WHILST ON HOLIDAY, it occurred to me that my posts regarding photographic tonal ranges and realistic images had been poorly illustrated. Graphs are one thing; but a simple photograph, as they say, is worth a thousand words.
I was musing on the problem last Saturday night, which just happened to coincide with a bright full moon, in a dark blue sky, which many photographers attempt to capture. Moreover, my room’s balcony looked onto the sea and I found myself with a perfect vantage point for capturing the moonlight shed upon the water.
Such a scene is not easy to accurately capture. It is impossible using conventional photographic techniques (without resorting to darkroom tricks); but the new digital medium provides a means of coming close to revealing nature’s beauty by allowing High Dynamic Range (HDR) images to be produced.
You might think that taking a night-time photograph would require a high ISO setting; but the fact is that nature’s moonlight can be far more powerful than any studio lamp. That was the case, on Saturday, when averaging the scene’s highlight and shadow spot readings indicated 1/1000th second at f16 using 200 ISO.
The result (inset above) is fairly pleasing; but it is not an accurate representation of the original scene. The moonlight, because of its power, has bled across the clouds, masking their definition and diluting the blue of the sky. Moreover, the tonal range from the 200 ISO setting has been unable to capture the foreground’s shadowy detail – and that elusive ripple in the middle portion of the moonlight’s reflection upon the water is almost blown.
The image inset here is a much more accurate rendition. It was produced by combining the first image with a sequence of six more shots, made by bracketing the original exposure and taken in quick succession. By combining all the different tones captured by the camera’s sensor into a 16 bit colour space, capable of representing each value individually, we can finally achieve the missing foreground detail and reproduce the scene’s original colours and surreal mood (produced by the moonlight playing on top of the low-lying clouds) accurately.
In this case, the seven different photographs were combined using Photoshop; but HDR images are supported by all good digital image processing software.
When undertaking HDR processing, it is important to ensure that you balance your photographic sequence around the average exposure indicated by your highlight and shadow readings to obtain an accurate image (just as you would normally place the mid-tone in the centre of the ISO slope). However, just as you can arrange to place the mid-tone differently for artistic effect, you can also weight your bracketing towards under or over exposure to introduce specific moods or effects in your final HDR image.
