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Reciprocity

From: Foundations of Photography: Exposure

Video: Reciprocity

In program mode, when you half-press the shutter button on your camera, your camera meters the scene and calculates a shutter speed and aperture that will be adequately exposed, neither too bright nor too dark. In shutter priority mode, as you've seen, you can dial in a shutter speed, and the camera will choose a corresponding aperture that is neither too bright nor too dark. In other words, in both of these modes, your meter protects you from over- or underexposing. Well, let's look closer at what happens when I change shutter speed, when I'm in shutter priority mode. I'm back here with a scene we saw before, the toy with the spinning spaceships.

Reciprocity

In program mode, when you half-press the shutter button on your camera, your camera meters the scene and calculates a shutter speed and aperture that will be adequately exposed, neither too bright nor too dark. In shutter priority mode, as you've seen, you can dial in a shutter speed, and the camera will choose a corresponding aperture that is neither too bright nor too dark. In other words, in both of these modes, your meter protects you from over- or underexposing. Well, let's look closer at what happens when I change shutter speed, when I'm in shutter priority mode. I'm back here with a scene we saw before, the toy with the spinning spaceships.

Let's say I want to shoot it to freeze motion, so I dial in a shutter speed of say 1/500th of a second. So here I am in shutter priority mode. I'm dialing up to 1/500th of a second. When I half-press the meter, I see that the camera has chosen an aperture of f/2.0. Now let's say I decide I want to blur the motion instead, and so I want a slower shutter speed. So I'm going to slow my shutter speed by one stop. Remember, a stop is a doubling. So if I switch from 1/500th to 1/250th, which is a shift from faster to slower, I've changed my shutter speed by one stop.

1/500th halved is 1/250th. So I'm going to slow my shutter speed down to 1/250th, and now I've gone to f/2.8. I half-press the shutter button again, I see that f/2.8 is what the camera chooses. My light has not changed, but the camera has chosen a new aperture. When I slowed my shutter speed by one stop, I allowed twice as much light to reach the sensor. So the camera picked an aperture that is one stop smaller than what I used before. My shutter speed allowed twice as much light, so to compensate, the camera has picked an aperture that blocks twice as much as my previous aperture.

If we go back to our Aperture chart, we see that an aperture change from f/2.8 to f/2.0 is a change of one stop. Remember, every time I meter, the camera calculates an exposure that will yield a good level of brightness. Since my light has not changed between these two shots, the camera is aiming for the same level of exposure. So when I changed shutter speed, the camera had to choose a different aperture from before, so as to preserve the same level of brightness. It can do this because shutter speed and aperture have a reciprocal relationship.

If I change one parameter in one direction, I can change the other parameter by the same amount on the other direction, and preserve the same overall level of illumination. Now this is not something unique to shutter priority mode. It's simply a fact of exposure. For any given level of illumination, there are many combinations of shutter speed and aperture that yield the same overall brightness. This is great news for you, because it means that one of those combinations will probably help you achieve the image that you see in your head. However, this also means that you cannot over- or underexpose in shutter priority mode.

If you change the shutter speed, the camera will always pick a corresponding aperture that yields a good exposure. If it can't, it will flash it at you, as you saw earlier. Now this doesn't mean that your camera will always take perfect exposures. Your light meter can still be confused. Sometimes, it will come up with an exposure that over- or underexposes things. But if you want to intentionally over- or underexpose beyond what your meter suggests, you'll need to learn some additional controls that we'll cover later.

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This video is part of

Image for Foundations of Photography: Exposure
Foundations of Photography: Exposure

64 video lessons · 84128 viewers

Ben Long
Author

 
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  1. 8m 44s
    1. Welcome
      1m 56s
    2. What is exposure?
      4m 8s
    3. A word about camera brands
      2m 40s
  2. 9m 32s
    1. What is a camera?
      2m 53s
    2. The shutter
      3m 53s
    3. The aperture
      1m 33s
    4. Exposure defined
      1m 13s
  3. 13m 50s
    1. Modes
      2m 7s
    2. Pressing the shutter button
      2m 54s
    3. Autofocus
      5m 22s
    4. Light metering
      2m 3s
    5. White balance
      1m 24s
  4. 29m 26s
    1. Shooting sharp images
      1m 58s
    2. Noting shutter speed
      4m 3s
    3. Taking control of shutter speed
      1m 30s
    4. Stop defined
      2m 50s
    5. Shutter priority mode
      4m 34s
    6. Exercise: Shutter speed
      40s
    7. Reciprocity
      3m 13s
    8. Controlling motion
      7m 8s
    9. Shutter speed increments
      2m 21s
    10. Exercise: Go work with shutter speed
      1m 9s
  5. 26m 3s
    1. Depth of field
      1m 53s
    2. How aperture is measured
      2m 42s
    3. Aperture priority mode
      4m 57s
    4. Lens speed
      53s
    5. Shooting deep depth of field
      3m 53s
    6. Shooting shallow depth of field
      2m 50s
    7. The depth-of-field preview button
      4m 24s
    8. How shallow should you be?
      2m 47s
    9. Exercise: Go work with aperture
      1m 44s
  6. 16m 26s
    1. ISO: The third exposure parameter
      6m 27s
    2. Assessing your camera's high ISO
      5m 32s
    3. Shooting in low light
      3m 32s
    4. Exercise: Shooting in low light
      55s
  7. 14m 30s
    1. White balance controls
      5m 37s
    2. Adjusting white balance manually
      4m 25s
    3. Shooting raw
      4m 28s
  8. 6m 3s
    1. How light meters work
      1m 47s
    2. Why are there different modes?
      4m 16s
  9. 33m 59s
    1. Exposure compensation
      4m 0s
    2. Intentional overexposure
      2m 40s
    3. Intentional underexposure
      1m 42s
    4. Controlling tone
      2m 31s
    5. The histogram
      10m 4s
    6. Real-world histograms
      5m 49s
    7. Tone and color
      2m 16s
    8. Auto exposure bracketing
      3m 58s
    9. Exercise: Go work with exposure compensation
      59s
  10. 12m 56s
    1. Dynamic range
      2m 24s
    2. Exposing for highlights
      4m 15s
    3. Fill flash
      3m 11s
    4. Three solutions to the same problem
      3m 6s
  11. 12m 26s
    1. Manual mode
      2m 6s
    2. Manual mode and light meters
      4m 52s
    3. Manual exposure exercise
      5m 28s
  12. 12m 1s
    1. Custom modes and A-DEP
      1m 39s
    2. Program shift
      3m 52s
    3. Exposure compensation with program shift
      1m 58s
    4. An exercise in reciprocity
      53s
    5. Scene modes and in-camera processing
      3m 39s
  13. 8m 16s
    1. Shooting with post production in mind
      3m 46s
    2. Exposure strategy
      3m 51s
    3. Goodbye
      39s

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