How do I bracket exposures by 1/3 stop for ColorChecker calibration?

Question

I'm photographing a ColorChecker for calibration and the instructions say to determine a starting exposure, then shoot additional exposures in 1/3-stop increments, covering two stops on either side of the starting exposure. They also suggest labeling each shot with equivalent ISO values such as 100, 80, 64, 50, 125, 160, 200, even though ISO should remain fixed.

I'm confused about what setting I should actually change. If ISO stays at 100, do I change only the aperture in 1/3-stop steps while keeping shutter speed fixed? For example, if my lens only opens to f/2.8, how do I shoot two stops brighter than a starting exposure if I can't open the lens any wider? Does that mean I need to choose a different starting aperture, such as f/5.6, so I can go two stops both darker and brighter?

user1943 · Answer

The instructions tell you to keep shutter speed and ISO constant, and change just the aperture — the f/stop. The number being recorded is the theoretical ISO number at which an image with a changed f-stop would be the same as the one with your actual ISO and original f-stop. So, for example, if your starting aperture is f/5.6 at ISO 100 (with whatever shutter speed is metered to be correct), at f/8, you would write "200" — because (for constant shutter speed), f/8 @ ISO 200 gives the same exposure as f/5.6 @ ISO 100 — each factor has been adjusted by one stop.

I think you're saying that you have a lens that you can't set wider than f/2.8. That suggests that, since you need to go two stops on either side, you need to start two steps up, at f/5.6.

So, make f/5.6 your initial exposure. Keep ISO at 100, and set the shutter speed to whatever is appropriate for the light, and then keep that the same. Then, as suggested, first go up in third-stop increments, to f/11 (two stops up from f/5.6. Like this:

aperture   (adjustment)   write this
  f/5.6         0             100

  f/6.3        +⅓             125
  f/7.1        +⅔             160
  f/8          +1             200      ← note a full-stop adjustment is
  f/9          +1⅓            250               exactly double
  f/10         +1⅔            320
  f/11         +2             400

  f/5          -⅓              80
  f/4.5        -⅔              54
  f/4          -1              50      ← or exactly half
  f/3.5        -1⅓             40               
  f/3.2        -1⅔             32
  f/2.8        -2              25

Presumably, these recorded values will be useful in the next step of calibration. (Why these particular numbers? See What is one "stop"?)

How do I bracket exposures by 1/3 stop for ColorChecker calibration?

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