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If you are new to the DSLR or other high tech camera, you may have come across the topic of light metering modes. While the general camera documentation does an adequate job of describing what each of the different settings can do (you DID read the manual, right?), it is often hard to understand what exactly the differences each mode offers and under what circumstances you would choose one over the other. Today I am going to introduce you to the three basic metering modes and show you why they differ:
To run this experiment, I set my Nikon D300 to Single area Autofocus so that I could ensure that my focus point remained constant (and to better demonstrate the difference between spot metering and the other modes). In the images that follow the focus element used is highlighted in red on the back of Piggy. This is the brightest area of the image with strong specular light reflecting off of the ceramic material. Then I changed the light metering mode between matrix, centre weighted and spot metering to take the example photos.
This is usually the default metering offered on most modern cameras. The camera evaluates the entire scene in the viewfinder and uses its high-tech computer chip to analyze the light, setting the exposure according to what it evaluates as the average, or middle grey value of the lighting histogram. In a high contrast scene, like that with Piggy, here, the camera makes exposure compromises that makes sure the middle exposure is covered, but leaves parts of the image overexposed and underexposed. This metering mode gives the best average exposure estimation for all of the scene.
As you can see from the above image, the Matrix metering has tried to balance the overall exposure to the extent that the deep shadows are somewhat preserved but at the expense of the highlights (shown in red) which are blown out.
In this mode the metering is weighted towards the value around an 8mm circle in the centre of the frame and all other areas of the scene, while considered, are given less value. The centre weighted mode is more predictable as far as how the scene will be metered, allowing the photographer to employ more control over the exposure of the image.
The downside to this type of metering, over the Matrix, is that the photographer must be aware of the lighting in the scene. He must also be aware that if the centre of the frame is largely dark, then the exposure will be increased according to the camera mode selected (i.e., if using Aperture priority, the shutter speed will be decreased). Because Piggy, the brightest object in the image, is near the centre, the brightest parts of the image are considered in making the exposure calculation. Had I wanted to bring out more of the dark areas I would have needed to make an exposure setting centred mostly on the dark part of the image, and lock in that exposure setting using the camera’s AE lock button.
As you can see above, since the centre of the image contains a large portion of both Piggy and the shadow behind him, the exposure is averaged around the lighting in that part of the scene. The darkest parts of the image, seen in blue in the back left, have not contributed much in the exposure calculation and remain under exposed. This image better preserved the specular highlights on the side of Piggy, and more of the shadows in darkness. You can see that the histogram is more clipped on the left hand side than in the image of the Matrix metering.
This is the fine tuned metering of the camera. In this mode, only the pixels around the focus point are considered for metering the exposure. As a result, the middle grey is determined by what you meter on. If you meter the highlight, as has been done in this image of Piggy, the entire scene will be underexposed but no highlights will be clipped because the brightest element in the scene (the highlight) was metered as being middle grey (similarly to what happens when one photographs snow).
As you can see, the highlighted area around the focus point has been perfectly exposed, but the balance of the image is grossly under exposed. It is critical that the photographer be aware of what this metering mode is measuring and ensure that the exposure is being taken on a part of the image that it is intended. Again, judicious use of the auto exposure (AE) lock can make all the difference.
No mode is universally superior to any other one, but your particular photography style may lead you to favour one over the others. Generally, for scenes containing even lighting, all modes will give great results most of the time. Different lighting calls for different compromises, especially when dealing with broad contrast ranges, such as the highlights and deep shadows in the picture of Piggy. Also your artistic intention for the image may require that exposure be made only on one part of the scene (backlighting is one example that comes to mind). I will be posting additional articles discussing metering modes in the future using other specific examples.
The best way is to practise. Take your camera out and experiment with different metering modes in different lighting situations. That is the best way to learn.
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