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Part 6 : Introducing the histogram

*Disclaimer: Notes written by Alistair Keddie.


Introducing the DSLR histogram, photography notes



Part 6 : The Historgram

At first sight, the histogram can appear quite intimidating. graph, or series of graphs mean? Its quite simple really as the histogram is showing us a graph of the tonal values captured in our scene. It does not relate pictorially to any particular object in our scene, its not that this particular peak on the graph represents that tree in our picture. Rather, its a measure of dark to light and shows us, on the left, the proportion of dark tones ranging from black, the darkest, going towards the right, through the mid tones and lights in our scene, ending with white, the lightest. We can then see at a glance the spread or tonal range across our scene and get a good idea from this if we need to adjust the exposure. For instance, in an under exposed scene the graph will be shifted towards the left, whilst an over exposed scene will be shifted towards the right. Its important to remember that there is no such thing as a perfect graph and that we are making photographs not histograms. Each histogram is unique to the tonal range of the scene and its up to us to judge how to adjust this. It is though a very useful tool to get to know and in no time at all you’ll be reading it at a glance.


an example of a histogram on your DSLR camera


Some camera’s will show you more than one histogram. The most useful in my opinion is the one for ‘luminosity’, that is, the white one. There are another three. A red, a green and a blue. These reflect the channels of light that our camera records. Like the human eye, the camera captures three wavelengths of light, red, green and blue (RGB). Out of these three we get the full spectrum of colour. The red, green and blue graphs show us the tonal range in those particular wavelengths. Whilst useful to know, particularly later in post processing I do not find this essential while shooting and tend to use only the white luminosity histogram.


Highlight Clipping

Another function we see on the preview screen when we turn on the histogram is a warning indicating highlight clipping. This occurs when the image is over exposed to such an extent that all detail has been lost, in effect, that all the pixels in that area are white. In this situation that area flashes on and off on my screen. If I print out my image, these areas will be solid white. Its also possible for this to happen in the shadows at the other end of the scale where its called shadow clipping where things would print as solid black. My camera doesn’t flash a warning in this respect but the software on my computer does when I’m processing my shots.


An example of highlight clipping on your DSLR camera after histogram selection


Exposing to the Right

Without becoming too technical, there’s a technique to be aware of called exposing to the right. Its a peculiarity of digital sensors that they record more tonal information at the lighter end of the scale than they do in the darks. Because of this, noise will, if its going to appear, begin in the darker tones and shadows. If the image we record is under exposed, that is too dark and we try to brighten it later we will amplify any noise. In effect we are taking our graph where everything is squeezed towards the left (darker) and stretching it up towards the right (lighter) to brighten the image up. Because all we have recorded are the darker tones, these have to fill in for all of the lighter tones we failed to record. This will become clearer when we discuss RAW versus JPEG but is to do with bit depth and works something like this. When shooting in RAW, the 12 bit sensor inside our camera is capable of recording 4096 levels or discreet tones. If our camera has a dynamic range of five stops (imagine the histogram divided into 5 separate areas) then the distribution of these levels are as follows. Within the first (which contains the brightest tones) there are 2048 levels. Within the second (which contains bright tones) there are 1024 levels. Within the third (which contains the mid-tones) there are 512 levels. Within the fourth (which contains the dark tones) there are 256 levels. Within the fifth (which contains the darkest tones) there are 128 levels. What this means in practice is that if we don’t record any image data in the brightest tones, then we are losing fully one half of the levels available to us. Thats a lot of information and leads to a technique where we deliberately over expose our image, that is, expose to the right but not to the extent where the highlights clip. We then adjust the exposure back down again later working on the computer, our digital darkroom where, because we have recorded so much more information our image quality increases.


An example of exposure to the right on your DSLR Camera


Bracketed Exposures

We occasionally come across scenes outwith the dynamic range of the camera. That is, regardless of what we do with the exposure meter, some part of our shot is either under or over exposed. The classic scenario being when shooting a sunset. We expose for the sky and the ground comes out black or expose for the ground and the sky is overblown. One approach to solving this is to use graduated filters. These are darker by a set number of stops at one end and gradually clear at the other. Using these can help to balance out the contrast in the scene and give us an even exposure. There is another technique we can employ if we don’t have these filters. This technique also helps in situations where filters would be no use. For instance, in an interior shot where we want to show the room and the garden outside the window. If we expose for the room, the garden outside is overblown or alternately, the garden is fine but the room is too dark. In these situations we can use a technique called Auto Exposure Bracketing (AEB) and our camera has a function to allow us to do this. Usually found in the menu, when we turn this on we can set three points, sometimes more, on our exposure meter. These can be 1 or 2 full stops or any of the increments between but you’ll notice that they mirror each other. Once set, the next three shots taken will use those exposure settings taking one in the middle, one under and one over capturing a much wider tonal range than a single shot. We then combine these together on the computer. This technique works best when using a tripod. It is however beginning to creep into some consumer compacts as a new auto mode called HDR Scene (High Dynamic Range). To turn off AEB either access the menu or switch off the camera to cancel.


Th screen on a DSLR Camera showing the selection of exposure





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