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.

Dynamic Range, IQ and Tonal Depth Explained ( Olympus E450 owners)

Jargon Busting!


DR: Dynamic range. Seen a histogram? This is worth delving into on your camera display and at least on Olympus viewer on your PC. It relates very firmly to the actual digital data and the quality of the capture of the scene.

IQ No, your camera is not getting more intelligent, this is just a good old subjective reference "Image Quality"

Tonal Depth: this is just as important as dynamic range: given a hue or short range of light intensity, this is how many tones can be captured (and displayed!) such that the nuances of colour and shadow over shapes, textures and colour gradients like a sunse, add to the perception of quality and the technical accuracy of the shot..


How to Define Image Quality?

Well beauty is in the eye of the beholder and you could say therefore there is a dichotomy between technical accuracy and æsthetic value:

Most shots on the Dp forum showing Olympus DR etc are very dull subjects: standard test cards, studio objects or just desk foibles and views up the street from the office or shop with the new E5 on test!

However given some form of interesting content then you can say that the key contributors to image quality are:

1) representation of the scene in either true-to the eye or, a one that gives some æsthetic which impresses at first glance.

2) sharpness : usually the subject of the photo should be sharp: and when enlarged detail remains sharp, although this is a technical sharp: see the debate on how many mpx . I like many slightly blurry portraits - it is one of my own styles. Shaprness acheived by appropriate shutter speed, tripod, image stabilisation, correct focus and associated aperture mediated DOF:

3) Appropriate Depth of Field: the subject is either the only area which is represented sharp or the whole image is sharp ( by in large these two are what is regarded as contributing to good IQ.) DOF is a whole blogg on its own and can be mimicked in image software like gimp.

4) Accurate capture of shadows and highlights. The range apparent to the eye or a range which represents some æesthetic in the image.

5) A good representation of the colours and tonal depth of those colours across their hues and light intensities in the range between the white highlights and black shadows.

Rocket Surgeons Versus Picasso De Vanci

I guess the arguement could rage between the "technicians" and the "artists" on what IQ really is, but as in a previous blog on what makes a good picture, there are both technical elements and æsthetics which you could get rated by 1000 people and come up with an IQ rating if you must.

Certainly a good technical image will lend itself to representing the scene as intended or being worked up in after effects to a new æsthetic which may have been limited if the technical qualities were not there.

My favourite portrait photographers, including Anton Cobijn - the Joy Division and U2 image man, have often rather poor technical prints: very short DR, grainyness, blur, innapporpriate DOF , underexposure. However to me they are really striking.

Way Back When....

In the old days of film, given you had a reasonably good camera, there were two main areas affectting the quality of your colour prints.

1)IThe film as bought: how good the film was relative to its' ISO. it's colour "hue" representation, tonal depth and most of all the grain and noise.

2) At the Lab: The quality of the developing of the film ; the accuracey of the test exposure meter/print; the quality of the paper and chemicals, and the freshness of both.


Now we can't blame it on the lab or the shop or the film maker!

We have responsibility for the quality of our images, but it is luckily much easier : especially with Olympus which has such a good JPEG rendering engine and actually RAW files which are sensibly worked up from the totally raw data. Unlike some camera makes, you start with a reasonably sharpened ORF with a little more sharpening on the JPEG.

The 400 series do not have the absolute best DR and Tonal depth but for the price the overall image quality even with the so called "kit" lenses, is really stunning.


What is dynamic range and how do I see it in an image?

Well you see it most immediately in the histogram as mentioned, but wait up: take a look at the image and the exposure just after you have taken it, and compare it to what the eye sees. Try and alter exposure and focus until you capture near to what the eye sees.

Now look into the shadows and highlights of the image: there is a function in "INFO" which displays highlights and shadows ( I believe it is above 251 and below 5 respectively, more on numbers later.)

The key to understanding DR is that it describes these two extremes: what the camera can pick up in nuances before shadows become black and highlights become white.

Another related issue which is unfortunetly prominent in some conditions and exposures on the E450, is that in being devoid of nuances at the highlight or shadow end, flecks of noise, usually reddish, creep in. More on noise later.


Numbers:

Dynamic range is defined in digital terms as the range from 0 to 255, with 0 being pure black on the left of the histogram or curve (graph) and white highlight being 255 on the right. In black and white, everything in between is a shade of gray in a BW image for example.

Some cameras do not acheive a complete capture of this possible range: they will therefore lose some tones into being just black or white : so for example 250 upwards or 10 downwards. Also some exposures in the E450 will not capture the range: this would be under or over exposed for the situation.

Finally relating to this last point, some subjects or scenes either lack light intensities from the whole range: high key/ low key , or sometimes the sun is just too strong for the camera to "find" grades in tones from about 240 upwards. The same is true in very dark images, where there is not enough light / long enough exposure to give any detail in the shadows.

The relative proportions of image area which are captured in the image are then shown as the peaks on the y axis.

To capture the very best dynamic range for the conditions on the E450, I recommend using bracketed shots on programm or shutter priority, such that the aperture setting is changed. Further to this, if you want to capture the best exposure and "local" dynamic range for a given subject within the fram, like a face in a crowd, or a bird in a shaft of sunlight, then use the spot meter function. The camera will then interpret only the correct exposure and DR for that small area of the shot. ( see earlier blog)

Tonal Depth

This is actually not justt the y axis of the histogram : it is more the accurate representation of the spectrum of colours and their light intensity or saturation.

Shadows are black? Well many argue they are blue tinged, brown or infact grey and not pure black. But at first glance , total shadow point of below 5 on the scale will appear black to the eye.

The whole thing is easier to understand in the LAB system of colour separations than in the monitor oriented RGB scale. In RGB all the colours combine : all off, "0" black, all on 100% light intensity, 255 white. In LAB the blue-yellow and red-green primary range are separated and represented as the opposiste they actually are: where there is yellow, there can be no blue!

The third channel is pure BW, grey scale. In LAB you get a better idea of shadows and highlight areas, and how this helps add depth to an image, while you also have a better level of control in colour balance ( a whole other topic!)

It can be worth converting to the LAB system to work up some images: especially high key and low key, or subtle tonal range images and then finally converting back to RGB JPEG at the end. Nearly all monitors are RGB so you see what you are going to acheive when the "separations" are viewed together.

Why is Tonal Range Important to Me?

What you can do is refer to books which discuss and illustrate "polarisation". This shows how good tonal range is necessary to help with depth and sharpness in a pciture and perception of good image quality. You can reproduce it by super compressing a JPEG at a small size at 50 ppi. Save a copy of a portait ( face only) and then repopen it to see the poor quality and then look at the historgram.

There are large areas of the histogram with zero values: colours become clustered into individual peaks and there is an appearance of "super pixelisation" , box like areas of single colour. There is a sharp grading between colour or shadow areas, with contours often appearing on faces like some maps and weather charts!


Think about making this gradually better, in more detail , and then you start to realise that tonal depth is important for representing a 3D scene. Also it is important for displaying hues and saturations of colours across the historgram. The graph would have more peaks and a rounder curve between peaks given sublte grades of colours, such as in skin tones or fabrics with light coming from a low sun for example.