Metering Techniques:
The Duplex Method For Determining Exposure For Backlit Subjects With an Incident Meter


In my Introduction to Handheld Exposure Meters and my How to Use an Incident Meter For Digital Photography tutorials, I explained that incident light meters cannot be used to meter backlit scenes. That isn't entirely true!

When used normally, an incident meter will give incorrect exposure of backlit scenes. This is because an incident meter measures the light that falls on the front of the subject. In a backlit scene, the main light comes from behind the subject, where the incident meter is blind.

In my other tutorial on backlit scenes, I taught you a technique in which you average an incident reading and a reflected light reading of the scene. It works wonderfully, but can only be used with meters that can also do wide-area reflected light metering as well as incident metering.

This tutorial will teach you a technique for determining exposure for backlit subject matter using only an incident meter. This technique is called the Duplex Method. It was invented and named decades ago by Jack Dunn, a British engineer who did pioneering research into exposure metering in the mid 20th Century. Dunn wrote, with George Wakefield, a book called "Exposure Manual." If you're as big of a meter geek as I am, you want to get hold of this book.


The Problem:

In a backlit scene, an incident meter used in the normal manner will give readings that overexpose the photograph. This is because the meter's white dome will be in the shade due to the main light source being behind the meter.


A backlit scene that is overexposed because exposure was determined with an incident light meter.

This photograph, in which the bright sun comes from behind the sublect, is overexposed because I determined exposure with an incident meter using the normal metering technique for an incident meter. I stood next to the tree and pointed my meter's white dome back at the camera.


Equipment Requirements:

Normally, an incident meter has a white dome covering the light sensor. Many meter manufacturers offer a "Flat Diffuser" as an accessory. This flat diffuser is required for the Duplex Method.


Incident light meters with the normal spherical diffusers mounted on them.

Incident light meters with the standard "Spherical Diffuser" dome.


Incident light meters with flat diffusers mounted on them.

Incident light meters with the "Flat Diffuser."

Most meters, like the Minolta meter shown here, have a separate flat diffuser that replaces the standard dome.

The Sekonic has an ingenious system that doesn't require the removal of the dome when you need a flat diffuser. Instead of removing the dome, the you lower the dome into the meter head by turning the knurled ring around the base of the dome. When lowered, the dome then functions identically to a meter with a flat diffuser. This is nice because you don't have to carry an accessory, and there's no risk of losing it or the dome when out photographing. Several meters made today have this feature.


How to use the Duplex Method:

1: Mount the flat diffuser on your incident light meter (or lower the dome if your meter has that feature).

2: Take a meter reading from the subject position, with the flat diffuser pointed directly at the camera. This is just like doing a normal incident light reading, except you're using the flat diffuser instead of the dome. Remember the reading it gives. If your meter has a memory function, you can use that.

3: Take a meter reading with the flat diffuser pointed directly at the source of light. Outdoors, this is usually the sun. This reading will be higher than the first reading you took. If it is a sunny day with a clear, cloudless sky then the sun direction reading may be 4 or 5 stops higher than the camera direction reading you took in step two.

Remember this reading, or put it in your meter's memory if it has one.

4: Average the readings. If you're using a modern meter with a memory function, then it probably also has an Averaging button. This makes it super math!

5: Set your camera to the average reading and take the picture.


Don't Worry if it Doesn't Look Perfect

The image that results may not look perfect on your camera's screen. That's ok. This method is designed to ensure that the full range of tones are captured by the sensor. To do so, the final image may look underexposed; especially if the difference between the two readings that you took is very large.

In such lighting, it is impossible to get a "perfect" image right from the camera. The goal is to produce a RAW file that can be edited to bring out full detail in the lightest and darkest areas of the image. This technique does that. Notice that I said "RAW File," not JPEG. You must shoot RAW and edit the image using RAW conversion software, like Adobe Lightroom or Capture One.

On the next page, I'll show you some real-world examples of photographs of backlit scenes made using the Duplex Method to determine exposure.


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©2018 Christopher Crawford