After posting my assignment pictures on Flickr and adding to the OCA group on there, I got some feedback regarding the exposure of a couple of my images.  Essentially they were underexposed and it was suggested I review the Histograms for the images to recognise this and to allow me to make adjustments as appropriate.  Having never looked at Histograms I had no idea how to interpret what I was seeing.

I found the following tutorial which has helped me understand this:

I’ve summarised what I’ve learnt from this below:

Histograms essentially show you how many pixels of an image are at different tonal points (tone is the intensity of colour/light within an image).  On my camera these appear as an RGB Histogram and a separate Exposure Histogram.  The scale runs from 0 to 255 on the tonal axis.  0 is ‘little’ 255 ‘lots’.  The higher the graph, the more pixel of the image are within that tonal range.  So, on an Exposure Histogram a peak towards the left of the graph (nearer to 0) would mean there are a large number of dark pixels.  A peak towards the right of the graph (nearer to 255) would mean there are a large number of white pixels.

A Histogram’s appearance will depend very much on what you are shooting – shooting something with a lot of contrast may well show peaks towards either end of the graph.  Shooting a more balanced scene with less contrast would tend to show a more even distribution, with a peak more towards the mid-point of the graph for the mid-tones.

A quick and dirty example of the Exposure Histogram on the back of my camera is shown below:

There are a lot of dark tones here – from the black on the TV, speakers and media stand (and various black boxes) – so the Histogram is reflecting this with a peak toward the left-hand-side.  There are some mid-tones from the browns and other colours – hence the smaller peak in the middle of the graph.  Finally, the cream paint for the background is shown by a series of peaks toward the right-hand-side of the graph.  I think in this example the image is slightly under-exposed, with detail lacking in the black regions due to the lack of contrast provided by under-exposing the image.

I’m going to leave the Histogram on in my camera for a while so I get used to interpreting it to ensure pictures are properly exposed.