I am trying to understand how to get perfect exposure using gray card. With my Sony a6500 using ITU709 gamma, I locked exposure by doing spot metering on the gray card. When importing in final cut pro the waveform monitor show IRE value near 25 for the gray card. Shouldn't it show between 40 and 43 for 18% reflectance mid gray? final cut waveform scope

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    Was the gray card completely filling the frame when you set exposure? Or were you using the frame shown to the right with the much brighter grass occupying almost half the field of view? – Michael C Jun 19 '18 at 1:19
  • Isn't IRE a video-signal measurement? I'm struggling to understand the relevance here. Why do you care? – mattdm Jun 19 '18 at 14:55
  • mattdm you are correct that the IRE unit is typically used in video. I am trying to fix the exposure when post processing. I think the other way would be to verify that the luminance of the area on the gray card have a value of 117 on the 0 to 255 scale but I am not sure if this is correct and IRE is easy to visualize with the waveform monitor. – skyde Jun 19 '18 at 15:44
  • @MichaelClark I was using the frame shown above. I assumed the card only need to be on the center focus point and close to the subject. Maybe I should have used digital zoom to make sure it fill the frame? – skyde Jun 19 '18 at 15:46
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    @skyde Are you trying to fix photos in post that you have already taken, or are you planing on taking photo that are not properly exposed in the future and fixing those is post? The goal is to properly expose the photo when you take it so you do not need to "fix" it. – Alaska Man Jun 19 '18 at 17:14

18% reflectivity means it should reflect 18% of whatever linear light is actually on it. Gamma should compute 117 at 46%, but your picture shows the card to be in some shadows, so then it can't be 18% of the brightest 255 (expecting gamma to 46%). But the histogram won't be the exact computation, because the camera is busy doing other things too, like your contrast setting, and white balance, and your color profile like Vivid, etc. The purpose of those settings are to shift the histogram. Mild settings in the bright normal light should be near ballpark, but probably not exact as planned. It is not an adequate calibration solution.

  • you are saying the contrast and color profile might affect the real curve, so we cant simply rely on the gamma curve. Is there a way to fix exposure in post reliably ? What if contrast is set to default and color profile to neutral? – skyde Jun 19 '18 at 15:51
  • NOT speaking of exact histogram values nor of post processing, but in use, the 18% card should be metered in same light as the subject. Then Kodak always said of their 18% card that then camera exposure should be increased 1/2 stop. This is because almost all reflected meters use 12.5% (Sekonic, Nikon, Canon, etc). Adjusting exposure by eye works pretty well in post processing raw images. – WayneF Jun 19 '18 at 16:20
  • with gamma of 2.2, meter reading of 12.5% would give IRE of 40 and 18% would give IRE of 46. – skyde Jun 19 '18 at 20:20

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