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Histograms are available on the vast majority of cameras and they are a simple way of assessing two important things. If you never use them, or you're not quite sure what they're supposed to show, read on to get up to speed – and scroll down to the bottom for your handy cheat sheet.
How to understand a camera's histogram
A camera's histogram is a graphical representation of the tonal range in your image. In other words, it shows you how much of your scene will record as a shadow, how much as a highlight and how much in between.
The left-hand side corresponds with shadow detail and the right-hand side with highlights, and you can view this either as you compose your image or afterwards when playing back your image (or both). Usually this histogram will come up when you press your camera's 'Disp', 'Info' or directional menu-pad buttons, but this may not be the case on your particular model, so check your manual if you're unsure of how to access this.
Examining this will give you a better idea of two things. The first is the overall exposure. As you adjust your camera's aperture, shutter speed and ISO, you should see the information within this graph move. As you increase the exposure – such as by using a longer shutter speed or a wider aperture – you should see this move towards the right, away from the left-hand side of the scale. This is because more of the scene will record as a midtone and highlight, and less as a shadow. Likewise, decreasing the exposure will move this tonal spread away from the highlights and down towards the shadows.
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With an average scene, one that naturally contains a varied mixture of highlights, shadows and midtones, you'll be aiming to keep this spread so that it fits somewhere in the middle of the scale, and this should give you a good exposure. Not every scene is the same, however, which is where the second important thing histograms tell you comes in: how much detail is being recorded and how much is lost.
Once the graph starts to show lots of tonal information bunched up to either side, to the point where it's falling off the edge and reaching the top of scale, details in this region are not being properly recorded. Too much of this on the left-hand side means that it's shadow detail you're losing, and too much on the right will indicate highlights are blowing their details. So, the key is to ensure your chosen exposure settings don't allow this to happen.
It's important to remember that different scenes will have different histograms, and there's nothing wrong with there being more details on one side than the other. Indeed, if you always try to adjust your exposure to keep everything as much in the centre as possible, you'll find that some scenes just don't look right.
For example, if you're out shooting a scene in the snow, you'll probably want more information to be on the right-hand-side of the scale, as your scene will be filled with more highlight detail than usual. If you try to squeeze all of this down to the middle of the scale for balance, you'll find that those whites are being recorded more as greys.
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Histograms are particularly useful as they are not affected by the brightness of your camera's LCD screen or electronic viewfinder (if your camera has one). Sometimes, raising the brightness of one of these might be necessary in order to maintain a clear view, perhaps when photographing outdoors when it's sunny, but this can make it more difficult to see whether you are giving the scene the right exposure.
What they are affected by, however, are other capture settings you may have chosen, such as contrast or dynamic range optimisation settings. If you set a high-contrast capture setting, you'll find that there's more information being recorded in both the shadows and highlights at the expense of mid-tone detail, than if you were to use a more standard contrast setting. Conversely, dynamic range optimisation settings may bring highlight and shadow details more towards the middle of the graph to prevent these from being lost.
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