Camera Metering and Exposure

What is camera metering and camera exposure about?

Ever have problem with the photos that the subjects look too dark and hardly recognizable? This is all about the metering and exposure. In order to get a photo with good sharpness and brightness, the exposure to the sensor needs to be correct.
Exposure is identified as the amount of light permitted into the sensor (aperture setting) combines with the amount of time the light goes into the sensor (shutter speed setting). It is also affected by the sensitivity of the sensor (ISO setting).
In other words, in order for the exposure of a photo to be correct, the aperture setting and the shutter speed setting must be correctly synchronize.

Exposure is measured by EV (exposure value). EV is referred as 0 at the setting of aperture = 1, shutter speed = 1s and ISO = 100. For each stop drop in either section, the EV will increase by 1. For example, if aperture is drop one stop from 1 to 1/1.4, shutter speed remains as 1s and ISO remain as 100, then the EV value is 1. If, the aperture is 1/1.4, the shutter speed drops one stop from 1s to 1/2s and ISO remain as 100, the EV will become 2.

The higher the Exposure Value simply means the photo is over-expose and we need to set to higher shutter speed, smaller aperture and low sensitivity. On the other hand, if the exposure value is negative, then the photo is under-expose and we will need to set bigger aperture, slower shutter speed and higher sensitivity accordingly.
As shown here, the exposure differences in each stop in aperture setting is actually same as the exposure differences in each stop in shutter speed setting.

For example, if aperture setting drops one stop from f 1/5.6 to f 1/8, the difference of exposure is actually same as shutter speed setting drops one stop from 1/30s to 1/60s. EV increase by 1 whenever the aperture drop one stop and the other setting remain, EV also increase by 1 whenever shutter speed drop one stop while other setting remain. The difference of exposure is the same.

By understanding how exposure works, we get to know how to change the shutter speed, aperture and sensitivity setting depending on various situations.

For example, in a certain portraits photo shoot, the exposure metered shows that the aperture needs to be at F8 and shutter speed at 1/125s. However base on the EV, we can change the aperture three stop higher to F2.8 and subsequently change the shutter speed setting three stop lower to 1/1000s. The exposure of this particular photo will be the same; however we get to use a bigger aperture setting.

Metering is how the camera detects the amount of light on the subject and determines the number of EV needed for a proper exposure.

There are a few types of metering mode available in DSLR nowadays

  1. Matrix metering - Different types of cameras have different types of matrix metering. Each having different algorithm and zones of metering and the system will decide on the suitable EV for correct exposure.

  2. Center weighted metering - This metering mode will emphasize on the exposure at the center of the photos.

  3. Spot metering - This mode meters at the center spot/certain spot of the camera. The small area of metering is generally just 3~5% of the frame.

With better understanding of Exposure Value and how it works, we get to control the parts of the photo that we wish to emphasize on and need it to be clearly exposed. For example, in portraits photo, you should meter on the face of the person with either center weighted metering mode. For an even better result, you can even do spot metering on the eye of the person. This is to ensure that the exposure is set or metered to the brightness on the person. With proper metering, the subject will looks clear and sharp in the photos.

This explains on how to use spot metering. First, aim the spot metering point directly at the eye of the subject. Press and hold the metering button while changing the composition of the photos. Then, press the shutter.



 

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