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Using The Histogram

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Exposure Control With Digital Cameras

How to ensure you always get the correct exposure using the histogram display on your digital camera.

Digital cameras have made photography easier in many ways. With film, we used to rely on light meters, bracket our exposures and even fiddle with development times to ensure we got the correct exposure. With digital, we can approach exposure in exactly the same way, if we wish, and get excellent results. However, to do so ignores one fundamental difference.

Within a second of taking your photograph, you can look at it on the camera's preview screen, along with a histogram that tells you exactly what your exposure is like.

Using the histogram could make every other form of exposure calculation obsolete, except that there are times when you only get one chance to take a photograph. If it's a grab shot or a decisive moment, the exposure has to be correct first time and so your knowledge of exposure techniques is fundamental.

On the other hand, if you have even a few seconds before taking the photo to make a few test exposures, the histogram can ensure you get it right.
A camera with a good exposure system is still invaluable, but by looking at your histogram you can double check what you have done.


What Is A Histogram

A histogram is a graph of all the exposure values in a scene. If you photographed a perfectly even red wall (or orange or grey), the graph would show a single line extending to the top.


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The photograph at the top of this article has a wide range of exposure values or tones, from pure whites in the icebergs to pure blacks in the rocks and foreground.


The histogram reflects this by having values on the very left (Value 0 or black) and the very right (Value 255 or white). It also shows all the values in between.


The fact that the histogram is higher on the left indicatess that there are more dark tones than light tones in the photo, but this isn't really very important. What you need to note is that the histogram sits reasonably comfortably within the graph's edges.

If you have a photograph with a histogram like this, then you can be sure you have a ‘correct' exposure.

Why Can't I Use The Preview Image?

All digital SLR cameras and most up-market compact digital cameras offer a histogram display. Sometimes it is laid over the top of the photography, sometimes underneath. Some histograms show an average of the red, blue and green channels, others show three histograms.

For the purposes of this article, we're just going to talk about the average histogram which ranges in exposure values from 0 (black) to 255 (white). All you need to do is refer to your camera manual so you know how to turn it on.

Some photographers simply look at the image on their camera's preview monitor and assume that if it looks right there, it will be okay. This isn't a bad technique because a lot of the time it will be correct. However, there are many problems to deal with.

For instance, do you find it easy to look at your preview monitor in bright sunlight? How does it compare when you're looking at the monitor inside or at night? The same exposure can look a lot brighter or dimmer depending on the ambient lighting. In comparison, the histogram gives you exposure information in a way that can't be misunderstood.

Some cameras feature exposure warnings where areas of the photo that are over- or underexposed are highlighted (perhaps as a red overlay) on the monitor. This is a great feature because it is showing your visually what the histogram would tell you.

How A Histogram Can Help

Histograms can help you correct your exposures in a very simple way. Let's see  how.

Dark, underexposed photographs have histograms which are jammed up against the left side of the graph.

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Images which are light and overexposed, have histograms jammed up against the right side of the graph.

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If you've taken a photograph, looked at the histogram on your camera's preview monitor and seen the histogram jammed up one end of the graphor the other, chances are your exposure is incorrect.

If the histogram is jammed up to the left, you need to add exposure. You can do this manually or by dialing in exposure compensation. Try one stop (+1 EV), taken another photo and look at the histogram. Eventually the histogram should move to the right.


If you have a histogram jammed up to the right, you have an overexposure problem. You need to reduce exposure so the histogram moves to the left.

Once you have the histogram in the right position, everything should fall into place (in terms of exposure, that is!)

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The theory sounds simple and it is. However, there will be many times when the histogram is jammed up against both sides of the graph. You're in a no-win situation because if you increase the exposure to lighten the shadows, you will blow out the highlights even more. The same happens in the opposite direction if you reduce exposure.


The solution to this type of situation is to make a value judgment - what are the most important exposure values in the scene? If you're photographing a person, then their face is usualy the most important part of the scene and you should ensure it is correctly exposed, even if the background is overexposed.


Technical Stuff

The histogram represents an exposure range of five stops (EV), which is a measurement of exposure values. If your camera is set to save JPEG files, then it will take the exposure information and record it with those five stops of exposure. Anything that is lighter than this range is recorded as white; anything darker is recorded as black. This can be a problem later on if you wish to lighten or darken these areas because there won't be any detail in these areas to work with.


The solution for many photographers is to switch their camera to save in RAW format. The RAW format can save from seven to nine stops of exposure information, even though the histogram is still only showing five stops.

By shooting in RAW, you can process your file twice, once for the highlights and a second time for the shadows. The two images can then be merged together in Photoshop or Photoshop Elements to end up with an image that has detail over a wider exposure range.

However, even if you choose to shoot in JPEG, using the approach outlined here to produce an optimum exposure will work in the majority of situations - and if it doesn't, well at least now you will understand why!

Peter Eastway Uses

Peter Uses
AIPP

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