Cine Meter / Cine Meter II: Computed Brightness

TL;DR version:

On iOS 8.4, 9.0, or 9.1? Update to the current version of iOS.
Update to the latest version of Cine Meter or Cine Meter II.
Turn off “Use Computed Brightness” if it’s on.
Check exposure meter accuracy and recalibrate if needed.
Re-test the exposure meter when you update iOS versions, and switch “Use Computed Brightness” on if the exposure meter isn't working properly with it off. Recalibrate if needed.

The short version:

If you are running Cine Meter 1.6 or Cine Meter II 1.9 on iOS 9.0 or 9.1, and have a 64-bit device, “Use Computed Brightness” must be turned on. (64-bit devices are the iPhone 5S, 6, 6 Plus, and newer; iPad Air and newer; iPad Mini 2 and newer; iPad Pro; and iPod touch 6G.)
Otherwise, turn “Use Computed Brightness” off. Cine Meter 1.7+ and Cine Meter II 1.10+ will compute brightness automatically when needed.
If you’re on iOS 9.0 or 9.1, please update iOS to the current version for best results.

Current versions of Cine Meter and Cine Meter II use “Computed Brightness” automatically when needed to work around problems with camera brightness metadata on certain devices running iOS 8.4 through 9.1. When this happens, you’ll see “computed brightness” in the status line below the exposure readouts.

There is still a “Use Computed Brightness” switch in settings, in case you need to force computed brightness on (if a new iOS version makes the brightness metadata incorrect or inconsistent). If it was switched on previously, the new versions don’t turn it off automatically, so you won’t be caught by surprise (you need to recalibrate the exposure readings when you change “Use Computed Brightness”).

You should turn the switch off with Cine Meter 1.7+ or Cine Meter 1.10+, and double-check your exposure calibration. Also re-check metering accuracy when you upgrade iOS; if it gets worse, switch “Use Computed Brightness” back on again... and recalibrate as necessary.

Always recalibrate exposure after changing the setting for best accuracy.

(Color metering in Cine Meter II is not affected by “computed brightness” issues, and color metering recalibration is not required.)

The long version:

Cine Meter and Cine Meter II normally use brightness metadata provided by the iPhone’s camera. This brightness metadata is incorrect on 64-bit devices running iOS 9.0 or 9.1, even if those same devices work perfectly well with iOS 8. Affected devices don't meter accurately, and they'll typically read about 9 stops too high in subdued interior light. This problem only occurs with iOS 9.0 and 9.1 on 64-bit devices, and it's fixed in iOS 9.2.

Cine Meter version 1.6 and Cine Meter II 1.9, added a new setting: “Use Computed Brightness”. It’s enabled automatically when running those versions on iOS 9.0 or later, and causes the apps to compute the scene brightness from the iPhone camera’s own exposure settings and image content, instead of trusting the brightness metadata. (Certain older devices running iOS 8.4 report “infinite” brightness, which is easily detected and automatically corrected for by computing brightness, even if the switch isn't set.)

With Cine Meter 1.7+ and Cine Meter II 1.10+, the setting still exists, but it's not normally needed: when an affected device is running iOS 8.4, 9.0, or 9.1, computed brightness is automatically used. The switch is provided in case a future iOS update breaks brightness metadata (again) and you need to manually force the app to compute brightness, before the app is updated to deal with the new iOS version automatically.

How to determine whether you need “Use Computed Brightness” or not?

You see inaccurate or inconsistent exposure readings.
The exposure readings are stuck (typically at “too bright” readings).
You turn on the debug window, and Bv shows as INF or -INF

For example, this test works in iOS 9.0 or 9.1:

For Cine Meter II, make sure you're using the front or back camera, not Luxi or Lumu.
Press and hold the live image for a few seconds to display the debug data overlay.
Cover the camera lens so that no light gets in (do this inside a dark room for best results, not outside in daylight).
Read the Bv value (Cine Meter II screens shown; Cine Meter’s screen are similar but not identical):

If Bv is a negative number, typically -5 to -8, turn “Use Computed Brightness” off.
If Bv is a positive number, typically 1.3 to 2.9, turn “Use Computed Brightness” on.
When you’re done, press and hold the debug overlay until it disappears.

Note: changing “Use Computed Brightness” won't change the Bv reading. It only changes whether or not the app uses Bv!

In all cases, waveform monitor and false-color displays, and (with Cine Meter II) Lumu incident readings and Luxi color-temperature measurements are unaffected; it's just exposure readings based on Bv—brightness value metadata—that are affected by this setting.

*** After changing the setting, recalibrate for best results! ***

(Why not just turn “Use Computed Brightness” on for all devices and be done with it? Devices that work without it will give better readings, accurate to +/- 1/10 stop, whereas computed brightness readings are only accurate to 1/3 or 1/2 stop in most conditions, and are sometimes even farther off, due to the exposure program used by the iPhone camera.)