Can You Really Use A Light Meter App?

The Studio Setup

If you have been following my blog for any length of time, you have probably picked up on the fact that I am a big fan and proponent of using a light meter. With Sekonic's release of the 478D they increased the functionality of what a meter can do and made it more affordable. But what if you don't have $389? Can a free light meter app, or one that costs $4.99, do the job? That's what I set out to explore and the answer may surprise you.

Quick Reality Check
While the apps I tested cost next to nothing, let's not fool ourselves into thinking that they are really that inexpensive. As of this writing, they both require a $549 dongle. (Otherwise known as an iPhone). The cost of the dongle can be subsidized by paying for a phone contract, but it is a real part of the total cost, albeit a hidden one. If you are considering buying an iPhone to use as a light meter, I'd recommend saving yourself some money and buy the 478D. If, on the other hand, like many of us in the film community, you already have an iPhone, then the negligible added cost of these apps may be worth it to you, if you like the results that follow. :)

The Setup
I decided to compare 5 light meters in 2 different environments to evaluate the results. The 5 meters that I'll be looking at are the Sekonic 758Cine ($822), 478D with Spot Attachment ($498 total), Pocket Light Meter (Free - $4.99 + Dongle), Cine Meter ($4.99 + Dongle), and the internal "brain" of the Canon 60D ($1,299). The first test environment was in the controlled environment of my studio. The second environment was in my kitchen - uncontrolled lighting, using the overhead 5000k CFLs, lovely green spike and all. For every measurement I took a reading from the same area on a Kodak Grey Card.

The Studio Results
First up the Sekonic meters:

Incident Readings (Using the default profile)

Spot Readings (Using the default profile)

If you are wondering why the incident and spot readings of the 758Cine are different by 6/10's of a stop, it has to do with the way mid tone is calculated. If you want to learn more about it, you can do so here. The spot meter of the 478D is 5 degrees; it sees a wider field of view than the 1 degree of 758Cine. That means it is averaging more information, which results in a smaller difference in this application. The results from these readings say I should expose at F4 using a 180 degree shutter, EI 1250, and shooting at 24 frames a second.

And how does that translate to the 60D?

Exposure set according to the Sekonic meters

The exposure looks nice and healthy for the contents of the scene. But what does the camera say I should do? (By the way, the difference between 1/50 and 1/48 shutters is negligible when it comes to exposure readings).

Exposure set according to the full auto mode in the camera.

Hmm, interesting. According to the brains of the camera, the previous exposure was underexposed by about 1 1/3 of a stop. Fortunately, nothing in the scene is clipped, and since I was shooting in stills mode, I can use the RAW file to process the image correctly.

(You might also like: Can Your iPhone Really Be A Light Meter?)

And now on to the iPhone Apps...

Pocket Light Meter Results

The Pocket Light Meter is not far off from the Sekonic meters; it differs by only 1/3 of a stop.

Cine Meter Results

The Cine Meter nailed it. And to my surprise, even the waveform is correct- falling around IRE 50. Very impressive, if you ask me!

The Uncontrolled Results
Controlled environments are one thing, but what about the real world? Lets take a look... First up the Sekonic meters:

Incident Readings (Using the default profile)

Spot Readings (Using the default profile)

I had to bump the ISO up to 2500 to get to the F4 that the 60D needed using the kit lens. But other than that, the readings look healthy.

What that looked like on the 60D...

Exposure set according to the Sekonic meters

Again, using my meters, I get a good exposure. Nothing is crushed, and there is only very minor clipping in some of the specular highlights coming off of the microwave. What does the camera think the exposure should be at?

Exposure set according to the full auto mode in the camera.

It looks like the camera again wants to over-expose the image. This time by about 2/3 of a stop. (Which is a great argument for not relying on the auto mode in cameras...). Fortunately, this is in RAW, and the histogram shows that there is very little clipping, so I can still recover a usable image with some added time in post processing.

What do the iPhone Apps have to say about it?

Pocket Light Meter Results

The Pocket Light Meter remains consistent, and is only off by 1/3 of a stop. Not too shabby! (Actually less than that, as the spot meter results from the Sekonic meters were F 2.8 8/10).

Cine Meter Results

This time the Cine Meter is off a little bit too, but as it is less than 1/3 of a stop. It is hard to complain when the app was only $4.99 (plus the dongle).


My Thoughts On The Results
I must admit that I am VERY surprised by how close these iPhone apps are to an external spot meter. They even did better than the camera "brain" of the 60D! Both apps did well enough that I can comfortably say that you can use these apps to spot meter, provided that you take the following into account:

- Test the meter app out with the camera you will be shooting on and calibrate it as needed. Both the Pocket Light Meter and the Cine Meter offer the ability to calibrate the meter to an external meter or camera. (See their included documentation). If set it up properly, the exposure differences should be even smaller, to not existent.

- Only meter known reference sources. This can be the biggest gotcha, as it is possible to fool the iPhone app into giving you an incorrect reading. (As demonstrated in the pictures below). To avoid this from happening I recommend metering a grey card in the key light. Alternatively, you can also meter skin tones in the key light, as long as you keep in mind where the different lightness values of skin tone fall in the Zone System. :)

Spot Readings (Using the default profile) (F 11 9/10 [F 16] & F 11 6/10) 

Pocket Light Meter Results (1.5-2 stop difference)

Cine Meter Results (1/3-2/3 stop difference)

So will I be getting rid of my 758Cine or 478D? Not any time soon. While these results do have me thinking twice, right now, I'm not comfortable enough relying on a mass produced iPhone camera that was intended for the much more forgiving consumer market. However, I will be continuing to use both of these apps along with my meter, to get a better feel for them. (Maybe after repeated long-term use, I'll be thinking differently...). I am always looking for ways to make my work belt lighter. So it is appealing to me to have my iPhone double as a meter. After all, I'm using it already with a number of other apps for the work I do.

Ryan's iPhone Apps

The other thing that I appreciate about the Sekonic meters is the ability to create custom profiles, map the dynamic range of different camera systems, and then load them at will. I also appreciate the option to use the incident meter when needed (e.g. to measure contrast levels, or to get foot candle readings). (But maybe someone will figure out a way to make an incident attachment for the iPhone?). And with the 478D, I have really enjoyed being able to load in filter packs, and dial in customized settings.

I am VERY impressed with where technology has come, and I appreciate people like Adam Wilt who are innovating, pushing tech forward, as well as making it more accessible, and easier to use for everyone. If you have had your eye on a light meter, but haven't had the money to get one yet, then I would recommend that you get your hands on one of these iPhone apps (if you already own an iPhone). They are a great way to start learning how to use a light meter. (By the way, I recommend paying the $4.99 for the Pocket Light Meter. Even though you can get it for free, supporting the developer is a great way to ensure its continued development and support).

Are either of these apps appealing to you? Will you be switching to solely using an iPhone app for your light meter? Are there other light meter apps that you would recommend?

Until Next Time - Get Out There And Shoot!

Ryan E. Walters, Cinematographer

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