Where to start with exposure?

February 27, 2013  •  1 Comment

In the last post I discussed the three elements of the exposure triangle: ISO, Shutter Speed, and aperture. This time I want to give my ideas about how to get you images properly exposed in the camera.

Previously I suggested that most advanced photographers do not use the Auto or Programmed modes of exposure because they prefer to retain creative control over the exposure triangle.  The reasons for this desire are apparent when you consider the pros and cons of each element of the triangle. High ISO allows shooting at high shutter speed or low light conditions but can add digital noise to the image. Low ISO reduces digital noise from the sensor but creates the need for a longer shutter speed or larger aperture. High shutter speed can be used to freeze a subject in motion or reduce the possibility of blur in the image caused by camera shake but requires a higher ISO or larger aperture to capture the image.  A slower shutter speed can smooth a waterfall but requires a low ISO, small aperture, and possibly, a filter to limit the light getting to the sensor.  Large aperture (lower f-stop) can be used to lower the depth of field and blur the background but may require a fast shutter speed to limit the light to the sensor.  A small aperture (higher f-stop) will increase the depth of field but require a higher ISO or slower shutter speed.

The pressing question is how do we estimate the proper exposure and how do we adjust the components of the exposure triangle to reach this exposure while using the components to achieve the best creative effects?

In the olden days, photographers followed the sunny f/16 rule to estimate the proper exposure in the outdoors. The sunny f/16 rules says that under bright outside light the proper exposure will be when the shutter speed is equal to the ISO of the sensor. In other words on a bright day, shooting at f/16 and an ISO of 100, the shutter speed should be about 1/100 second. If you want, for creative reasons, to vary the f-stop or shutter speed you do so by using the concept of "stops" of light.  A stop is a change in the exposure triangle that effectively doubles or halves the exposure.  Doubling the exposure time (shutter speed) from 1/100 to 1/50 of a second doubles the light reaching the sensor and is said to increase the exposure by one stop. Doubling the the time again to 1/25 of a second increases the exposure by 2 stops. Exactly the same effect is seen when increasing the ISO.  Changing from ISO 100 to ISO 200 doubles the sensitivity of the sensor and increases the exposure by one stop.  Changing the ISO or shutter speed create simple exposure changes and shooting at a faster shutter speed, say 1/200 second and a lower ISO say 50 create no change in exposure - one is increased by one stop and one decreased by one stop resulting in no net change.

So far it sounds easy -- but the confusion occurs when we want to change aperture. Aperture changes also occur in stops that double or halve the amount of light reaching the sensor. However, aperture changes do not follow the simple arithmetic of doubling or halving the number like ISO and shutter speed do. The standard f-stops follow the numeric sequence: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.  Moving up the series of standard, full-stops effectively cuts the light reaching the sensor by half. So, using the sunny f/16 rule, if we needed to double the light reaching the sensor, we could open the aperture to f/11. This would accomplish the same thing as increasing the ISO from 100 to 200 or the shutter speed from 1/100 to 1/50 of a second. The concept of working in exposure stops allows us to change exposure by varying one or more of the elements of the exposure triangle or keep the same exposure by changing any combination of elements in the opposite directions. 

In the next posting we will wrap-up the discussion of exposure by examining the metering modes available on modern digital cameras and the exposure modes that allow creative imaging without using the Automatic or Programmed modes.


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