Cameras and Film

Free Incident Light Meter (With Purchase of iPhone 4)

So my birthday is coming up. This was fortuitously timed with the almost complete exhaustion of the battery in my Apple iPhone 3G (not GS…I’m behind the times by two generations). So my wife sent me down to the local AT&T store to buy myself a birthday present, which I was happy to do. Behold, my new iPhone 4! Woot woot. There it is, above.

Wait, what’s that piece of paper taped to my new phone? Why, it’s a free incident light meter of course!

There are a number of light-meter apps for the iPhone, although none of them would work on my 3G. So that was the first thing I looked for when I upgraded my phone. However all of those meters are reflective light meters. To use them, the phone’s camera is aimed at the scene to be photographed, and measures the light being reflected from the objects in the scene. Just like the meter found in all cameras.

Reflective light meters are certainly convenient, but they have some drawbacks. They are easily fooled by back-lit situations, because the sun or bright sky causes the meter to underexpose. They also vary as the scene varies, making your images inconsistent from one to another. And they are also fooled by very bright or very dark scenes: that wedding dress in a white room is going to underexpose and make everything gray. Same with the dark forest.

An incident light meter measures the light falling on the object, not the light reflecting from the object. This means that you get the proper exposure, no matter how light or dark your actual subject matter is. This is done by placing the incident meter at the subject’s position, and pointing it back toward the camera. The meter determines how much light is falling on the subject from the camera’s direction, and proper exposure is achieved. A white translucent dome or disc is placed over the sensor, to average the light sources and avoid errors.

Using an incident meter is also useful because you can meter for the shadow side of something, without worrying about any back-lit light sources causing your meter to underexpose. I do this all the time with portrait photography. I place my subjects with their backs to the sun, and then use an incident meter to measure the light falling on their faces. I use this exposure reading, and let my highlights blow out if necessary. Since I’m shooting film, I’m not too worried about overexposure: film can handle a lot more highlight information that digital can. Here’s an example where I metered for the shadow side with an incident meter, and then shot in manual mode. I didn’t have to worry about exposure as long as I kept my clients facing the right way. (By the way, my work is filled with examples of back-lit images such as this, some strobed and some without. You can see more here.)

So incident meters are useful for film photography especially. They take a little more effort, but the results are worth it. Wouldn’t it be great if you could use your phone for an incident light meter, and not have to carry a dedicated meter around with you?

(read more…)

That’s where that little piece of paper comes in. I did a search for iPhone light meters and came up with several: some were free, some were cheap. There was even one that claimed to have an incident light meter setting! Yay! Until I tried it. Boo! The designer was obviously confused about what an incident meter was, and just switched the meter reading from the main (rear) camera to the front one. That’s not a functional incident light meter, and it’s useless that way.

However, with the addition of a piece of typing paper taped over your iPhone 4′s front lens, you have both an incident meter and a reflective meter, and don’t need to fuss with putting some sort of diffusion over the main camera lens. You keep the main lens free to take pictures, and can always remove the tape if you need the front lens for something. The key is to find a light meter app that will allow you to switch between the front and rear camera lenses. Then just leave the tape there all the time. As a general rule, you’ll have to determine what sort of exposure correction you need to do with the incident light meter. You could even have one meter app for reflective readings, and a second app for incident readings. Leave the second app with the exposure compensation and the front camera dialed in, and you don’t have to fiddle with settings.

As luck would have it, I found a meter app that not only allows me to switch between front and rear cameras, but also doesn’t need any compensation! Coincidence? I’m not sure. It turns out the rear lens measures reflective scenes pretty well, and the front lens measures accurately with the paper taped over it. Other apps didn’t respond the same way, and would require exposure compensation.

The app is called Pocket Light Meter, made by Nuwaste Studios. It’s free with advertisements, or $0.99 without. I’m not affiliated with this company, I’m just telling you which one worked best in my experience. FotometerPro, by Kit Da Studio, is the app that inaccurately claimed to have incident metering. I found it slow and occasionally inaccurate, even with reflective metering.

This combination of printer paper, tape, and iPhone 4 app yields an incident meter reading that is usually within 1/3 stop of my Gossen Luna-Pro F, and also my Minolta digital meter. That’s pretty impressive! I’ve been wandering around the house and yard for the past two days, checking the readings against my ‘real’ meter. It works! Your printer paper might be a different density than mine, your adhesive tape might also have different characteristics than mine, so your mileage may vary.

Now I’m off to Alaska for two weeks, and I’m not bringing a meter. I will rely on my Mamiya 645AF for reflective in-camera readings, and will use my iPhone for incident readings. I won’t get rid of my Luna Pro, but for travel and always-with-you metering, this is perfect!

P.S. If you don’t have a real incident light meter to use for calibrating your meter app, there are a couple of ways you can find the right exposure compensation adjustment:

1. Grab a camera and select spot-metering if it has it, and go outside. Put the sun behind you, and hold the palm of your hand in front of you, in the sunlight (or ambient cloudiness, depending on the weather). Get a meter reading off your hand, and mentally adjust so that you overexpose one stop. That’s your meter reading. Now point your diffuser-covered lens at the sun/bright spot in the sky, and measure. Adjust your exposure compensation accordingly. For example, on a sunny day you might get a reading of f/16 1/250 @ ISO 100 from your palm. Mentally adjust that to f/16 1/125 @ ISO 100 (one stop overexposed). And then you measure with your iPhone, and you get a reading of f/16 1/60 @ ISO 100. That’s one stop overexposed, so you need to adjust your exposure compensation to -1 stop. This palm-metering technique works regardless of your skin color, by the way, because palms tend to be the same brightness regardless of the color of the rest of your skin.

2. No camera with a built-in meter? Just take your iPhone meter outside on a sunny day, at noon. Get a meter reading by pointing the diffused lens at the sun. Using the sunny-16 rule, you should get a reading close to f/16 1/100 @ ISO 100. If not, adjust your exposure compensation until you do.

Update: here is a screen shot of the app, measuring bright sun on the front lens. The image is the blurry paper. The reading is within 1/3 stop. Cool huh?

. . . . . . . . . . .

Posted in nerdy technical stuff | 16 Comments »

• 
  Digg
• 
  Facebook
• 
  Delicious
• 
  StumbleUpon
• 
  Twitter
• 
  LinkedIn
• 
  Technorati
• 
  reddit
• 
  Mixx
• 
  Newsvine