Cine Meter II: Brightness Correction

Adjust Brightness Correction (requires version 4.0 or later): Some iDevices introduced after mid-2014 (iPhone 6 and later) are too sensitive to bright light: at high light levels, such as outdoors in full sunlight, Cine Meter II may read up to 2/3 stop too high. This should not be a problem for any iDevice with an A11 or later processor running iOS 14 / iPadOS 15 or later, but iDevices with earlier processors may be affected. This “brightness bump” is a small error, but if it bothers you, you can correct it.

How to see if it affects your iDevice: Take readings with Cine Meter II and compare them to the readings from a different metering device (a light meter, DSLR, mirrorless camera, or video camera). Your iDevice’s front and back camera may be affected differently, so test them individually.

Take readings at light levels below ISO 200, 1/60 sec, f/2.8 (roughly 55 foot-candles or 600 lux): match the readings between the two meters, either by calibrating Cine Meter II to match the other meter or setting Cine Meter II’s COMPENSATION control for a temporary match.
Take readings at light levels above ISO 200, 1/60 sec, f/11 (about 930 fc or 10,000 lux): Cine Meter II should match the other meter. If it consistently reads higher by 1/3 stop or more, your iDevice has a brightness bump.

What to do if you see a brightness bump:

Ignore it: the exposure error is small enough that it took five years for anyone to notice it. Unless you are using Cine Meter II to calculate lighting ratios right around the bump, or you are using Cine Meter II to expose film with Zone System precision, it won't have a significant effect on your work.
Correct it: you can determine where the bump occurs on your iDevice’s cameras, and correct it.

How to correct the brightness bump: Cine Meter II’s Brightness Correction panel lets you set the starting and ending brightness values (BVs) for the brightness bump, and the amount of correction to apply. To use it, match Cine Meter II to a reference device (another light meter, DSLR, mirrorless camera, or video camera) at a low light level, then compare the readings of the two devices across a range of brightnesses, from low to high.

The highest brightness level where the readings match is the starting bv.
The bv correction is the difference in stops between the two meters’ readings at the higher levels.
The lowest brightness level at which that difference is reached is the ending bv.

Example with both meters reading in decimal third stops. In reality, you will see more variations in your readings. Small variations are normal and are to be expected: the two meters’ readings may not change from one value to the next at exactly the same brightness level, and your iDevice’s bump may not be as well defined as shown here. The values shown are for illustration only and may not line up with the same brightness values in your measurements.

The easiest way to generate a controlled range of brightnesses is with a motion picture lamp in a dark room. I’ve used a 30-watt LED panel and a KinoFlo Diva-Lite 400 with good results.

You can test using either incident or reflected readings. Brightness correction is different for your iDevice’s front and back cameras, but it’s the same for both incident and reflected readings using the same camera.

For incident readings: start far away from the lamp and move closer to it to increase light levels, aiming both meters directly at the light and taking readings as you go.

For reflected readings: set the lamp close to a white wall, washing the wall with light, from dark to bright:

Wall wash with 30 watt LED. Blue tape dots at one-stop or half-stop intervals help when aiming meters.

You can also stick a tape measure on the wall to use as an aiming target.

Aim both meters at the same point on the wall, aiming far from the lamp for low light levels and closer to the lamp for higher light levels. (Tip: when testing with a camera, use a zoom lens on the camera and the zoom setting in Cine Meter II to focus them both on the same small area of the wall.)

Step-by-step instructions:

First, calibrate Cine Meter II to match your other meter as closely as possible at or below ISO 200, 1/60 sec, f/2.8; roughly 55 foot-candles or 600 lux. (If you're using a camera with an f/4 zoom, stay below ISO 400, 1/60 sec, f/4.)
Set both meters to measure aperture (tap and hold Cine Meter II’s APERTURE readout until it turns gray; set a DSLR or mirrorless camera to shutter priority mode). Set Cine Meter II’s Aperture Increments (in Settings > Display & Control) to match the style of your other meter; typically Fractional Tenth Stops for another light meter, and Decimal Third (or Fourth or Sixth) Stops when using a camera as the reference meter.
Set both devices to ISO 200 and 1/48, 1/50 or 1/60 sec (use 25 or 30fps for a cine-style meter). Those settings should give you a useful range of apertures. If you find that you need more light at the dim end of the test, try a higher ISO; if you find your metered value's aperture getting too small at the bright end, try a lower ISO or higher shutter speed.
Then tap to open Settings, scroll down to SETUP, and tap Brightness Correction’s Set Up to display the BRIGHTNESS CORRECTION panel:

The current f/stop is shown in the style you selected (here, fractional tenth stops). The white BV number after the aperture is the brightness value you’ll use to set the controls. (The gray number after the arrow is the corrected BV: the original BV minus the correction.)
Turn the switch on, and set BV CORRECTION to 0.0.
Adjust the light level you're measuring until BV is 4.0 or less.
Now, while watching both meters, increase the light level by half a BV at a time. For example, move the meters closer to the lamp for incident readings, or aim them at increasingly brighter parts of the wall for reflected readings, starting at BV 4.0, then moving to BV 4.5, BV 5.0, BV 5.5, and so on. The two meters should track each other when you start doing this. At some point, usually around BV 5.0 to 7.0, Cine Meter II may start to read higher than the other meter. The highest BV at which the two meters agree is your starting bv. Set the STARTING BV control to that value, or as close to it as you can. Set ENDING BV to be half a stop higher than STARTING BV.
Now increase the light level by 3 stops or more (3 BVs or more), and adjust BV CORRECTION until both meters read the same again. Look for the best overall match at that light level and brighter; typically BV CORRECTION will be 0.4 to 0.6. (If you get very close to your lamp, position is critical: make sure both meters’ incident sensors are exactly the same distance from the light, or that both reflected readings are aimed at exactly the same spot on the wall.)
Then start decreasing the light level, and see when the readings stop matching again (typically at a BV 1 to 3 stops above starting bv). The lowest BV at which the meters match is your ending bv. Set the ENDING BV control to that value.
Finally, move through the different light levels, from less than starting bv to more than ending bv, and make sure that your corrections work well. Make small adjustments if you need to. Don’t worry if you can’t get a perfect match across all light levels. If your readings match within about 1/3 stop, that’s good enough for practical purposes.
Switch to your iDevice’s other camera and repeat, if needed (on 3.5" and 4" iDevices in landscape orientation, the button is hidden behind the Brightness Correction panel. Rotate to portrait orientation to show it).
When done, tap and hold the bottom of the Brightness Correction panel to close it.