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Turn Your Camera Meter Into
An Incident Light Meter:
Using an ExpoDisc
Jim Doty, Jr.

ExpoDisc from Expo Imaging. Photo copyright Jim Doty Jr.


There are a lot of advantages to using an incident light meter, and you can read about them here.  But what if you don't have an incident light meter? Or you are packing light and don't want to take your incident light meter with you? There are several less expensive alternatives, and one of them is to use an ExpoDisc. A photo of one is above, sitting on an 18% Gray Card.

As you can see, they look like a filter with a translucent, honeycomb-like surface which is a specially designed light transmissive layer. An ExpoDisc goes on the front of your lens, just like a filter, but they have a compression fit (friction) instead of filter threads.

An ExpoDisc needs to be the same size as the filters you put on the lens so that no light leaks around the edges into the lens. However, you don't need one for each of your lenses. Just buy one in a size that fits a lens you always have with you and use that lens whenever you use the ExpoDisc to meter.  That may mean swapping lenses from your "metering lens" to the lens you will use to take the picture (if it is takes a different filter size), but that's a lot cheaper than buying a bunch of ExpoDiscs.

Using an ExpoDisc

An ExpoDisc is ridiculously simple to use. Just put one on your lens, point the lens at the light source (turn off autofocus), and take a meter reading with your camera.  Lock in that exposure reading, take off the ExpoDisc, turn around and face your subject, and take the picture.

Photogrrapher Using an ExpoDisc. Photo copyright Jim Doty Jr.

Photographer Pointing an ExpoDisc
At the Light Source

Other than that, you use an ExpoDisc just like you would an incident light meter, the only difference being you are pointing the surface of the ExpoDisc at the light source (usually), instead of the white dome of an incident light meter.

The same caveats apply to using an ExpoDisc as when using an incident light meter: (1) The ExpoDisc must be in the same light as the subject, (2) the direction you point the ExpoDisc matters, and (3) you must do some exposure compensation for very light and very dark subjects. (4) Just like with an incident light meter, an ExpoDisc will NOT work if your subject IS the light source. Rather than repeat all of the relevant information here, all of the details are in the article on Using an Incident Light Meter.

Setting Your Camera's White Balance

One advantage to using an ExpoDisc Neutral is that you can use it to set the white balance on your digital camera, something you can't do with an incident light meter. Point your ExpoDisc at the light source, turn off autofocus (so the lens isn't helplessly trying to focus on the ExpoDisc), meter, and take a "picture" of the light coming through through the ExpoDisc. You won't get a "photo" of the light source due to the translucent layers. You will get a plain looking, even, medium toned image. Use this "picture" to set the white balance on your camera (read the manual to see how to do this with your specific camera model). Your photos will have a nice, neutral white balance. If the light intensity or light source changes, you will need to re-meter, take a new "picture" through the ExpoDisc, and set the white balance again.

If you prefer a warm white balance, rather than a neutral white balance, you can get an "ExpoDisc Portrait" instead of an "ExpoDisc Neutral". Not only do some portrait photographers appreciate a warmer white balance, some landscape photographers do too.

Exposure Articles

Why Is Exposure So Important?

Speaking Your Camera's Exposure Language: Aperture, Shutter Speed, and ISO

Using Reflected Light Meters, Part One

Using Reflected Light Meters, Part Two

Simplify Your Life with an Incident Light Meter

Turn Your Camera Meter Into An Incident Light Meter: Using an ExpoDisc

Using a Gray Card, Incident Light Metering on the Cheap

The RAW versus Jpeg Exposure Advantage

Purchase Link

You can buy an ExpoDisc in the size of your choosing by clicking the Amazon link at the bottom of this page and search for ExpoDisc.

More Info

The most detailed information about using an ExpoDisc, gray cards, incident light meters, and camera meters is in my book, Digital Photography Exposure for Dummies.

You can read more about the ExpoDisc at the company's Web site.

December 26, 2010
Updated May 17, 2013

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