To turn off HDR on an iPhone, the process depends on whether you want to turn off HDR for photos or videos:

• For HDR video:

  1. Open the Settings app.
  2. Scroll down and tap on Camera.
  3. Tap on Record Video.
  4. Toggle off the switch for HDR Video (which is 10-bit High Dynamic Range including Dolby Vision).
     This will stop the iPhone from recording videos in HDR mode.

• For HDR photos:
  Most recent iPhone models (like iPhone 13 Pro and newer) no longer allow users to fully disable HDR for photos. The iPhone automatically applies HDR when it deems necessary.
  Some models allow toggling off Smart HDR or Auto HDR in Settings > Camera.
  However, in newer models, HDR photo capture can't be completely disabled.
  You can turn off viewing full HDR in photos (not the capture itself) under Settings > Photos > View Full HDR to see photos in standard dynamic range.

In summary, you can turn off HDR video by toggling it off in camera settings, but full disabling of HDR for photos is generally not available on the latest iPhones, as the system applies HDR automatically for best results.