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Metering mode

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In photography, the metering mode refers to the way in which a camera determines the correct exposure.

Examples of metering modes

Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes.

Spot metering

An image taken with spot metering. Notice how the flame is correctly exposed, while the background has become completely black.

With spot metering, the camera will only measure a very small area of the scene (1% to 3.5% for 35mm cameras and about 0.7% to 2.3% for Half Frame D-Slrs). This will typically be the very centre of the scene, but some cameras allow the user to select a different off-center spot, or to recompose by moving the camera after metering. A few models (including the Olympus OM-4 and Canon T90) support a Multi-Spot mode which allows multiple spot meter readings to be taken of a scene that are averaged. Both of those cameras and others also support metering of highlight and shadow areas.

The foundation of spot metering is for the photographer to determine the normal exposure of a simple (single-toned) subject. For portrait photography it could be the forehead of the model. Once the normal exposure is determined, it will ALWAYS create an 18% gray image tone for film and a Gray Scale density of 128 in Digital Photography where 128 falls in the middle of a 000 to 255 (000 is pure black and 255 is pure white). This normal exposure must be interpreted by the skilled photographer before the picture is taken. In the above example, if the model is Caucasian, then the aperture has to be opened-up by one stop to create a correct light gray image tone of 36% (grayscale density of 178) for the flesh tone before the picture is taken. Standard subject tones separated by one stop (double or half) built around the 18% gray image tone include 4.5% black, 9% dark gray, 18% medium gray, 36% light gray and 72% white. To convert Normal exposure to correct exposure, the photographer can use the following guidelines: For white surfaces the photographer opens-up by 2 stops. For light gray surfaces the photographer opens-up by 1 stop. For medium gray surfaces the photographer shoots as is. For dark gray surfaces the photographer closes-down by 1 stop. For black surfaces the photographer closes down by 2 stops. Spot metering is the foundation of the zone system of light measurement for distant subjects or when the photographer is NOT under the same light as the subject.

File:Grays-5.jpg

The corresponding grayscale density for digital photography is 028 for 4.5% black (approx), 078 for 9% dark gray, 128 for 18% medium gray, 178 for 36% light gray, and 228 for 72% white. To find tones in a photo editing programs such as Adobe Photoshop or photo elements, simply desaturate the image (remove color), then use the eye-dropper to pick the tone (5x5 average) then click the foreground. The number such as 128Red, 128Green, and 128Blue will appear on the color picker screen.

Illustrating an 18% gray image tone with a grayscale density of 128 in Photoshop.

Center-weighted average metering

In this system, the meter concentrates between 60 to 80 percent of the sensitivity towards the central part of the viewfinder. The balance is then "feathered" out towards the edges. Some cameras will allow the user to adjust the weight/balance of the central portion to the peripheral one. One advantage of this method is that it is less influenced by small areas that vary greatly in brightness at the edges of the viewfinder; as many subjects are in the central part of the frame, consistent results can be obtained.

Average metering

In this metering mode the camera will use the light information coming from the entire scene and and averages for the final exposure setting, giving no weighting to any particular portion of the metered area.

Partial area metering

This mode meters a larger area than spot metering (around 10-15% of the entire frame), and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly. Like spot metering, some cameras can use variable points to take readings from, (in general autofocus points), or have a fixed point in the centre of the viewfinder. Partial metering is found mostly on Canon cameras.

Multi-zone metering

This mode is also called matrix , evaluative , honeycomb , segment metering, or esp — (electro selective pattern) metering on some cameras. This metering mode was first introduced by the Nikon FA, where it was called Automatic Multi-Pattern metering. On a number of cameras this the default/standard metering setting. Here the camera measures the light intensity in several points in the scene, and then combines the results to find the settings for the best exposure. How they are combined/calculated deviates from camera to camera. The actual number of zones used varies wildly, from several to over a thousand. However performance should not be concluded on the number of zones alone, or the layout.

Many manufacturers are less than open about the exact calculations used to determine the exposure. A number of factors are taken into consideration, these include: Autofocus (AF) point, distance to subject, areas in focus or out of focus, colours/hues of the scene, and backlighting. Multi-zone tends to bias its exposure towards the autofocus point being used (whilst taking into account other areas of the frame too), thus ensuring that the point of interest has been exposed for properly (is it also designed to avoid the need to use exposure compensation in most situations). A database of many thousands of exposures is pre-stored in the camera, and the processor can use a selective pattern to determine what is being photographed.

Some cameras allow the user to link or unlink the autofocus and metering, and allow the option of locking exposure once AF confirmation is achieved, AEL, (auto exposure lock). Using manual focus, and on many compacts/bridge cameras, the AF point is not used as part of the exposure calculation, in such instances it is common for the metering to default to a central point in the viewfinder, using a pattern based off of that area. There is considerable variation from different manufacturers as to how multi-zone metering is implemented, and even from the same maker in their model range, and how much "priority" is given to the AF point itself.

However, some photographers, including some advanced photographers and some older photographers who are accustomed to more traditional metering methods, may be uncomfortable with multi-zone metering, this tends to stem from a lack of clarity about "how" the camera reacts in certain situations. In general the most advanced metering is found on single-lens reflex cameras.

Some users have problems with wide angle shots in high contrast, due to the large area which can vary greatly in brightness, it is important to understand that even in this situation, the focus point can be critical to the overall exposure.

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