Saturday, March 28, 2009

Metering Modes

In order to produce an image that is not too bright (overexposed) or too dark (underexposed), all digital cameras need a way to access accurately how much light is present in the scene that is being photographed. Now cameras have built-in light meters that measure the light reflected from the subject or from the scene as a whole. The problem for the photographer is that the brightness can vary significantly from one part of the image to the next. For example, the sky is far brighter than the land during sunrise or sunset. If you were to set the exposure manually, the area you should set the exposure for depends on the result you want; normally you would set the exposure for somewhere between the two extemes. The way a camera arrives at this compromise depends on the type of metering system it uses.
How the camera measures light:

The exact method that different cameras use to measure exposure varies from model to model - but essentially there are three types of metering system commonly found on modern cameras. Three modes present in Sony Cybershot DSC-H2 are

1. Centre - weighted metering:

In this mode, the metering is biased or weighted to the centre of the view-finder (three boxes shown in the picture) and then averaged for the entire scene. This mode is very useful when taking portrait shots.

2. Spot Metering:

Spot metering in most cameras takes a reading from the very centre spot (black dot shown in picture) of the viewfinder (In Sony Cybershot it has flexibility to move this spot) and sets an accurate exposure for that part of the image only. This metering mode should be used when the subject is correctly exposed, no matter how much (or how little) light is falling on the surrounding areas, or during close-up photography.
In the above picture spot metering was used and a part of this picture was overexposed.

Both these pictures were shot at same time and I used spot metering mode for both but position of the spot was different. Spot was on the sky for the picture on left and it was on the bird for the picture on right.

3. Multi / Matrix metering:

This is most sophisticated of all the metering systems, and is the 'default' mode for the most cameras that offer metering option. It works by taking multiple readings from from various points in the scene, while also assessing the position of the main subject, foreground and back-ground, overall brightness, any front - or backlighting, colour and so on. The camera then compares these readings with a database of thousands of 'typical' photographic scenes and selects the exposure that most closely resembles one in its database.

My experience on Metering:
  • Pick the "Center-Weighted Metering" mode when you want the camera to meter the light in a broader area than with spot metering but not the entire frame. With center-weighted metering, you can focus on a larger portion of your frame. This is a good option when you’re taking group photos or portraits.
  • Select the "Spot Metering" mode when you have one small subject from which you want to record the light value. Spot metering is handy when your subject is in front of a bright background, such as a sunset. By choosing to spot meter the light from the subject only, your camera will adjust the settings, making your subject visible as opposed to creating a silhouette.
  • Choose the "Multi Metering" mode (multi-segment metering) for a good all-around photo. In most digital cameras, this is the default meter setting and it measures the light reflected in all parts of the frame and uses the combination to select the light value. For most purposes, matrix metering is sufficient.

Next I will discuss about Histogram