 that is still out of my price range -- I won't pay more for software than I spent building the computer! The problem I have with HDR images is that people tend to abuse the process, resulting in images which have no grounding in reality. You only have to look at a few photos in the HDR pool on Flickr to see what I mean, although some people do use it sensibly and create fantastic looking images

I've recently upgraded the copy of PSP I use to the latest version (bizarrely called X2 and available from Corel) as the University I work for have a license allowing me to buy a copy for the grand sum of £2. One of the new features is HDR Photo Merge. So I have three differently exposed shots of the same scene and some HDR software, the approach to rescuing something from the original photo is now clear. On the left you can see my initial attempt at HDR. This is the raw output from the HDR photo merge, before I've spent time fixing the colours. What is important is that while it looks a little washed out there is detail throughout the whole photo. Some of the clouds are still blown out but only as badly as in the photo exposed for the sky anyway. Unlike most other HDR software PSP has a simplified interface for choosing the correct settings, allowing you to alter just the brightness and to clarify the image. Apparently these settings equate to setting the gamma and exposure and a local contrast operator. For the curious I used 60% for both settings to get this image. PSP can actually suggest settings for these two options if you don't want to spend hours tweaking the things manually, although it's suggestions of 36% and 27% respectively left the image a little dark for my liking.

The final step was to return some of the colour to the image that had been lost during the HDR merge. I could have done this in a number of ways using histograms, curves, colour temperature, channel mixers etc., but I opted for the simple Smart Photo Fix dialog in PSP. This allows you to alter the overall brightness of the image, the brightness of the shadows and highlights, saturation and sharpness. I could have spent days tweaking these numbers, especially as I wasn't at all happy with the automatically suggested values. In the end I went with values of -10, -70, -20, 50 and 75 respectively to produce the final image. I'm pretty happy with the result, although the blue in the sky might be a little bright for some people. In the future I'm certainly going to try both HDR and the old fashioned layering of images in order to make sure I get the most from my photos, especially when the lighting conditions are tricky.

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