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Posts Tagged ‘EV values’

For many Schutzhund photographers, me included, really understanding the fundamental relationship between ISO, shutter speed and aperture so it becomes second nature has not been easy, especially when lighting conditions change on a dime and quick decisions are the rule. This past weekend, I did some more digging, and learned about EV values, which for me has been a missing link to this often perplexing equation. I now finally, finally understand that choosing the correct exposure for lighting conditions and what I am striving for creatively is a function of doubles and halves.

Before reading further you may wish to review my earlier post Figuring Out Aperture, ISO, Shutter Speed in Changeable Weather Conditions, as today’s post builds on the information presented there. Also, I wish to acknowledge Fred Parker of Fred Parker Photography, who penned a great article that explains EV values, along with shutter speed, aperture and ISO. His article is the primary resource for this post, and I encourage you to read it sometime. He also has an “Ultimate Exposure Computer” that is very helpful.

Often, you will see the ISO, aperture and shutter speed relationship depicted as an exposure triangle. Varying these settings allows photographers to manage exposure and creatively change the image. After reading Fred’s article, as well as some other sources, I see this relationship more like a square, with the fourth side reserved for lighting conditions (see the figure at the end of this post). Various lighting conditions are assigned numerical values (called exposure value or EV) on a scale of 1 to 23 (for ISO 100 film) (see bullet list below). Each step up represents twice as much light and each step down equates to one-half the light falling on the subject. This value is then converted into aperture and shutter speed settings to achieve the proper exposure for the chosen film speed.

Most daylight subjects / conditions fall within the rage of EV 11 to EV 16, as follows:

  • Light sand or snow in full or slightly hazy sunlight (distinct shadows):  16
  • Typical scene in full or slightly hazy sunlight (distinct shadows): 15
  • Typical scene in hazy sunlight (soft shadows): 14
  • Typical scene, cloudy bright (no shadows): 13
  • Typical scene, heavy overcast: 12
  • Areas in open shade, clear sunlight: 12

For a full listing, including evening, night and indoor lighting values, see the Wikipedia article on Exposure Value.

To review, film speed (or ISO) represents the sensitivity of the film to light or in the digital world the sensitivity of the light sensor in the camera. These speeds are 25, 50, 100, 200, 400, 800, 1600 and 3200. Each step up from 25 to 50 to 100 to 200 and so on is twice the preceding number and represents twice the sensitivity to light. With all other exposure settings the same, each step up will take half as long to reach the same level of exposure as the previous step. Conversely, each step down equates to one-half the sensitivity to light of the previous step and will take twice as long to reach the same level of exposure. Remember that the faster the ISO, the more noise in the picture. The equivalent artifact in film is called grain. For a great explanation of digital noise and ISO, see the Cambridge in Color’s tutorial on Digital Camera Image Noise. Most experts recommend keeping the ISO as low as possible. This doubling or halving the light holds true for aperture and shutter speed settings as well.

Shutter speeds are expressed as a fraction of a second (except those for very long exposures that are more than a second): 15, 8, 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 and 1/8000. As you move to the right, each value is approximately twice the preceding value and represents half the light or moving to the left twice the amount of light. Shutter speeds at 1/1000 will stop most action, while slower speeds such as 1/8 will blur even slow moving subjects. Here’s useful tip from Fred to avoid motion blur:

“If your shutter speed is slower than the reciprocal of the focal length of your lens, you must use a tripod…For example, if you are using a 200 mm lens, your shutter speed must exceed 1/200…If your subject is moving, double this shutter speed…If you are moving, triple the speed.” 

Aperture settings are the ratio of the focal length of the lens to the diameter of the lens diaphragm opening, hence the term “focal number” or f/stop or f/number. The designation “f/2”, for example, means the diameter of the aperture is 1/2 the focal length of the lens. These are expressed as 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64. Each number to the right is one half the amount of light; conversely, each number to the left allows twice the amount of light. The lower the number, the more wide open the aperture and the more light is allowed into the camera sensor. Notice that the progression of f/stops approximately doubles the numbers 1 and 1.4: 1, 2, 4, 8, 15, 32, 64 or 1.4, 2.8, 5.6, 11, 22, 45. Combining the two series yields the entire range of aperture settings or f/stops: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64. With ISO, shutter speeds and aperture settings, there are intermediate steps as well. To keep things simple, they are not addressed in these posts.

Exposure Square - Double or Half the Light (or Sensitivity to Light - ISO)

So, how does a photographer put all this together? Well, you’ll have to wait for my next post, which will be up later this week!

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