Deep dive into N-Log and its use of ISO

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Horshack Veteran Member • Posts: 9,787
Deep dive into N-Log and its use of ISO
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Ever wonder how N-Log works and what effect its ISO has on dynamic range (DR), and how all this compares to shooting video with standard dynamic range profiles (SDR)?

Perhaps you've had the following questions:

  • Why does N-Log have a higher minimum ISO (800) than ISO 64/100 for standard definition (SDR)?
  • Why does video achieve the highest DR with N-Log at ISO 800 but the highest DR for still images is at ISO 64/100?
  • Should I use N-Log when shooting NRAW video?
  • When Nikon added ISO 200-640 (Lo 2.0 to 0.3) support for N-Log to the Z9 in Firmware 4.0, why did the Nikon's Firmware 4.0  notes warn of reduced DR when shooting at those lower ISOs?

Here are the answers:

  • N-Log's ISO 800 is not really ISO 800. It uses a sensor ISO of 100 and selectively pushes the exposure digitally via a tone/gamma curve, the same as you would do exposing a still image for the highlights and pushing the shadows/midtones in post.
  • This is done to simulate exposure/metering of the midtones, otherwise the midtones for a "correct exposure" would look too dark on the viewfinder/LCD
  • If you increase N-Log's ISO beyond 800, the sensor ISO used is ISO 100 + equivalent number of stops above N-Log's ISO 800. For example:
    • N-Log ISO 800 is sensor ISO 100
    • N-Log ISO 1000 is sensor ISO 125
    • ...
    • N-Log ISO 4000 is sensor ISO 500
  • When shooting NRAW video with SDR tone, the sensor ISO matches the actual exposure ISO, same as for stills. When shooting NRAW video with N-Log, the sensor ISO is 100 used for the shooting ISO of 800 (same as non-NRAW N-Log), scaling up as described in the previous bullet point.
  • For cleaner deep shadow performance, use a minimum ISO of 500 for SDR and ISO 4000 for N-Log (Z8/Z9), both of which correspond to sensor ISO 500, which is the first ISO of the dual gain ISO logic.
  • You don't need to use N-Log for NRAW. However, using SDR and pushing from low ISOs creates a shadow tint in Davinici Resolve that doesn't exist in N-Log, which you'll need to correct in post. I believe this tint is automatically corrected for the N-Log case, either in-camera or in Davinici (more experiments needed).
  • When shooting non-NRAW (ie, H.264/H.265), N-Log is required over SDR if you want the full DR of the sensor represented in the video file (post-processing required). Same concept as photo raw vs jpeg, except N-Raw squeezes the extra DR into a smaller DR deliverable format by compressing the tones, whereas a still-image raw uses an entirely different format (NEF).
  • The new ISO 200-640 (Lo 2.0 to 0.3) support for N-Log in Z9 FW 4.0 use ISOs below 100 (pending a few tests to confirm)

To demonstrate the dual gain taking effect for SDR vs N-Log:

8K NRAW SDR, ISO 400 vs 500, >5EV push in post (Animation)

8K NRAW N-Log, ISO 200 through 4000 for >5EV push in post (Animation)

More demonstrations to follow as time permits.

Nikon Z9
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