Introduction to Exposure - Page 2
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* All elements of exposure work together. *

The shutter.

  • The Shutter
  • .......... Controls how long the sensor is exposed to the light.
  • .......... The shutter is like a door in front of the sensor and is normally closed and only opens for for exposure.
  • .......... The time the shutter is open is expressed in seconds or fractions of a second.
  • .......... It is operated by "the button" or a remote controller or the self timer.

* All elements of exposure work together. *

  • Shutter speed settings.
  • ............. (1) Must be set to ensure correct exposure along with aperture and Sensor ISO settings.
  • ..............(2) Must be fast enough to prevent camera movement resulting in un sharp photographs.
  • .............. a guideline is that 1/focal length of the lens (in 35mm terms) = the shutter speed.
  • ............... eg. On a Canon 400D (Rebel) the sensor is smaller than a film (35mm) sensor
  • ............. .. so you multiply the lens focal length by 1.6
  • .............. with a lens set at 40 mm (~60mm equivalent) the shutter speed used should be 1/60 second or faster.
  • ..............(3) If required must be sharp enough to stop subject motion.
  • ..............(4) Can be used creatively. eg, long exposure of car taillights in night scenes.

  • "London Girls"
  • Here the shutter speed is short enough to capture the set, including the man looking at the girls.
  • But not fast enough to completely stop the motion of the girls.
  • Larger Pic is available by clicking on the picture above.
  • Canon 350D (Rebel XT) with 17-85 IS lens (~28-135)ISO 1600 1/10 @ F6.3 Focal length = 17 mm (28) Hand held.
  • The lens is image stabilized (IS) which means it can be used with much lower shutter speeds. I would not expect reliable results at anything less than 1/30 of a second.

  • Canon 350D (Rebel XT) with 70-200 F4 L (Non IS) lens.
  • ISO 400 - 1/1600 @F10, Focal length = 94 mm (~150 mm equivalent) . Hand held.
  • Here a high shutter speed was used, by the camera, to stop the action.
  • Camera was used in Sports mode.
  • Slightly larger pic available by clicking on the pic above.

* All elements of exposure work together. *

The sensor sensitivity (ISO)

  • .
  • The Sensor converts light into an electrical signal which can then be saved.
  • The sensitivity of the sensor can be adjusted so it needs less light to create the electrical signal.
  • The unit of measure for sensitivity in photography is the ISO.
  • In a Canon 350D (Rebel XT) the ISO values can be 100, 200, 400 , 800 and 1600.
  • Each value is double or half the sensitivity of adjacent value(s)
  • Increasing the ISO allows (1) Faster shutter speeds and/or (2) smaller apertures.
  • Or the ability to take photographs in almost any conditions.
  • .
  • The only problem is that when you make the sensor more sensitive , "noise (grit?)" is added to the photograph.
  • The noise shows up in areas of continuous tone - like a sky - See the large London girls pic.
  • .
  • I try to use the lowest ISO possible as it allows photographs with less noise.
  • However I will increase it as need be to the photograph the scene or action.

* All elements of exposure work together. *
Onto Page 3 - We put it all together.
 
© Richard Taylor 2007