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fredag 9. september 2011

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.

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