EXERCISE: Histogram

AIM: You will need to find and shoot the three most basic categories of scene by contrast.  These are low-contrast (this is, flat in appearance, with a low dynamic range), average contrast, and high contrast (with a dynamic range greater than that of the camera).  Then for each of these picture situations, shoot not only an averagely exposed version, but also one that is approximately one f-stop darker and another that is approximately one f-stop brighter.  This makes nine images in all by the time you have finished.

The lower the contrast (the ‘flatter’), the more the values are squeezed together in the histogram, whereas a high-contrast image will be spread right across the histogram, right up to the edges.  Varying the exposure will shift the bulk of the values either left (with under-exposure) or right (with over-exposure).

It has taken me all week, but I think I actually get this now!  To a point where I sat and explained how to read a photograph’s histogram to my husband, and he is an engineer!

So, for this exercise we need to take three different photos, one with low-contrast, one with even-contrast and finally one with high-contrast – sounds easy enough, but in order to satisfy the exercise requirements, we need to make the histogram move from left to right (depending on exposure) now that could be a little more tricky and I am skeptical by the results.

For all of the photographs taken during these exercises, I have chosen to shoot in manual (my preferred shooting mode).  I have kept the aperture as wide as possible and governed my exposure by using the Shutter Speed, increasing and decreasing the setting by 1-stop as necessary.  The images shown below are the RAW, unprocessed files, and the screen shots have been taken from Photoshop.

Low-Contrast

A low contrast image, due to either the subject matter or lighting techniques, has a flatter than average look, meaning that it isn’t too light or that it isn’t too dark, and looks, well average.  Freeman (2008 p.22) states that; ‘The parts of the scene that looked average in tone – neither bright nor dark – would also look average in the image.’

The narrow peak on a histogram confirms that the tonal range within the scene being shot is small, and could resemble the image below.

Low Contrast Histogram

Low Contrast Histogram

Dennis.  (2010) Histogram of Normal Distribution [Online Image].  Available at: <http://dennisivan89.blogspot.kr/2010/07/ap-186-activity-5-enhancement-by.html&gt; [Accessed 8 February 2013].

My low-contrast images have been taken of a flagstone on a pavement.  I stood in the shade to assist in flattening the light within the scene, and I came up with the following results.

Shutter Speed 1/25; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/25; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

This correctly exposed shot is represented by a good example of a low contrast histogram.  The peak is fairly central, and quiet compact showing that the tonal range within the shot is limited.  The height of the peak also shows that the tonal range is within the dynamic range of the camera.

There is really nothing special about this shot, which is the exact look I was hoping to achieve.

Next we have the over exposed shot of the pavement.

Shutter Speed 1/13; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/13; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

Here we can see that the histogram has not changed shape at all, but has moved over to the right of the axis, which is what we would expect to see from an image that was over exposed.

The photo looks quiet washed out and we have lost quiet a large amount of detail in the paving slabs, but not enough to produce highlight clipping.

The final image in this sequence is the under exposed shot.

Shutter Speed 1/50; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/50; Aperture F5.6; Focal Length 300mm; ISO 100; Metering Spot; WB Sunny

Here, as expected, the peak has moved over to the left of the axis, which has been achieved by increasing the shutter speed, which in turn has let less light onto the sensor, thus under exposing the image.

As with the over exposed shot, due to the even tonal range, there is no shadow clipping present.

The even, flat tones within the paving stones have produced really good low-contrast results for this exercise; I could not of hoped for a better representation of the histogram as received here.

Average Contrast

An average-contrast image offers a tonal range that spreads wider across the histogram than that of a low-contrast image, and is usually represented by an evenly toned image that includes similar colours, similar sized objects, or similar sized banks of colour.  A histogram representing an average contrast image, could look like this:

Average Contrast Histogram

Average Contrast Histogram

dun Epic Games.  (2011-2012) Histogram [Online Image].  Available at: <http://udn.epicgames.com/Three/TexturingGuidelines.html&gt; [Accessed 8 February 2013].

The histogram for average contrast images are wider than those of the low contrast histogram, as the tonal range available within the scene is greater, producing more colours and textures, therefore producing more information for the camera to process.

My photos in this sequence have been taken from the 30th floor of my apartment building.  It is around mid-morning; so the sun is nearly at is peak in the winter sky, so the natural light is quiet bright and neutral.

The first image is my ‘perfectly’ exposed shot.

Shutter Speed 1/1000; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/1000; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

There is a vast difference in the low contrast histogram and the histogram produced for the average contrast image, and the first noticeable thing is the increase in the size of the data stream.

Of course, when comparing the two images, this is obvious as there is so much more ‘data’ present in this shot.

From the detail of the histogram, you can see that the tones within this image are within the dynamic range of the camera and that they are spread quiet evenly across the axis.  The sky is a little bright, especially in the left hand corner; hence the slight spike to the right of the histogram, but it is still within a comfortable limit.

The second image is of the over exposed shot, what a difference 1-stop of exposure can make!

Shutter Speed 1/500; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/500; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

I expected to see more movement in this histogram, as it is very similar to the one produced for the well exposed image, and what I really did not expect to see were so many highlights, which have been represented by the areas of red in the photo.

In hindsight, this image was always on the lighter side, but it isn’t until the camera breaks the data down into a histogram that you can see how much lighter some of the areas are.

There is some movement within the histogram, as it has become flatter than before, and some of the channels have squashed up closer to the right of the frame, so the exercise worked and we received the results we expected too.

The final image in the sequence is of the under exposed shot.

Shutter Speed 1/2000; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/2000; Aperture F3.5; Focal Length 28mm; ISO 100; Metering Spot; WB Sunny

Possibly not the best histogram to show under exposure, but you can read that there are parts of the image that are black, there are just not as many of these areas as I expected.

One thing that is very interesting is the movement of the graph away from the right hand side of the frame, indicating that there is no pure white left in the photograph.  This confirms that our eyes will read a scene differently than a camera’s sensor, as when I look at this photo I can still see quiet a lot of white present in the buildings and sky.

Of the three photos in this sequence, this is my favorite as I am always drawn towards darker images that are slightly under exposed.

High contrast

“Contrasty scenes bring the risk of losing detail because their brightness range is likely to be beyond the dynamic range of the camera’s sensor” (Freeman 2008 p.26).

Scene’s that are predominately dark, or have lots of black in them, would be classed as high-contrast, although bright images can be classed as High Contrast also.  The histogram below represents a high contrast image, as the tonal range within the scene has produced a dense block of data across the spectrum.

High Contrast Histogram

High Contrast Histogram

Snell, K.  (2009) High Contrast Histogram [Online Image].  Available at: <http://community.spiritofphotography.com/index.php?topic=361.0&gt; [Accessed 8 February 2013].

Freeman makes comment, that the critical judgment within High Contrast images, is deciding on what tones should be preserved.

My high contrast images have been taken of a statue that sits outside a building here along the back streets of Ulsan.  The images were taken in the shade, to assist me create the right atmosphere, and although they might not meet the exercise requirements exactly, it was the best I could do under the freezing weather conditions we had on the day of my photo shoot.

As with the other three exercises, my first shot is that of the correctly exposed image.

Shutter Speed 1/50; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/50; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

The histograms produced for this shot is very similar to those produced during the average contrast images exercise, but in this sequence the spikes are much more prominent.

The histogram shows that the image is well exposed, with a broad range of tones across the axis, which are within the dynamic range of my camera, although there is quiet a high spike present, which would represent the darker areas to the left of the frame.

Secondly we have the over exposed image.

Shutter Speed 1/25; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/25; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

Surprisingly, in the over exposed image there is a large area of highlights present on the statues right facing arm, which is confirmed in the histogram by the spike in the right of the graph (right = white).

We can also see that much of the dark shadows have been eradicated from the image by the slower shutter speed, as the graph has moved well away from the left of axis.

Finally we have the under exposed image.

Shutter Speed 1/100; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

Shutter Speed 1/100; Aperture F4.5; Focal Length 50mm; ISO 100; Metering Spot; WB Sunny

As with the over exposed image, I am again surprised by the results of this graph, as the histogram does not show any major signs of under exposure, and the dark areas from the correctly exposed image are very similar to the ones we can see here.

Again, this exercise has shown that what we see with our eyes is definitely different to what our camera’s sensor sees, as when I originally looked at the scene for taking these images, everything appeared much darker than the results have shown in the photograph, and the histogram in my camera also showed greater signs of under exposure.

Conclusion:

I have had such an interesting and enjoyable few days learning about and using histograms in my photography.

The thing that has amazed me the most is the differences between what our eyes see, what our camera’s see and how that is determined in our image software.

It is not until you conduct an exercise like this that you can see how much a scene can change just by changing your exposure by 1-stop to either under or over expose your images, and now that I understand how to read histograms – which I had all wrong before – it is defiantly something I will try and make use of in the future, it may not be something I do with every image I take, but I will not be afraid to start using this function more if the necessity arises.

Source:

Reference:

Dennis.  (2010) Histogram of Normal Distribution [Online Image].  Available at: <http://dennisivan89.blogspot.kr/2010/07/ap-186-activity-5-enhancement-by.html&gt; [Accessed 8 February 2013].

dun Epic Games.  (2011-2012) Histogram [Online Image].  Available at: <http://udn.epicgames.com/Three/TexturingGuidelines.html&gt; [Accessed 8 February 2-13].

Freeman, M.  (2008) Mastering Digital Photography.  East Sussex: The Ilex Press Limited.

Snell, K.  (2009) High Contrast Histogram [Online Image].  Available at: <http://community.spiritofphotography.com/index.php?topic=361.0&gt; [Accessed 8 February 2013].

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3 Responses to EXERCISE: Histogram

  1. profstoff says:

    Julie – Have you got your under and over exposed mixed up? If the shutter is open for longer, the sensor gets more light so more exposure, resulting in a brighter image. Under exposed is the opposite, less light and a darker image.
    Chris

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