Saturday, March 7, 2009

Exposure Value (EV)

This how to calculate the image parameters you need for a photo:

First there is the EV value, which is determined by the current scene you are about to shoot. Here are the EVs for different scenes using ISO100:
  • In the full sunlight the average EV can be 15, and may reach 16 in sand or snow.
  • Cloudy days can range about 13.
  • Shooting in the shadow of a sunny day can be in the range of 12.
  • Sunset scene is 12 EV.
  • Moon can be in the range of 14.
  • Shooting under the moonlight can be in the range of -4.
  • Indoor shots can range between 5-7 EV.

Definition for EV:
  • 0 EV means using ISO100, exposure of 1 second and aperture of f/1.0
  • 1 EV is said to increase the exposure one step, is achieved by increasing one of the parameters (ISO, exposure time, or aperture) by one step too. (May be adding 1/3 step for the three).
  • 10 EV for example can be achieved by increasing some of the 10 steps in each of the parameters. For example increasing exposure time by 5 stops, aperture by 4 stops and ISO by 1 stop.

ISO stops:
  • Decreasing EV with one stop means multiplying the ISO number by two. Example shifting ISO100 to ISO200 means decrease of one stop. Also shifting ISO200 to ISO400 is one stop down.

Exposure time stops:
  • Increasing EV by one stop means dividing the exposure time by 2. For example decreasing the exposure time from 1 to 1/4 seconds means moving the EV two stops up.
  • Stops are always approximated for ease of memorizing. So instead of saying 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024 we say 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000.

Aperture stops:
  • Increasing EV by one stop means multiplying aperture number by root two. For example increasing the aperture from f/1.0 to f/1.44 is increasing the EV one stop.
  • Stops are approximated for ease of memorizing. So instead of saying f/1.414, f/2.828, f/5.656, f/11.314, f/22.627 we say f/1.4, f/2.8, f/5.6, f/11, f/22.

  • If you got an f/2.8 lens (lucky you), and you are going to shot indoors, so say the EV is 5. f/2.8 means EV 3. So, say you will shoot with 1 sec and ISO100, you will get an over-exposed photo with +3 EV. Stopping down the exposure time to 1/8 will make a balanced photo with +0 EV.
    But normally, shooting indoors most probably will include people and you cannot always afford shooting people in 1/8 sec because it will blur any tiny motion. Also without a tripod or stabilizer the rule of thumb says that you should use exposure time near 1 over focal length. Say you are shooting with 70mm focal length with a steady hand; you will need about 1/60 sec exposure. 1/60 is 3 stops beyond 1/8, these means an under-exposed image with -3 EV. To make it up you may shift the ISO from ISO100 to ISO800. If your length is at f/4 you need to shift ISO to ISO1600 to keep shooting with 1/60 sec exposure.
  • Another example while shooting in a sunny day, you need to achieve 15 EV. With f/4 (4EV) and ISO100, you will need exposure time to be 1/2000 sec. This means that in order to capture motion (use longer exposure) in sunny days, you must shift up the aperture and also widen the depth of field.

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