Exercise: Your camera’s dynamic range

I have been going on about this exercise in various forums and now have to write about it.  I am going to give it my best shot but I am still not sure of the correct interpretation of the notes.

This exercise gives us a practical and empirical way to discover the dynamic range of our own cameras.

In order to do this we need a subject that itself covers the entire range of light from bright white to dark shadow.  It is suggested that we place a piece of white card into the image to get the bright end of the scale but since I was shooting a white building it that was not required.  Having shot the photograph, exposure reading were required from a few areas, in particular the lightest and darkest areas with a few in between.

My image is below:

The image has the exposure readings from my notes appended and it can be seen that from the brightest area, the white columns to the darkest area, a black window in hard shadow, there is a difference of 8 stops.  Whilst the measurements I took may seem a bit random and it might have been easier sticking to one aperture and showing the change in speed it doesn’t invalidate the readings.  I used a simple on-line calculator to input the various exposure settings and get the stop differences (rounded to the nearest whole number).

So from this example it would seem that the dynamic range of my camera is 8 stops… a little low for a high end SLR (Canon 5DII) so I must examine why.  My thoughts are that although the building is a bright white it isn’t pure white and the stone is matt which will reduce its light reflecting quality; this would probably be worth 1 stop of light value.  In addition, there weren’t any truly black areas as even a black window in shadow is going to receive reflected light from the surrounds and the shiny ground and this would be worth 1 more stop of light.  In the dark areas I had no difficult in raising the brightness to examine the detail which makes me think that I wasn’t completely at the limits of the available dynamic range.

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Exercise: Your tollerance for noise

The term ‘noise’ in the way we are using it here originally came from the world of radio… more specifically old fashioned High Frequency radio use.  The transmitted signal would be received but those long wavelengths were also coming from lots of other sources; car engines, refrigerators, overhead power cables, the atmosphere, thunderstorms and even outer space… indeed almost any electrical effect, natural or man made.  If these other sources were strong enough they would swamp the transmission and the strains of the BBC World Service would fade into the hiss.

The effect of image noise is similar but a better analogy is to remember our days of using wet film.  In order to shoot in low light we used fast film that had large crystals of the light sensitive silver halide in the film emulation, big enough to react quickly to the low light levels.  The disadvantage was that these fast films produced grainy, blurred negatives.  When we wind up the ISO setting of our digital cameras we get a similar effect, albeit for a different reason.

When we increase the ISO setting of our cameras we are asking the electronics to amplify the light signals received by the sensor so that we can get a viewable image.  This amplification (increase in sensitivity) has a down side.  The millions of light sensitive pixels on our sensors cannot all be made perfectly.  Some will react  to light a little more than its neighbour and some a little less.  As we amplify the output of the sensor these differences become more apparent and errors appear.  We see this as noise.

An acceptable level of noise will depend on our personal tolerance to imperfections.  As a general rule however, if the level of noise exceeds the level of detail in the image it will probably be unacceptable and visa versa.

This exercise asks us to shoot an identical image throughout the ISO range of our cameras, varying the speed to obtain the same EV (Exposure Value) but going no lower than ½ sec to avoid introducing long exposure noise.  Below are my images with areas of the ceiling, the dark doorway and the bust details magnified.  The images were shot in RAW but haven’t received any additional processing, just a basic conversion to JPG.

1/2 sec, f22, ISO 100

At ISO 100 the ceiling looks smooth, the dark areas unblemished and the detail is clear.

1/5 sec, f20, ISO 200

1/10 sec, f20, ISO 400

By ISO 400 a small amount of noise is appearing in the smooth ceiling area but not enough to affect the natural texture, indeed without the ISO 100 shot to compare it might well have gone unnoticed.  The door and bust details are generally unaffected.

1/20, f20, ISO 800

1/40, f20, ISO 1600

For a lot of cameras, ISO 1600 would be almost unusable but I am fortunate to be using the Canon 5DII and even at this level the noise isn’t too noticeable, until you examine the smooth ceiling which is now quite obviously suffering from a rash of speckles.

1/80, f20, ISO 3200

By ISO 3200 the ceiling is developing it’s own patterns caused by the noise that has appeared.  The frame of the door has also become quite speckled, however the texture and colours mixed in with the bust do a good job at disguising the effect.

1/160, f20, ISO 6400

ISO 6400 is the practical maximum of my camera.  It does have specialist levels of H1  (ISO 12800) and H2 (ISO 25600) but these should be used in conjunction with the High ISO speed noise reduction C.Fn settings which would skew the findings of this exercise.  At ISO 6400 noise is apparent in most areas of the image and it gives the entire image a mottled, rough textured finish.  The overall image is still quite acceptable at a reduced size but close up it definitely suffers.

High ISO settings will always degrade an image to one degree or another, particularly in areas of smooth texture that will become artificially grainy.  However, if the situation demands a speed or aperture setting that forces the use of high settings then it is the lesser of the evils.  In some situations, the grainy quality introduced by a high ISO can help to create a period feel to an image but usually when combined with a form of monochrome to complete the illusion.

The images above had only the most basic RAW to JPG conversion but with a little effort even a high ISO image can be corrected to become quite acceptable.  Below are two magnified images taken from the set above, one is from the ISO 100 photo and one from the ISO 6400 one.

The advantage of using good RAW processing software can’t be overemphasised when dealing with high ISO images.  If you haven’t quite worked it out yet… the top one is the ISO 6400 one.

Exercise: Highlight clipping

This is a scene of high contrast and I have photographed it from the point just below which the camera indicates hightlight clipping.  From this light level I then replicated the shot through +1 to -3 EV giving me five shots.  Detail from each shot, an area of colour and an area of near white is shown below for comparison.

+1 EV

EV 0 (exposure just below the white clipping point)

EV -1

EV -2

EV -3

Looking at the magnified areas above we can note the following.

At +1 EV the white areas of stone are completely lacking in detail such that different areas of stone blend together as if they were one.  Except where shadow indicates breaks, it is impossible to tell the edges of the stone.  Clicking on the +1 EV image to expand it, lines of green fringing can be seen around the block (top left corner) and red fringing on the shadow it creates.  The colour sections shows a little desaturation as it is washed out by the overexposure.

Examining the other images the amount of detail that becomes obvious on the white increases as photos are stopped down but by the time they get to EV-3 things are getting so dark that the detail is becoming lost in the gloom. The coloured fringing is generally present throughout but by -3 EV the green has almost disappeared however the red is still present.  The colour saturation increases until the lack of exposure at -2EV and beyond makes it too dark to be obvious.

As is suggested, on this blowup I used the Recovery option in Lightroom to regain some of the detail from the +1 EV image.  Certainly, this option has done a lot to recover the information from the clipped areas; the edges are more obvious and much more texture is visible.  There were no discernible strange effects from putting the slider to maximum.

Finally, I used all of the available options in Lightroom to adjust this +1 EV image in an attempt to recover the maximum amount of information from the RAW photo.

Exercise: Sensor linear capture

One of the most appealing aspects of wet film is it’s natural response to light, in that it tends to mimic the capability of our eyes.  Not so the digital sensor.  Our eyes have the ability to compress the extremes of light and dark into a larger dynamic range than is available to the digital sensor.  This allows our eyes to cope with a wide range of light levels but to achieve the same with a digital camera we have to resort to electronic manipulation.

In the great majority of cases, our cameras process the raw digital interpretation of a scene before we are presented with it.  This ensures that the result is much more to our liking than the basic data would have appeared.  Of course it is possible to process this raw data ourselves using suitable software in such programmes such as Photoshop or Lightroom.

To examine how the raw data would be presented without processing this exercise asks us to open a picture and adjust it’s curves to simulate the raw data.

This is the original picture with it’s histogram attached.

With the curves of this picture adjusted like so…

the picture looks more like it would have done without any processing.

With the two images together I now return the dark one (left) back to resemble the normal one (right) by moving the curve up and left.

The result of this adjustment is to lift the dark areas of the image but the side effect is the noise that was previously hidden in the shadows had appeared.  Below is a comparison with the original image.

The noise is quite evident when viewed above with a section of the original image.

For all practical purposes, the processing done by the camera to create a good looking image is beyond the understanding of most photographers.  Even when converting a RAW image file to save as a TIF or JPG, the photographer is mainly tweaking a pre set series of values that have been chosen by the software designer.  However, having access to the RAW file allows us much greater artistic freedom.  Though we must endeavour to remember the limitations of the digital sensor, particularly when eyeing up a scene of high contrast and large dynamic range.  What our eye sees is not necessarily what the camera will be able to record!

Addendum:  The course notes relating to this exercise would seem to be in error.  They state that ‘for the linear image the histogram shows the tones to be squashed strongly to the right’, whereas in fact they are moved to the left as can be seen above.  In addition, the notes add that the linear image ‘has most of the levels available to represent the tones devoted to the brightest parts of the image’ whilst it can clearly be seen that the linear image has more dark tones.

Exercise: Editing… desigual

In New York recently and since Christmas is approaching I was out and about looking for some night images at 3am.  What I actually found was the Desigual store.

As soon as I saw it I knew I wanted to shoot it… it stood in the shadows with glimpses of the splashed paintwork decorating the sides.  What really attracted me were the windows which had been turned into lightboxes and glowed different colours in a psychedelic display.  I started by shooting from different angels, timing my photos to capture the changes in colour.  When I had tried a couple of different angles I started to try to include the chimney that was erupting steam from a manhole in the street.  A lot of buildings in New York are heated by a network of steam pipes that are often allowed to vent via temporary chimneys… they are classic NY!

I took 57 images of Desigual which I downloaded into Lightroom and put into a folder with a date time group.  This set was taken whilst I was shooting my Histogram exercise so appears as a sub folder of that folder.

The first thing I need to do is put all 57 into a Collection so that they are separate from the other sets that I shot on that evening.  I called the collection desigual and once created it sorted the images out of the original folder of 272 pictures.

Once I separated the desigual collection from the rest I added keywords.   This is not an essential step in the editing process for this particular exercise but will allow me to search for these images more easily in the future.

My technical edit comes next and is my opportunity to get rid of those photos with errors such as poor focus, camera shake, poor exposure etc.  I set my computer up using both screens (one beside the other) to view a close-up (loop view) on the left screen with the film strip below which allows me to move from one photo to the next and the overall image on the right screen.  Photographs that get cut at this stage are deleted from my hard drive however if I make a mistake I have a complete backup of all my images on a second hard drive.  I had been using a tripod for this shoot and with good exposures I only found 5 shots that needed deleting.

My system of editing now needs me to rate the photos so that I can start concentrating on the best.  This is before any adjustments so I rate them with an eye on how they might look after some alterations.  I use the star system and give between 1 and 5 stars.  Out of this shoot I ended up with 8 *, 13 **, 14 ***, 16 **** and 0 *****.  I was not surprised to get this mix. I often shoot with an eye for an HDR composite so some shots are deliberately over or under exposed and without the final adjustments it is rare for me to give a shot a 5 star rating.  I will review the star allocation after I do some developing in Lightroom and raise or lower as required.

My next task is to adjust basic settings like exposure, white balance, clarity, vibrance, curves and lens corrections.  As I finish these changes (some of which I copy and paste from one shot to another to save time) I change the star rating as required.  At the completion of this stage I then filter the photos again for ≥ ****.  This gives me only 21 images out of an original 52.  Amongst these 21 shots there are about 5 view points so now I want to select the best of these viewpoints.  Although could do this using the split screen view it is fairly limiting in size so I lock the main shot into my second screen and then select the comparison shots in my first screen.  When I pick the best of a group I up the score to *****.

My ***** pictures number 6.  Out of them I must pick just one.

The final choice came from 3 aspects.  I was able to include a great swirling steam cloud into the shot… something that wasn’t present in some aspects.  The colours are great and the proportions of the shot good.  So this is my final choice:

Exercise: Histogram

Ever since I first saw this little graph on my first digital camera it has fascinated me.  I have always used it as an indication of the level of clipping that has occurred but I am sure it has more uses than that so this chapter should be interesting.

Firstly… what is the histogram?

Basically it is a representation, in graph form, of the light value that each pixel on the sensor sees.  Here I have put on my kitchen table the inside cover of our course note folder which is grey with a bit of black mounting board on one side and some white paper on the other.  Below it I put the histogram that this picture produces in CS4.


On the histogram the horizontal axis represents the level of brightness, black on the left and white on the right.  The vertical axis represents the number of pixels.  The histogram shown here is common to most that I have seen in that it restricts the top of the graph so with a lot of a single brightness level the spike will be cut short by the limitations of the graph display.  So, whereas it might be thought that there is a lot of grey and white but only a little black we know from the photo that this is not true.  What this histogram is showing us is that the black is very pure black so the spike is narrow whereas the grey and white are spread over several shades which is why the spikes are broader. Were we able to see the top of the graph we would find that the black spike is taller than either the grey or white since the pixels seeing those shades are spread over a wider area.

That aside, we can tell that both the black and the white are within the dynamic range of the camera since the spikes fall within the edges of the graph.  Should either spike go beyond the sides of the graph we would know that some clipping has occurred, in that the camera hasn’t been able to record light levels that fall outside the graph.  Those areas where this occurs will look plain white or black.

The exercise asks us to shoot the three most basic categories of scene by contrast showing the histogram and an example of the changes that occur with +1 and -1 stop of Ev.

Low Contrast Ev0:

This scene is dominated by the plastic ball the girl is looking through.  Apart from the hole the plastic has given the rest of the image a very similar light value so the histogram has a definite peak which shows this.  The darker area through the hole gives rise to the smaller bump to the left of the histogram.

 

 

Ev+1:

At +1 stop the histogram has moved to the right (the bright side) and now falls off the side of the graph showing that there is some clipping of the dynamic range in the brightest areas of the picture.  The small bump representing the dark area has flattened a little as the change in light has brought more detail out of that area and it now covers a greater dynamic range.

Ev -1:

At -1 stop the main hump of the histogram has predictably moved left and the smaller hump has grown a little and shows some clipping at the darker end of the dynamic range.

 

 

 

 

Moderate Contrast, Ev0:

This shot of the Empire State Building reflected in an office block has moderate contrast.  This is seen by a fairly flat histogram that extends the length of the graph and only spills off the ends by a small amount.  The vast majority of the photograph falls within the dynamic range of the camera and is covered by the histogram.

 

 

 

Ev+1:

At +1 stop the graph has moved to the right and now we can see that there is a peak on the right edge of the histogram which indicates clipping of the bright areas of the image.

 

 

 

 

Ev-1:

At -1 stop the graph has moved left and the clipping of the bright areas has been cured but now we have the same problem at the dark end of the dynamic range.  This is a clear graphical demonstration of the need for accurate exposure for subjects which cover a large dynamic range.

 

 

 

 

High Contrast Ev0:

For High Contrast shots the histogram forms a U shape with peaks at either end of the scale showing that there are a lot of extreme darks and lights within the shot.  I have expanded the size of the graph for this set for clarity.  So the dark hair and T shirt of this hi-key portrait are all in the very low end of the range whilst the hi-key background, being bright white, is at the very high end of the range.

 

 

 

 

 

Ev +1:

At +1 stop the histogram has moved right as we expected and from the graph it looks almost as if there is no bright whites at all!  We know that this is untrue just by glancing at the photo.  It looks this way because most of the white has been pushed off the end of the scale.  However, when I took the screen shot I rested my cursor on the right edge of the histogram to get a count of the pixels at position 255 (the far right).  This indicates that there are 17,975,601 pixels at this level, an enormous number and easy to miss because the peak is so narrow and right at the edge of the graph.

 

 

Ev-1:

When we look at the -1 stop image it is obvious from the photo that the bright white background is showing a bit of fall off in the bottom right corner.  This is backed up by the number of spikes that are showing at the right end of the histogram.  The count at position 255 is still very high but 7 million less than +1 stop image.  As we expected the graph has moved left and the mid tones have taken up a stronger and darker presence.

 

 

 

 

The histogram is a valuable tool when shooting in difficult light conditions but it has limitations.  The most serious one is the lack of impact that the graph shows for a highly over (or under) exposed image as it only appears as a thin line at the far end of the scale.  As can be seen in the Ev+1 high contrast portrait above, the vast amount of over exposure is not obvious from the histogram.  A very useful tool to back up the histogram on the camera back is the highlight alert.  This setting causes the areas of highlight clipping to stand out by being outlined or in the case of my camera by flashing.  Now the photographer has a visual clue to back up the histogram as to where and how much of the image is beyond the dynamic range of the sensor.

Exercise: Your own workflow 2

This is a similar exercise to the previous one but differs in that it is not a time constrained shoot, will consist of an unpredictable number of images and that I should produce a workflow specifically for this shoot.

Whereas I used my last trip to shoot a portrait session in Pasadena, LA, this trip takes me to Johannesburg in South Africa so I thought a day at the Pilanesberg National Park would give me an opportunity to shoot some wildlife.  My workflow for this shoot was as follows:

  • Check gear.  This was to ensure that I had the right lenses, in particular my 100-400mm lens.  I charged my batteries, cleaned the lenses, zeroed the camera settings and wiped my cards of old photos.  I checked that I had both my 8 Gb cards to ensure enough space to shoot without compromise.  Knowing that I would probably be shooting at long focal lengths I packed a travel tripod.
  • I joined a couple of friends interested in photography and booked a day drive around the Pilanesberg National Park.
  • I shot some 12 Gb of RAW images on 2 cards with only 1 set of batteries.  Because of the space limitations inside our vehicle I used my tripod as a mono-pod to stabilise my big lens.
  • During the shoot I deleted poor shots as I went but mainly reviewed the last few images whilst we were on the road to the next location.  I only deleted gross errors realising that some elements of an apparently poor image might still be recovered at the post processing stage.
  • Once back home I created a folder for the shoot and downloaded all the images through Lightroom.  The shoot totalled 464 images.  During the download I added key words to the images for future searches.
  • The aim of my first quick review was to delete images that were obviously not worth keeping… this reduces the count to 356 images.
  • I then created a Collection for the shoot named DPP Ex2 and made it the Target Collection to aid selection.  I ran through all 356 images and put 80 into the Collection.
  • Using the 80 shots in the Collection I gave Flags to the 40 that I was going to post process.  I was mainly aiming to adjust the light levels, clarity, saturation, sharpness, luminance and Profiled Lens Corrections where this helped the image.
  • After adjusting the photographs in Lightroom developer I gave the images a final Star rating to help me choose the best for my blog. For the final selection I colour coded them Red and then used the filter to present my final 18 which I am posting in a gallery below.

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So… how well did I anticipate my needs in the various stages?  I had all the equipment that I wanted and everything worked as advertised so I definitely ticked that box.  My downloading and processing work was considerably quicker than on Ex 1 but that was mainly because I could do the all the post processing within Lightroom rather than using the 3 different programmes that were required for my portrait shoot.

I can’t say that this examination of my workflow has changed my habits much.  I have always been fairly good about checking and packing what I need for a shoot so that hasn’t changed much.  My download and post processing workflow has become a little slicker but that is as much because I am getting used to new software (Lightroom is a new addition) as it is to thinking through the process.