Exercise: Histogram

Aim
The aim of this exercise was to increase familiarity of histograms. I sometimes check the histogram on the computer when reviewing images, and occasionally do so when shooting, but it's not something I do too regularly - I tend to rely more on how the image 'looks' rather than what the numbers tell me. Nonetheless, it's useful to become familiar with histograms and in particular, to be aware when the data is being clipped - either at the shadow or highlight end of the spectrum.

Software comment
I have been somewhat delayed in finishing this exercise (and also working on the first assignment) because I have built a new computer and installed Lightroom 4 for the first time. I've had it up and running a few days now and it's quite different to DigiKam which I used previously. It will be a bit of a learning curve, but I feel that it's an appropriate time for me to be upgrading to industry standard software. So this is the first exercise I am using Lightroom 4 for.

Low Contrast Scene
For my low contrast scene I have chosen a fairly bland view of the base of a large tree, with grey rocks and dirt  in the foreground. It is not particularly inspired, but fits the brief!

Correctly exposed, Low contrast image with histogram

-1 stop EV, Low contrast image with histogram

+1 stop EV, Low contrast image with histogram

For these three low contrast images, the histogram shows a fairly uniformly triangular shape. For the 'correctly' exposed image, the triangle is roughly centrally located. For the -1 stop EV 'underexposed' image, the triangle is pushed to the left, and for the +1 stop EV 'overexposed' image, the triangle is pushed to the right. None of these images has the 'wrong' exposure though, they are just variations depending on what I want the scene to look like (medium grey for the 'correct' exposure).

Average Contrast Scene
For my average contrast scene I have chosen a view over the Brisbane river with buildings, sky and river.

Correctly exposed, Average contrast image with histogram

-1 stop EV, Average contrast image with histogram

+1 stop EV, Average contrast image with histogram

 For these three average contrast images, the histogram shows a flattish, spread out shape. For the 'correctly' exposed image, the distribution is roughly centrally (slightly left) located. For the -1 stop EV 'underexposed' image, the distribution is pushed to the left, and for the +1 stop EV 'overexposed' image, the distribution is pushed to the right. All three images have some 'shadow warnings', as indicated by the triangle shown top left on the histogram. This displays in blue on the image when the mouse is hovered over the triangle. An example is shown below:

-1 stop EV with shadow warnings highlighted in blue

The shadow warnings are useful for highlighting where an image is being clipped at the low end. This means data is lost effectively, and no amount of post-processing will get any detail back in these parts of an image. I sometimes use the functionality on my camera to check for clipping when I am out shooting so that I can adjust my exposure or lighting if necessary to avoid excessive clipping.

High Contrast Scene
For my high contrast scene I have chosen a view looking up at a large tree, with large areas taken up by branches, and light filtering through the foliage.

Correctly exposed, High contrast image with histogram

- 1 stop EV, High contrast image with histogram

+ 1 stop EV, High contrast image with histogram

For these three high contrast images, the histogram shows a non-uniformly distributed bimodal shape spread across the whole spectrum. For the 'correctly' exposed image, the distribution is mostly located on the low end, with a spread of values all the way to the far high end. For the -1 stop EV 'underexposed' image, the distribution is pushed to the left, but still with some values all the way up to the high end, and for the +1 stop EV 'overexposed' image, the distribution is pushed away from the low end with more values at the highest end. The -1 stop image has some 'shadow warnings', mainly on the fringes of the tree and leaves. The 'correct' and +1 stop images have 'highlight warnings', as indicated by the triangle shown top right on the histogram, mainly in the sky. This displays in red on the image when the mouse is hovered over the triangle. An example is shown below:

+ 1 stop EV with highlight clipping warnings shown in red

The highlight warnings are useful for showing where an image is being clipped at the high end. This means data is lost effectively, and no amount of post-processing will get any detail back in these parts of an image. I sometimes use the functionality on my camera to check for clipping when I am out shooting so that I can adjust my exposure or lighting if necessary to avoid excessive clipping. Highlight clipping can be particularly bad in skies, when a graduated neutral density filter can be used to decrease the dynamic range of the image. 'Control the highlight's', or 'expose for the highlights' are good mantras for digital photography (interestingly, I understand it is the opposite for B&W film photography, where it is better to slightly overexpose, and control for the shadows). 

Summary
This exercise is good for increasing awareness of histograms but more importantly, exposure and how to correctly expose images, which I believe we get to more in the next chapter.