Friday, January 24, 2014

High Dynamic Range

I've been experimenting. In truth,what happened is that I discovered a way to get my camera to take a burst of three, or five, or seven, or even nine photographs at different exposures, all in extremely rapid succession.

This is one of the fundamentals of HDR photography.

The basic idea is that the human, and presumably the caprine, eye captures images that are processed by the brain in a way that allows us to see detail in dark shadows and also in bright highlights. A photograph simply cannot do this, and a camera set to show detail in the shadows tends to blow out all highlights in great areas of featureless white. Similarly, taking a picture where the highlights still retain some detail will plunge dark areas into a black, featureless morass. Digital photography, unlike ye olde silver halide, loses all image detail for ever once the brightness of any particular pixel rises to 255 or drops to zero.

It is possible to dredge additional detail out of a digital photograph by messing with the histograms in Adobe Photoshop, or whatever other photomanipulation software is to hand. Provided there's some digital information, it can be increased or decreased to make a visible difference to the photograph.

So we find digital photographers cropping areas of their pictures, enhancing the subject and pasting it back into the background that's been tweaked in a different way, and thus producing improved images at the expense of masses of work in the virtual darkroom. This isn't dishonest, by the way. Photographers have always adjusted images by enlarging, cropping, dodging, burning, and messing with colour, brightness, and contrast, even in the darkroom with actual chemicals. Who never sent their negatives off to TruPrint for enlargements to be done, and got them back a different colour from the original enprint? Anyone younger than about fifty, I guess.

Enter High Dynamic Range. The basic idea is to take a blast of identical images at different exposures, and then use computer software to combine them into a single image. It'll even align the pictures in case you've not been using a tripod and they're all framed slightly differently. The program picks the highlight detail from the underexposed images and the shadow detail from the overexposed. What you get is supposed to represent more accurately what the brain perceives. I've been using EasyHDR, but there are plenty of others out there.

In the picture of Dubai's skyline above, the buildings are all in shadow against a sunlit sky. Single photographs either cast the buildings into silhouette, or else blow out the sky to a featureless overexposed white.

Halo around the motorbike caused by
amateurish overuse of HDR.
Word of warning, though. It's easy to overdo it with HDR. Imagine a dark subject against a light-coloured background. The computer program correctly overexposes the dark subject, but this tends to spill into the background, and you get a characteristic halo around the subject. In my book, that's heavy-handed overdoing it, and is a reason why I avoided any contact with HDR photography for several years.


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