Extended dynamic range using Layers from Bracketed Exposures 1.

One criticism of the 43 sensor is that it has limited dynamic range compared to mid to high end APS-C sensors. This means that highlights become white blocky areas and lose realism, and shadows the same in dark slabs. In fact the criticism is that the top of quarter tones, light with detail, and the bottom of three quarter tones actually disappear into highlight and shadow in an otherwise well exposed shot, leaving an area which is unrealistic, in being blocky, posterised or marked by noisey speckles.

In a perfect world we would have sony's sensor qualities in the D7000 in four thirds, and there will be progress in this at least within 12-14 megapixels according to many enthusiasts who know a thing or two about quantum physics.

However we want to acheive what we can within our limits or as part of the challenge of mastering our equipment and post processing (post') technique.

A simple means to capture more dynamic range is to make bracketed exposures, and then layer them onto each other. The top layer or even several layers are made partially transparent.

The best technique is to use a tripod and take three immediate shots, especially in daylight so conditions and shadows will be as near as possible between each bracketed shot. Most all enthusiasts cameras bracket on increments (+/-0.7 is a good starting point, giving three bracketed shots). Otherwise cameras often have an exposure adjust, which allows for slight under and slight over exposure to +/- one exposure step. A simpler but less effective means of doing this is without bracket function is to set the camera to aperture priority and simply take one shot "right" and two other shots either side of that. Alternatively you can as I have below, set up manual exposures which vary the depth of field as well as the overall exposure.

From the cameras own LCD /LED screen it can be difficult to judge the exposure of each shot, but you can see that some are quite dark while others become too bright. You can look at the histogram to see if there is a large skew to the last 5% of either side of the scale. However some overly dark shots on screen actually have a lot of detail to be tweaked back out of them.

So I recommend setting up with a tripod and running lots of shots: get a feel for the depth of field ( we are going to consider a long depth of field which gives a sense of "realism" in combination to extended dynamic range). The second advantage of a tripod is you can shoot unusually long exposures in daylight @f18-22 or in addition an ND or polarising filter.

Example Using Gimp

This is my next experiment, having come a long way in adjusting jpegs in curve work, sharpness and so on. It helps if you have worked in layers before, but in fact this can be the simplest use of layers (as apposed to the long route to masking to blur a background in an earlier blog).

For my first ever venture into layering for extended dynamic range, I chose some night shots with very long exposures. I wasn't happy with the end results in either of two exposures : one was too dark and the other was light and soft with flat highlights around some nice period style lamps.

I wanted to try several exposures but on the laptop I wasn't pleased with either of the two "keepers" from the back of the camera view. However both had their merits. So my first fodder for a long awaited experiment.

So what came out of the box??

One too dark: maybe a lift would help it, but then it may lose depth and realism. The other one is about right, but a bit too soft and short a depth of field. Also the highlights are overcooked around the detail lamps I wanted.

Try One: Dark onto of lighter: shining through?

Okay, so these have uploaded not so great but you can see there is a subtle extension to the DR in the merged down shot, dark ontop of light with about the right transparency (sorry I forget if it is 60% + or under 30%: experiment yourselves, use undo to get back !)

The highlights are still blown and the detail is not sharp. A kind of worst of both worlds but the exposure is better.

Now the reverse: a thin, 24% light layer ontop of a 65:8:5 sharpened dark layer which was lifted slightly in the shadows. The light layer on top is slightly dropped on the highlight peak (lamps of course)

Now! Here I have had quite a plasticy looking "dark" layer once sharpened, but the feint light layer is merged down at 24% opaicity it unexpectedly softens it nicely. The main aim was to lift the shadows.

Any benefit over just one image worked up on curve and sharpening though?? Well as mentioned the sharpening which hardened the details on the lamps, balcony, woodwork etc would be too intrusive while the image with the best DOF viz the darker, if chosen would become noisy and dull when lifted so much!!

I have heard of layers being used from negatives in the dark room, suppositioning several exposures onto one paper. And of course it is the origin of the colour process!

So I am pretty pleased and once again I feel a little of the magic that people had 30 years ago and longer when an image would appear out the baths magically.

Here is the image larger below because I can see the compression is killing the IQ : however you can see what I mean above in all the shots.

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.