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fredag 19. august 2011

Hyper on Dynamic Range

Even photography itself, in the days of degeurrotype, was a novelty and a little bit of a gimmick.

Many of the gimmicks and fawcets of cameras, films, lenses, filters and now sensors in their path of evolution have come to roost as established styles and techniques.

Probably what is the latest of the emperors-new-clothes to many luddites, is Hyper Dyanamic Range in post processing and the latest enhanced, broad dynamic range nascent in-camera, surpasing that of the human eye even. HDR is here to stay.

DR- dynamic range a measure of the extremities of light intensity which can be captured on the sensor and subsequent image file for the given exposure. This means that the nuances of light and dark extend so far as becoming pure white, or pure black shadow at a given upper and lower level of light intensity.

What is actually equally important to image quality, is tonal depth. Despite modern digital cameras proporting to have the same technical tonal depth in number of bits, different cameras render these better in RAW and in-body jpeg. Combining a wider DR with excellent TD means that more detail and nuances of colour and brightness can be captured.

How does this add up to the pretentious and contentious "IQ" ? Well you can argue a lot about subjectivity in this, but if you are a 10 - 12 mpx Four Thirds / mFT owner then you know all about limited dynamic range. Blown highlights where large blocks of white make a very intrusive presence on photos from scenes with high contrast taken on an average or subject spot exposure.

High or broad DR means you are capturing more tonal depth at the extremes of the intensity of a scene. This can now surpass the ability of the human eye, leading to a slightly surreal or jamais vu feel to especially dawn and dusk images. The additional nuances allow us to see more detail and more texture. This means the eye, sorry brain, picks up on additional qeues for depth and interest in a shot, and basically makes the perception of realism. Alternatively the surreal or the presque vu or jamais vu come through.

Okay now I am getting a little arty and subjective about how good DR and corresponding TD make for a good shot. Essentially good DR also means that more of the scene can be reported as being properly exposed or at least "pleasantly". When combining several images of different exposures as layers in post processing software, the very best of each can be employed in a final image.

Technically you can measure DR and tonal depth as finite, and DR from the various tests is often worth looking up for a camera you want to buy, and comparing to in particular the excellent DR of the Nikon D7000 and the Nikon D3S and then to rival models vying for your hard earned wonga.

But what about doing your own HDR on a four thirds camera? We want to avoid both blow highlights AND heavy shadows with in my case, the Olympus / OEM CMOS "red spotted dabs" syndrome in shadows and highlights.

Here is my next experimental path in The GIMP, starting asap, as expressed by JPMATH on flickr.

jpmatth (70 months ago | reply)

here's the entire process, simplified where possible:

* set the 20D's auto exposure bracketing to 1 stop.
* stand really still, and squeeze the shutter until three shots are taken (that would be 0, +1 stop and -1 stop). try not to move the camera (didn't have a tripod with me).
* developed each RAW file with the same settings (with recipes in DPP), and exported to highest quality jpeg (to retain the EXIF data).
* opened the three jpegs in gimp as three separate layers, with the 0 exposure at bottom, then -1 on top of that, then +1 at the top of the stack.
* moved the layers around until each element of the scene lined up as much as possible (because i was handholding, each exposure was a few pixels off from the others even though they were taken milliseconds apart)
* added transparent (black) layer masks to all but the bottom layer
* painted white on the layer masks where i wanted sections of each layer to show through.
* flattened everything and cropped the edges where the misalignment was obvious.
* a little unsharp masking to hide the slight softness of the misalignment
* saved to a final jpeg with quality at 90, no smoothing, subsampling at 1x1,1x1,1x1 and DCT method at floating point.

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