How to shoot the sequence of adjusted exposures wanted for a ColorChecker?

Question

I'm trying to shoot a sequence of photos needed for calibration. As instructed, I'm in manual mode, with fixed ISO 100 (camera dafault), and other values read from the lightmeter ... and now:

Can you explain this passage?

Once you’ve determined your starting exposure, you will shoot additional exposures, incrementing the f-stop value by 1⁄3 f-stop using the manual controls on your camera. Shoot two stops on either side of the starting exposure and keep track of the exposures as you go along. I find it convenient to indicate the exposure change on the target itself by writing on a paper attached to the target. I write down the difference in exposure as a change in ISO. In other words, if I stop down one-third from the ISO 100 reading, I write 125: 2⁄3= 160, 1 stop = 200, and so on. The beginning of the sequence would go: 100, 80, 64, 50.

My ISO is 100 and it's not changing. Do I understand this passage as instructing me to change eg. F 2.8 to 3.2 and then to 3.5 then to 4.5, etc. 4 ... until I reach to F5.6? and these changes applied to the card as an ISO in a modified F? How then to do these measurements down by 2 stops (in the opposite direction from F2.8), as ISO must be constant and my lens can't be set lower than f/2.8?

I don't understand... Do I change time or aperture? Is my starting point from 4.0 and then 2 stops up and down?

Answer 1

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"?)