Cine Meter icon
Cine Meter for iPhone: details & info copyright © 2013 Adam J. Wilt  
 
A light meter with a waveform monitor and a false-color picture
 
 HOME   SUPPORT   CONTACT 
 CONTROLS   HOW TO…   FAQ 
 DETAILS & INFO 
Cine Meter's Limitations, Tips and Tricks for using it, How It Works, and assorted Observations.


Limitations

Cine Meter can only do what an iDevice (iPhone / iPad / iPod touch) lets apps do: lock and unlock exposure and white balance, but not preset either one to a known value; and read out scene brightness values. These limitations define what Cine Meter can—and can’t—do.

UNDERSTAND THESE LIMITS:

The light meter is absolute—you can count on its readings to mean what they say, regardless of circumstance, since they’re calculated from the camera’s reported brightness value. Once you calibrate Cine Meter to your reference standard, it should always give you correct readings.

However, the picture, false-color picture, and waveform monitor displays are only relative—they show scene brightness values relative to other levels in the scene, but the levels of those images and waveforms depend on how the camera sets its exposure, which often differs from the brightness value the camera reports to the light meter, as described in How It Works.

For this reason, you can’t directly compare light meter readings with waveform or false-color levels!

You can’t preset an exposure level or a white balance and then use the displays to show you absolute levels. iPhone / iPad / iPod touch cameras don’t let you manually preset values; they only let you lock in the current auto-exposure or auto-white-balance setting, so you can’t set the scopes for, say, ISO 800 @ 1/48 sec @ f/4.0 with a color temperature of 3200K.

The only way to set a particular exposure level is to trick the camera by showing it the “correct” light (for example, using a gray card) and then locking its settings. Once locked, you can then look at the pictorial displays to see how tones and colors in a scene render relative to the locked settings.

That’s the key to getting the most out of Cine Meter: treat it like a point-and-shoot camera with exposure and white-balance locks (which is really what it is, only with fancier readouts), and you’ll be able to “fool” it into doing what you need.


When Cine Meter isn’t actively running, your iDevice releases the camera for other apps to use, and when Cine Meter regains control, the previous exposure and white balance will have been lost (the camera remembers that it was locked, and it will lock exposure and/or white balance again as soon as Cine Meter regains control, but the camera will lock in whatever values it sees at the moment, not what it was set to before!

To prevent mistakes due to these random values, Cine Meter always unlocks the camera when it launches or wakes up. Once you’ve locked in an exposure or a white balance, don’t turn your iDevice off, let it go to sleep, or close Cine Meter. It’s OK to display the settings pages; just don’t exit the app or let your iDevice shut off.


The waveform monitor scale runs between 0% and 100%; there are no “superblacks” or “superwhites” shown. Cine Meter gets an 8-bit RGB image from the camera, and the 0%-100% range seemed like the best fit for the camera’s exposure tendencies.



Tips and Tricks

If you want to “dial in” an exposure, shoot a gray card (or your hand, or something in the scene) while watching the waveform monitor and meter readings. Tilt the card or shade the subject as needed to get the exposure you desire, then use to lock it into place when you get the value you need. Often it’s easier to dial in an exposure using the spot meter than the matrix meter mode.

Similarly, when you need to preset a white balance, the best way is to use while shooting a white or gray card under the desired lighting conditions. Once locked in, you can then aim the camera at other parts of the scene to judge the color of the lighting.

I often set an exposure on Cine Meter by progressively covering the lens with my finger until the exposure opens up to where I want it, or aiming my iDevice more and more towards a light source until the picture gets as dark as I want it.


A gray card is a handy thing for setting exposure and white balance, but there's never a gray card around when you need one. In a pinch, use a white card or paper: set white balance lock on it, then tilt or shade it until exposure drops about two stops before setting auto exposure lock.


iDevice cameras use fixed-aperture lenses, and vary shutter speed and gain (ISO) to set exposure. They also record the scene’s “brightness value” in camera metadata, and it’s that brightness value that Cine Meter uses to calculate exposure. When you change ISO and shutter speed settings in Cine Meter's light meter, you're changing the inputs for the standard APEX equation that calculates the displayed aperture value, but you're not affecting how the iDevice camera itself sees the scene.

By the same token, setting Meter Compensation only affects Cine Meter’s exposure readout; it has no effect on the camera’s own determination of proper exposure, or how its picture is captured or displayed.


When Cine Meter is running, it’s flogging the camera hardware, the CPU, and the GPU mercilessly, so that you can see images updating in a satisfyingly real-time manner. It will, however, run down your battery in the process, so you'll probably want to shut it off when you're not using it.

Just hitting the Home button on your iDevice, or turning it off, will put Cine Meter to sleep.

Just be aware that turning off your iDevice or moving Cine Meter into the background causes the camera to forget any locked settings (as described in limitations), so you’ll either want to keep Cine Meter awake and displayed if you have locked settings you need to preserve, or be prepared to re-acquire those locked settings when you start Cine Meter up again.

If you’re worried about battery drain but need to hold your locked settings, tap to display the Settings and Info page. The camera is still running and delivering frames, but Cine Meter won’t process them for display, reducing the workload on both the CPU and the GPU.


 

How It Works

iDevices have automatic cameras in them: you can lock and unlock exposure, focus, and white balance, but you can’t set them to any predefined values, and you can’t control the programming they use to determine their exposure values.

The cameras have fixed apertures and vary both gain and shutter speed to control exposure.

The cameras record scene brightness values (Bv) in their EXIF metadata. It’s that brightness value that Cine Meter uses to compute its light meter readings, and (once you've calibrated Cine Meter for the camera in your particular iDevice) those readings match those of external meters very closely.

So far, so good. However, the camera’s actual exposures do not track Bv precisely.

When the camera reaches its low-light or bright-light limit, or when exposure is locked, Bv will still change (you can verify this yourself with Cine Meter's onscreen data display; tap and hold the camera picture for two seconds to see it). My iPod touch 4G’s camera will report Bv, when locked, to about 2.5 stops of overexposure and roughly 5 stops of underexposure.

Even with the camera unlocked, Bv will vary even when the camera’s exposure settings don’t. The camera’s internal auto-exposure programming has some hysteresis in it; it’ll hold a setting until exposure differs by a third of a stop or so before it changes. It won’t chase minor variations in light level, instead holding a more visually-pleasing constant exposure as long as possible. Practically speaking, that means the light level can vary by as much as two-thirds of a stop before you'll see the camera change its own exposure settings. But all the while, Bv is changing in response to the scene.

Confusing enough? Wait, there’s more: even when the camera's reported exposure setting isn't changing, the brightness levels in the picture may vary. I've seen the waveform levels suddenly jump  5% on my iPod touch 4G, even though the camera's metadata swears that ISO, shutter, and aperture remained constant. My iPad 3's camera will vary picture levels considerably, with midtones changing from 45% to 55% in several discrete steps, even though the camera claims to be fixed at ISO 1600, 1/24 sec, f/2.4. Does the camera have an additional, undocumented gain stage in it? Does it implement dynamic range optimization or dynamic contrast control? Whatever the mechanism is, the camera can't even be trusted to report how it has come up with its final exposure, not with any consistency and repeatability.

Because the camera runs its own exposure program and doesn’t give us full and accurate metadata to relate that back to Bv, you can't directly compare levels in the displayed images to levels reported by the light meter. And since the camera’s metadata doesn’t give us the complete picture (pun intended) of how the camera produces its image, you can’t even relate levels in the picture to the camera’s own stated exposure.

In short, there’s simply no way to look at the camera's picture, its false-color version, or the waveform monitor levels, and relate them to any absolute measure of exposure.

Cine Meter is really two separate instruments in one package: (1) a light meter, and (2) a picture monitor with fancy display modes—and sadly enough, they don't always agree. They'll usually be within about a stop of each other, but  that’s the best you can hope for.



Observations

The colorimetry of iDevice cameras is not likely to match those of other cameras, especially cine cameras with extended red responses. Cine Meter’s RGB waveform monitor (WFM) is a very useful quick-and-dirty color-checking tool, but it’s not intended to replace a dedicated color temperature meter, a spectrophotometer, or even the view through the lens of your production camera.


You may sometimes observe that the camera picture’s exposure varies from what the meter readings indicate. It appears that at least some iDevice cameras have an additional processing stage, separate from the explicit gain and shutter settings: something like a Dynamic Contrast Control or Dynamic Range Optimizer, perhaps. If you move the camera rapidly between light and dark scenes, you’ll often see the exposure reading stabilize within a second, but the waveform and the image brightness may pump up or down over the course of three seconds or so (during which time the camera itself reports no changes in gain or shutter settings!).


iDevice cameras tend to be on the contrasty side, with some S-curving of highlights and shadows, typical of still cameras. Their gamma curves are not matched to Rec.709 broadcast specs, nor to any other cine camera curve. The WFM and False Color displays you see in Cine Meter are not direct matches for those you’ll see from your production camera (unless, of course, you’re shooting your show on the very same iDevice). That’s why you can adjust the False Color levels yourself: you can set your own trigger levels based on your needs and experience, and compensate for the difference between your iDevice’s camera and your production camera.


You might sometimes see “sticky texture” and fixed-pattern noise in the False Color view: patterns of red and yellow or blue and purple that remain even as the picture changes. It appears that some iDevice cameras use recursive (“3D”) noise reduction, and False Color makes those sticky textures visible.

The False Color view is only reporting what it sees; it gets a fresh frame to work with every time, and has no memory of previous frames. The static patterns are visible in False Color because False Color exaggerates the contrast between the pixels just below a false-color level and those just above it; in normal view these tiny differences (as little as 0.4%) aren’t distinct enough for the static patterning to be visible.

Not all iDevice cameras behave the same way. An iPod touch 4G’s camera shows a lot of sticky texture and fixed-pattern noise, while an iPad 3’s camera and an iPhone 4S’s camera do not.

On the other hand, the iPod touch will auto-white-balance cleanly in dim tungsten light and under “warm white” fluorescents, while the fancier devices insist on rendering the same scene as a dirty yellow. Sometimes you win; sometimes you lose...


© 2013 Adam J. Wilt.  Last updated 2013.01.16