TTL Metering and Exposure

TTL means "through the lens". This is the method of metering the light that comes to the camera's sensor through the lens. Any digital camera has a built in light meter which is used to set the correct esposure (shutter speed and aperture settings) at a given ISO. It is a reflection light metering system, i.e. your camera measures the light that is reflected from a subject, a house for example. The meter gets all the light and blends it to an 18% gray. You can change the way your camera mixes up the received light in order to produce 18% gray.

Main metering modes:

  • Spot metering. Your camera uses a very small spot of the frame to calculate the exposure. Usually a central spot of 1-5% of the whole scene in size. Some cameras allow you to move that spot off the central position. This mode is used when you need to focus on one specific area of the scene, for example if there is a strong back light, you cannot use the whole frame as your subject will be too dark. So it is quite an advanced metering mode.

  • Partial metering. It is similar to teh spot mode but the area used for light calculation is bigger, usually 10 to 15% of the frame. It is also used when you need to exclude some really bright areas of your image, the sun for example.

  • Average metering averages the whole scene to produce 18% gray and often it does not work well. If you have the sun in one corner, this mode will make the whole image underexposed because it used the bright sun for its calculations. Camera manufacturers know about this disadvantage of the average mode and they created a Center-weighted average metering, which covers a centrally located area of 60-70% of the image.

  • Matrix or Evaluative metering. Modern cameras with powerful digital processors have this mode. The frame is split to several zones and the processor computes how these zones are lighted. If one zone is overlighter with the sun for example, the computer knows that it should not take this brightness for its calculations. This mode  is recommended for those who strat their photography.

The metering system is not too sophisticated and can be fooled by very dark subjects, if you photographing a group of people in black suits, your image will be overexposed because the camera does not expect the black color, it expects 18% gray. The same is true when taking photos of snow, it will be underexposed. The camera does not expect all that white color, it expects middle gray and it reduces the amount of light that comes to the sensor.

You can use exposure compensation or a light meter to correct your camera's mistakes.

Exposure compensation range of adjustment goes from +2 to -2 EV in 1/3 steps. So you can adjust the exposure measured by the light meter by telling the camera to allow more light in (positive exposure compensation) or to allow less light in (negative exposure compensation).