Understanding Exposure – Part 2: Auto/Manual Exposure, Reciprocity, and Light Meters

The subject of exposure is very broad since it is the foundation of the medium of photography. To have more control over your camera’s output, it is best to understand the various concepts of exposure. Here are more information regarding exposure, from how its elements interact, to the various ways it is measured.

Cameras have automatic and/or manual exposures. By using the auto mode, the camera decides the best exposure settings based on the subject’s mid-tone.It may have aperture or shutter priority, meaning you have control over these particular settings while the camera still calculates the settings of the other two elements based on what you have chosen. With manual mode, you have to adjust all three settings yourself which gives you total control over the exposure.

To get a balanced amount of light, just remember that the smaller the aperture, the slower the shutter speed, and vice versa. Certain combinations between these two can result in achieving the same exposure, and this inverse relationship is called ‘reciprocity’. Note that for both f-stops and shutter speed, each measurement is half (or twice if you’re counting in the other direction) of the next. For example, f/2.8 would be twice the aperture diameter compared to the next stop which is f/, and a shutter speed of 1/250 would be half as fast as 1/500.

Here are some aperture and shutter speed combinations which result in the same exposure:

Aperture Shutter Speed
f/8 1/60
f/5.6 1/125
f/4 1/250
f/2.8 1/500

You might think you can just use any of these combinations and each image will look alike since the exposure is the same. Wrong. Aperture largely affects an image’s depth of field. If you choose the f/2.8 combination, your subject’s background will be largely out of focus unlike if you choose the f/8 combo. As for shutter speed, a 1/60 combination can capture camera shake or movement more visibly compared to the 1/500 which can freeze action.

Let us touch a little bit on this useful device called a ‘light meter’. This is used to measure the appropriate exposure of what the camera sees. It has a computer that guides you to choose what shutter speed and aperture would be best to use based on the available light and ISO. There are two general types of light meters:

Reflected-light meter – this type is used for all in-camera meters. It measures the light reflected on the scene that you want to shoot. These meters are attuned to give the ‘correct’ exposure for a regular scene with no extreme lighting scenarios. However, if the scene has areas that are very bright (giving off a higher reflectance), the reflected-light meter may overcompensate for this and there is a big possibility of underexposure. In instances like these, you might have to compensate for the brightness yourself by adjusting the settings.

Incident-light meter: this measures the amount of light hitting a subject. To get this, the device is placed in front of the subject and facing the direction of the camera. It then gives an approximate exposure setting to use. However, this might not be possible in the occasions when the subject is too far away, such as with landscape photography.

This entry was posted on Thursday, August 19th, 2010 at 9:01 am and is filed under Articles, Photography Basics, Photography Techniques. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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