DIY Incident Light Meter and Custom White Balance

An older mature photo professional showed me this trick of using a white Styrofoam cup over your lens as an incident light meter and to set a custom white balance. He was a portrait and event photographer in the film days when you couldn’t just pimp your image on the screen as we do today.

I was a bit skeptical because, being a digital kind of gal, always thought that you could just make the same corrections as long as you shot in RAW. Well, you probably can, but in this test, the one with the custom WB was definitely better than anything I could do in Adobe Camera Raw.

Here is how it works.

1. Set your camera to AV, TV, or P and the appropriate ISO, aperture and/or shutter for your situation.
   In other words, set your camera to normal semi-automatic settings. Don’t use M right now, we will use that later.
2. Take a white Styrofoam cup (or bowl for those bigger lenses) and put it over your lens.
   You will be taking a picture of the inside of the styrofoam cup.
3. Use your camera like an incident light meter in the position of the subject and take a picture of the inside of the Styrofoam cup. I’ll call this the reference image.
   This means you put your camera where your subject will be and point it towards where the camera will be. You are now taking a reading of the light that will be falling on the subject (called the incident light). In the normal situation, when you point your camera toward your subject you are reading the light that will be reflected off your subject (called reflective light). If you study lighting, this is there is a difference between incident light and reflective light in many, but not all, lighting situations. For best exposures you want incident light, it is just so darned hard to get most of the time so you approximate with reflected light and, hopefully, use your brain to apply some manual camera adjustments when you are in extreme situations.
4. In this step you will both set the custom white balance and the exposure using the reference image you just took of the inside of the Styrofoam cup.
   • First use your reference image, the one you just took of the inside of the Styrofoam cup, to set the custom white balance using the method for your camera. You just might have to dig out that manual. Usually it involves going to a menu somewhere and choosing the image to use for setting the custom white balance and then changing your WB setting from AUTO, CLOUDY, TUNGSTUN or whatever to CUSTOM.
   • Next look at the exposure settings of your reference image, f-stop and shutter speed, and using M on your camera, manually setup your shot using these settings.
5. Take the picture of your subject and be amazed.You will be amazed at how good and dead-on the result is.

Here is an example taken in a typical bad lighting situation, a dimly lit den. They were taken on my Canon 5D mk II at ISO 6400 (eat your heart out).

Auto image straight off the camera. Evaluative metering, auto white balance. 1/125 sec, f/5, ISO 6400

Manual edit of auto image using Adobe Camera Raw, EV +1, WB using eyedropper on the white mat on the wall as the reference point

Reference Image for exposure and white balance (the inside of the Styrofoam cup) 1/125 sec, f/4, ISO 6400

Image taken using the Reference Image for white balance and exposure settings – Straight off the camera 1/125 sec, f/3.5, ISO 6400

Hmm, it looks like the experimenter set the f-stop 1/3rd of a stop more open that would have been indicated. Never the less, the experiment is close enough.

Here they are next to one another

Custom WB using Styrofoam WB and Exposure method

Manually edited in ACR, EV +1, eyedropper WB adjustments

To my eyes, the one using the reference image is much better. First, there is less noise because I didn’t have to push the exposure value. Second, the skin tone versus the white wall color seems to get both better. The skin tones look about the same, but notice the greenish cast the wall in the manually edited version. Finally, I did nothing to the one with Custom WB. Here is a case where an ounce of planning gives us back time in the digital darkroom.

I understand that there are also lens-cap kind of looking things that are sold also for this purpose. Like this product ColorRight. I don’t know if this product will also give you the added benefit of acting like an incident light meter. Additionally, others have suggested that you can use a white plastic lid from a coffee can (do they make those anymore?).

Acknowledgments

Thanks to Lee Agnew for showing us this trick, George “Hutch” Hutchison for taking the pictures, and Joni Agnew for posing.